Time to Failure Series Continued
Electrical Signature Analysis – Part 10 Winding Shorts and Stator Mechanical Faults
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Homework:
Contact me and request the ‘MCSA Pattern Recognition Guide Excerpt’ for general reading on this topic.
Stator Winding Shorts
Electrical winding shorts cause variations in the magnetic field at the point of fault as the resulting arcing will cause high current at this point. As the fault is occuring in the stator, the faults related to winding shorts will have a forcing function relating to the running speed of the rotor times the number of stator slots. The winding fault is electrical in nature which results in line frequency sidebands of the stator slot passing forcing function.
(Running Speed x Stator Slots) +/- Line Frequency = Stator Electrical.
The fault then has a second, lesser, forcing function around the electrical forcing function of the running speed. Therefore:
Stator Electrical +/- Running Speed sidebands = Winding Shorts.
The detection of stator winding shorts with ESA requires frequent monitoring as detection online will indicate a later stage winding fault.
Stator Mechanical Faults
Stator mechanical faults are defined as loose coils, loose wedges or loose stator. The movement occurs with a forcing function of the line frequency. However, there will be no running speed sidebands (Stator Electrical only).
Small sidebands, or a raised noise floor around the Stator Electrical peaks, indicate a worsening condition which should be addressed.
Stator mechanical faults will normally be detected above 25% load and the fault should be at least -60dB to be considered serious.
Time to Failure Series Continued
Electrical Signature Analysis – Part 9 Dynamic Eccentricity, Misalignment and Unbalance
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Homework:
Contact me at hpenrose@alltestpro.com, and request the ‘MCSA Pattern Recognition Guide Excerpt’ for general reading on this topic.
It has been a long week on the road, so I have fallen behind. Therefore, I am going to cover the remaining topics for this week in this lecture.
Dynamic Eccentricity
Dynamic Eccentricity is similar to Static Eccentricity in that the rotor is operating off-center. The difference, however, is that there is a driving force causing the rotor to flex out of alignment versus the rotor being mounted eccentrically. One example of a cause for dynamic eccentricity is smeared rotor laminations. In this case, the motor will start with a relatively even air gap (it could also start with static eccentricity, multiple problems can exist in a motor), then, as eddy currents in the damaged laminations generate heating, the laminations will expand in that area, reducing the air gap on that side.
You will now have two forcing functions: Line Frequency and Running Speed.
The result is (RS * RB) +/- (N * LF) with RS sidebands, of lower magnitude, around the LF sidebands. N is an odd integer starting with 1.
Misalignment and Mechanical Unbalance
This one is potentially misleading as the basic signature for both is the same. However, there is a rule of thumb for evaluating which condition exists.
In high frequency FFT, you will have a pattern that appears as line frequency sidebands, 4*LF then 2*LF with the original sidebands existing around the point RS * RB. You can determine if the defect is misalignment or unbalance by looking at the demodulated current spectrum. If a peak exists at running speed AND you have the misalignment or mechanical unbalance signature described above, then the defect is most likely significant mechanical unbalance. Less likely, but possible, is severe misalignment.
The reason for the 1x peak is that the rotor is forced off center in one location.
Conclusions on Rotor Related Faults
As you may have noticed, all rotor related faults appear as rotor eccentricity. This re-enforces the previous lecture statement that these faults are detected due to rotor movement, radially, within the air gap. This results in an eccentric magnetic field, which causes ripples in the motor current.
Next week, we are going to work on AC induction motor stator faults such as loose coils, stator core movement and winding shorts, including how to determine the severity of these problems.
Time to Failure Series Continued
Electrical Signature Analysis – Part 8 Static Eccentricity
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Homework:
On the website http://www.motordiagnostics.com/motor_diagnostics_papers.htm download: “Applications for Motor Current Signature Analysis,” and, “Motor Current Signature Analysis and Interpretation.” These papers will help with the following Blogs.
Rotor eccentricity can be caused by a number of issues, such as eccentric machining of the rotor, end shield rabbet fits, machining of the rotor or shaft, foot flatness problems, twisted base and severe bearing problems. This causes the rotor to sit to one side of the stator bore during operation.
Each rotor bar passes each pole pair at running speed. This causes a base frequency of the running speed times the number of rotor bars plus and minus the line frequency of the motor ((RS x RB) +/- (N * LF) as the forcing frequency is related to the Line Frequency of the motor, which generates sidebands (The rotor pushes and pulls within the airgap at 2LF). N is an odd integer starting with 1.
Several problems arise from static eccentricity, including:
• Magnetic pulse waves that put electrical and mechanical stress on the winding;
• Danger of the rotor touching or rubbing the stator laminations; and,
• Additional stresses on the motor bearings, reducing bearing life.
Normally, eccentric rotor problems are found along with other electric motor problems, such as misalignment, bearing faults and mechanical unbalance.
For a free sample section of the ALL-TEST Pro Electrical Signature Analysis Pattern Recognition Manual, which covers AC Pattern Recognition for AC induction motors, email hpenrose@alltestpro.com and state "Blog ESA Manual" in the subject line with your name and company affiliation in the body of the email.
Time to Failure Series Continued
Electrical Signature Analysis – Part 7 General Rules for Analysis
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Homework:
On the website http://www.motordiagnostics.com/motor_diagnostics_papers.htm download: “Applications for Motor Current Signature Analysis,” and, “Motor Current Signature Analysis and Interpretation.” These papers will help with the following Blogs.
In the case of vibration analysis, a linear transducer using a piezoelectric crystal is used to measure direction and amplitude of primary mechanical and some electrical vibration frequencies. In the case of ESA, current signatures are obtained from small movements within the airgap that cause fluctuations in the motor current.
Rotor-related problems generally occur in such a way that the rotor bars are directly related to fault frequencies by the running speed of the motor. Virtually all of these conditions start with a base ‘center frequency’ of the number of rotor bars times the running speed.
Stator-related problems generally occur in such a way that the stator slots are directly related to fault frequencies by the running speed of the motor. As with rotor-related faults, the base ‘center frequency’ is the number of stator slots times the running speed.
Other problems, such as bearings, load-related problems, etc. can also be calculated due to variations in the airgap. Each of these problems cause flexion of the rotor shaft with the result of radial movement of the rotor within the airgap.
During this week, we will focus on the rotor-related problems such as static eccentricity, dynamic eccentricity and mechanical unbalance and misalignment.
Time to Failure Series Continued
Electrical Signature Analysis – Part 6 Broken Rotor Bars
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Weekend Homework:
On the website http://www.motordiagnostics.com/motor_diagnostics_papers.htm download: “Applications for Motor Current Signature Analysis,” and, “Motor Current Signature Analysis and Interpretation.” These papers will help with the following Blogs.
When a rotor bar breaks or fractures, a portion of the rotor winding circuit is open. This requires higher current to flow in the bars immediately surrounding the break in the circuit. This high current generates a higher than normal (unbalanced) magnetic circuit at the point of fault causing a wave of magnetic force to proceed with the fault. The result is a pulsating beat pattern in the operation of the motor that is directly related to the slip frequency and the number of poles: ppf = fslip x poles.
In the past, you could use an analog amp meter to observe the beat by the deflection of the needle during operation. This can even be observed on analog CT meters in motor control centers. Now, modern Electrical Signature Analysis methods can immediately identify the fault and magnitude of the fault, when presented as dB. As a fault peak becomes greater in magnitude, the peaks rise towards the peak magnitude of line frequency current. dB measurements, in the case of ESA, are measured from the peak of the line frequency current magnitude to the base of the line frequency current (the same will be for voltage when we approach that discussion).
As discussed in yesterday’s lecture, a beat frequency results in sidebands. As a broken rotor bar causes a significant change to the magnetic field, within the airgap of the motor, and is directly related to the line frequency of the supply voltage, ppf and resulting current, the sidebands MUST be ppf frequencies around the line frequency current. The magnitude, as determined by experiment (in a separate paper, to be presented in the next few months, we will be mathematically (theoretically) representing the values), appears to be -35 dB (dB down from the current peak) as a pass/fail value for sidebands related to the rotor bars.
Contrary to traditional thinking, Broken Rotor Bars Are a Progressive Fault.
What are the effects of broken rotor bars?
There are three specific faults related to broken rotor bars: Efficiency; Winding; and, Torsional. Each type of fault results in damage to the motor or driven equipment.
The efficiency of the motor depends directly on the amount of torque the motor can provide. The amount of torque depends directly on the amount of current that can be provided to develop a magnetic field, evenly, within the airgap of the motor. Broken rotor bars have two impacts on efficiency:
1. They interrupt the rotor winding circuit causing changes to the magnetic fields in the airgap (uneven transformer effect), generating increased magnetic field losses in the motor and reducing the ability of the rotor to generate torque.
2. The ppf current beat is the result of torsional pulsation caused by the variance in the magnetic field. The torsional pulsation represents stalling and starting of the rotational energy making the delivered torsional energy less than it would be in an even field.
The winding effect comes directly from the ppf current, as well. The result of the broken rotor bar is a magnetic force wave that rotates with the broken bar. As this force wave passes over each stator magnetic pole, the force wave puts physical movement stresses on both the windings and rotor. Picture a large football (either American or European) stadium. Now, picture the crowd performing the ‘wave,’ where they progressively stand up and raise their hands, then sit down again. This is, basically, what is happening to the windings within the motor, with the fault point drawing the conductors up into the wave, then letting the conductors fall. This movement puts wear and tear on the conductors and insulation system, eventually leading to a winding failure, usually at the point where the coils leave the stator slots. In another, worst-case scenario, a broken rotor bar may come loose and impact the windings directly.
The final(?) effect is the torsional effect on the load. The ppf beat directly effects the output torque of the motor and, in effect, impacts the driven load directly. For example, let us consider a motor driving a gearbox. When the motor is operating properly, and smoothly, the gears, even if worn, mesh together and an even amount of power is introduced into each gear tooth. Now, if we have broken rotor bars, we are introducing a pulsating power against the teeth at the ppf of the motor. The result is much like turning the gear with a hammer, with the eventual result of cracked or broken gear teeth. In the case of a belt, additional wear on the sheave and belt, itself. In a coupling, wear of the insert or damage to the coupling bolts, keys, etc. In addition, in belted applications, the angular torsional beats will generate increased wear and tear on the motor and load bearings.
Are we finished? No, there is actually one more problem that comes with the broken rotor bar issue: The supply side impact. Broken rotor bars, once they have progressed, will cause current pulsations back into the power supply, and, due to the reduced efficiency, will require more power to operate the motor and load. This puts additional stress on the conductors, connections, starters, contacts and transformers, based upon the size of the motor, severity of the fault and loading of the circuit.
How Common Are Broken Rotor Bars?
The good news is that broken rotor bars are a rare occurrence in small motors (< 600 Volts) and occasional in large motors (> 600 V). One plant that has all less than 600 V motor systems put it this way (they have maintained records for over 10 years): In 10 years, with an average rate of motor repair on their 30,000+ motors of 750 per year, they have only seen 3 cases of broken rotor bars, only during the repair process.
However, in power plants, chemical, petroleum and similar heavy industries that use a large number of >600 V motors, the rate is much, much higher. The primary cause being too many starts. As noted in Part 2 of this lecture series, each start generates a significant amount of heating within the rotor, which does not rapidly dissipate. Many large machines have a limit of one or two starts per day (some less). However, they may be started and stopped many times per day.
Rotor Bar Case Study: Pre-ESA:
In the early 1990’s, I was called out, on a Sunday, to a petro-chemical site to look at two 1500 hp, 3450 RPM, 4160 Vac motors on high pressure pumps (coke cutters) to perform vibration analysis as both motors were not producing enough power through the pumps to effectively perform their function. Upon arriving, I noticed the analog meters associated with each pump and saw the current fluctuating, on the worst one, from 180 Amps to 210 Amps in a regular beat. Vibration was performed and it was noted that twice line frequency peaks appeared with ppf sidebands (in vibration, electrical issues show as twice line frequency, which is different than the one time line frequency seen with ESA) and harmonics. Also, high peaks in acceleration that related to what might be rotor bar pass frequencies (# of rotor bars times RPM) with harmonics.
The motors were pulled and taken to the repair shop. There, once the motors were disassembled, it was noted that on the worst motor, over half of the rotor bars had failed! The other one had about 1/3rd of the bars broken. During the root-cause investigation, it was discovered that the motors, rated for one cold and two hot-starts per day, were being started a minimum of 12 times per day and a maximum of 24 times per day, seven days per week.
Total time to perform the investigation, once on site, was about 2 hours, not including shop time or root-cause-analysis.
Rotor Bar Case Study: Post ESA
In a recent case, a 500 hp, 3600 RPM motor on a compressor was showing signs of wear, as was the compressor. Routine vibration analysis identified RBPF and 2LF signatures, indicating probable broken rotor bars. The motor was sent to a repair shop who replaced the bearings, painted the motor and returned it stating that there were no other problems. The reliability program was brought into question because of the impact that the compressor has on production and the time it was out of the facility. The vibration group continued to monitor the fault, which appeared to progress.
ESA was performed on the motor as part of an onsite training course. Classic ppf sidebands at -38 dB were immediately detected (we could not have hoped for a better example). Using both the ESA and vibration data, the reliability manager had the motor pulled and sent into the repair shop. The repair shop again stated that there were no problems with the motor.
At this point, the method to evaluate the rotor bars by the repair shop was brought into question. There are several ways of testing for broken rotor bars through the repair process:
1. Visual inspection. Not very accurate.
2. ‘Growler’ test: Where the rotor is placed on a half-transformer fixture and current is induced. A medium such as iron filings or magnetic paper can be used to trace each good bar. Because of the energy used to cross the fault point, this (and the next method) method can go through the oxidized surfaces of a fracture and the fault missed. The best approach is to heat the rotor in an oven to about 210 degrees F prior to performing the test (expansion will separate the fracture).
3. Single Phase Test: Where 10% of the motor rated voltage is provided to two phases of the motor, in effect single-phasing the load. The rotor is hand-turned slowly with a clamp-on amp meter attached to one phase. Variations of more than 3% indicate a fault. There should be an increase and decrease of current once for each pole of the motor (always in pairs).
4. MCA Rotor Test: Using inductance, as outlined in last month’s series on Motor Diagnostics and Quantum Mechanics.
5. Dye Test: Similar to a visual test, however the dye is used to readily find the broken or cracked bar.
Trivia: The term ‘growler’ comes from when the system was used to detect shorts in DC armatures. The power would be induced and a hacksaw blade would be brought over the surface of each armature slot. The charging and discharging point (arcing) of the short would cause the blade to vibrate and make a growling noise.
As it turned out, the repair shop had used a visual check. The site used MCA to verify the fault, again (now three methods) and sent the motor to another repair shop. The new repair shop utilized a proper growler test method and identified two severely broken rotor bars.
In the continuation of this lecture series, next week, we will start investigating other faults that can be analyzed by ESA.
Time to Failure Series Continued
Electrical Signature Analysis – Part 5 Motor Losses (2) and Sidebands
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
In today’s topic, we are briefly going to describe two items that will be of importance as we move forward: Steady state losses in a motor; and, How sidebands are formed.
Steady State Losses
Although eluded to in a number of cases, throughout the past and present lectures, following are the losses that are common within an electric motor during operation (All losses are represented as Power or Watts):
• Stator I2R losses: The heat energy produced, in Watts, as current flows through the conductors of the windings. Calculated as current squared times the DC resistance of the stator circuit.
• Rotor I2R losses: The heat energy produced, in Watts, as current flows through the rotor bars of the rotor. Calculated as current squared times the DC resistance of the rotor circuit.
• Friction and Windage losses: The drag caused by air passing over the surface of the rotor and fans as well as the friction of the bearing surfaces.
• Core Losses: There are two types of core losses that occur both in the rotor and in the stator:
o Eddy Currents: Result from a magnetic field interacting with metal. This can also be referred to as ‘induction heating.’ In a stator, this can be found as localized heating when you have shorted laminations. The maximum allowable ‘hot spot’ in the stator is 10 degrees C. This heating will accelerate the degradation of electrical insulation at the point of fault. In a rotor, the effect of shorted laminations and eddy current heating is a ‘bowing’ rotor. This means that the side with the defect expands, while the side without the defect does not and the rotor ‘bends’ in operation.
o Hysteresis Losses: This is the heat generated due to the resistance in change to the magnetic field by the stator lamination material. This was covered more in-depth in the “Motor Diagnostics and Quantum Mechanics” series in July and early August, 2004.
• Leakage Losses: The final set of losses actually encompass all other losses in the motor including magnetic, fringing and capacitance losses.
The efficiency of an electric motor is defined as: ((kW input – Losses)/kW input) x 100.
Sidebands – How They Occur
One of the more interesting aspects of what we will be dealing with, while performing analysis using ESA, is sidebands.
In ESA we will be dealing with sinusoidal waves in the following manner:
• Fundamental Frequency: This is the primary frequency of the voltage or current. In the USA, this fundamental frequency (when dealing with an ideal power source) will be 60 Hz. When dealing with variable frequency drives, the fundamental frequency can be changed to some other frequency.
• Harmonic Frequency: Is a frequency that resides in the fundamental frequency. For instance, a fifth harmonic in a 60 Hz system would have a 60 Hz sinewave with a 300 Hz sinewave within it.
For now, we will relate this to sound: The carrier (modulated fundamental - Wc) frequency is induced. An event occurs in which a sound is introduced (harmonic Wm). The induced sound is considered a cosine function where (law of cosines):
S = (cosWc x t) + (1/2bcos(Wc + Wm)t) + (1/2bcos(Wc – Wm)t) [where t = time]
The strength of the harmonic is represented by b. When b is introduced into the modulated frequency, the intensity is proportional to the strength of the event, b2, at frequency Wc + Wm and Wc – Wm.
OK, now lets relate this to a series where we are looking at an FFT (Fast Fourier Transform) of a signal, such as current:
Suppose we have a modulated signal of 60 Hz, an event occurs that is related to the pole pass frequency of the motor (we will use 2 Hz from our previous example). We will then have a 60 Hz signal with sidebands of +/- 2 Hz.
Now, suppose we have a new harmonic, that relates to the line frequency of the machine being analyzed, of the line frequency times running speed frequency (example: 60 Hz x 30 Hz (1800 RPM) = 1800 Hz). An event occurs that creates a forcing function at this frequency. The harmonic is the fundamental frequency and the line frequency is the forcing function (b2)!! Due to the amplitude of the fundamental, the forcing function becomes very dominant and you will, most likely, not be able to see the fundamental (1800 Hz) but only the resulting sidebands of +/- 60 Hz. Therefore, the FFT will only show 1740 Hz and 1860 Hz.
This will become more important as we explore some of the discoveries that our engineers have uncovered while analyzing a large number of systems. These concepts will assist us in determining the severity of a fault in terms of decibels (db) due to fundamental and sideband signatures as they relate to the line frequency of the system.
Time to Failure Series Continued
Electrical Signature Analysis – Part 4 Motor Stresses, Losses and Torque (1)
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
There are a number of stresses and losses that occur in an induction motor that directly effect its life, electrically and mechanically. For today’s lecture, we are, again, going to assume a perfect motor and will deal only with the stresses and losses that occur in this type of system. As we move forward to discuss each type of fault detected with ESA, we will go into the real-life variations from the ideal.
While we are discussing this section, there are a few quick definitions that will have to be covered:
1. Torque (T): Is the rotational (twisting) energy produced by the motor. In English units, it can be represented as Torque (lb-ft) = (horsepower x 5250)/RPM; in Metric: Torque (N.m – Newton.meter) = (kW x 9550)/RPM.
2. Inertia (WK2): The resistance to change in speed. In the case of an electric motor, we will use WK2 = Inertia of rotor + ((Inertia of load x load RPM2)/Motor RPM2).
3. Locked Rotor Torque (LRT): The torque a motor will develop while the rotor is stationary and full voltage and frequency are applied.
4. Pull-Up Torque (PUT): The torque a motor will develop as it is accelerating to its breakdown torque.
5. Breakdown Torque (BDT): The maximum torque a motor can develop before it stalls, from a full load.
6. Full Load Torque (FLT): The torque the motor produces at rated speed at rated voltage and frequency under full load.
Rotor Losses
An electric motor comes up to speed at a rate depending on it’s inertia:
Seconds = (WK2 x Speed Change)/(308 x Avg Accelerating Torque (lb-ft))
The Average Accelerating Torque (AAT) can be calculated as:
AAT = (((FLT + BDT)/2) + BDT + LRT)/3
The actual rotor losses in kW-seconds are:
kW-sec = (0.00136 x WK2 x Ns2 x (initial slip2 – final slip2))/5910
The slip is represented as the per unit slip, which is %slip/100. So, 100% slip (stationary) = 1.
Therefore, on startup, as the motor goes from a standstill (100% slip) to full speed, heat is developed. The heat causes the rotor bars and end rings of the rotor to expand. This heat must then be dissipated, otherwise the motor would overheat. The heat dissipation is directly related to the mass of the motor materials. In smaller motors, the amount of material per horsepower is much greater than for larger motors, so smaller motors can be started more often than larger motors, under full voltage, full frequency.
You will also note that the heating of the rotor is directly related to the time it takes to accelerate from one speed to another. This acceleration time is directly related to the size of the motor and the load it is driving. As a result, some loads will reduce the number of starts that a motor is allowed. However, high inertia loads also take a long time to come to a stop, unless an outside source is provided, such as a brake. The good news is that, in very high inertia loads, different designs of motors are available that produce different values of LRT, PUT, BDT and FLT.
Inrush Losses
At the point in time where a motor starts, the rotor sees full line frequency and high current (I = E/Z – Ohms law) as the frequency both reduces the cross section of the rotor bar used (skin effect), increasing its basic resistance, saturates the core and changes the impedance of the circuit. This value of rotor current is seen on the motor leads, due to the transformer effect, as 4 to 8 times the nameplate current.
The result of the high current causes two additional losses:
kWrotor = Irotor2Rrotor
kWstator = Istator2Rstator
In addition, there is coil movement at this point, as the large magnetic fields, due to the high current, cause the turns within the coils to move inwards towards the rotor (the conductors actually move due to the high current – this effect can be seen in a short circuit condition if you have a cable and induce a sudden, direct short circuit, you can see the cable move). This flexion causes mechanical stresses on the end turns of the coils, but particularly on the insulation system as the coils leave the stator slots.
Other Losses:
Other losses include the hysterisis loss, which is higher at startup due to the increased magnetic fields. The cause is related to our previous lecture series and has to do with forcing dipolar spin.
Eddy current losses are due to currents in shorted laminations, which are higher during startup.
Leakage losses also increase. Leakage has to do with the capacitance of the circuit. Details can be obtained from the MCA Blog Lecture Series.
In effect, a high rate of losses occur on startup. The stresses also make this one of the most dangerous times for the motor, as weakness’, mechanically or electrically, can be forced and the machine fail. This causes many to consider inrush analysis as a good method for analyzing a machine. However, the high current and chaos that is occuring during this stage in the motor operation, will often mask many important issues within the motor, itself. We will discuss that as we move forward.
Tomorrow, we will discuss the operation, losses and stresses in a motor during steady-state operation. From that point we can begin our true discussion on ESA, including discussions on how to evaluate systems that are not operating steady-state. (Test methods applying steady-state operating conditions are great in laboratories. However, most real systems, even when considered steady-state, are very dynamic, and the potential for error increases).
Time to Failure Series Continued
Electrical Signature Analysis – Part 3 Electric Motor Theory (2)
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Today, we will begin to explore the operation of a three phase induction motor. The motor consists of the following components:
1. Stator Frame: Encases the motor. Acts to protect the winding, contains the stator core and is used to hold the end shields centered on either end of the stator.
2. Stator Core: Is made up of many layers of either laminated annealed steel or silicone steel. The layers average 0.019 to 0.049 inches thick to reduce eddy currents and are arranged (manufactured) in such a way that the grains allow for less resistance to the change of magnetic flux (hysteresis losses). The stator core both houses the stator coils (primary) and uses the ‘back iron’ (area between the stator housing and coil slots) to direct the magnetic fields into the airgap between the stator core and rotor.
3. Stator windings: Connected to create a north and south pole (pole pairs) separated by 120 electrical degrees. Each pole will normally be made up of a coil grouping, or number of coils, that can be calculated as follows: If a stator has 36 slots, and the motor is designed to be 4-pole, and you have 2 coil sides per slot (36 coils total), you will have: 36 coils/3 phases = 12 coils per phase; 12 coils per phase/4-poles = 3 coils per phase per pole. Therefore, you would have 12 groups of three coils in this motor.
4. Rotor: The rotor is held centered in the stator by the end shields of the motor and bearings mounted on a shaft. It is suspended in such a way that there is even clearance on all sides of the rotor in relation to the stator core. The clearance is defined as the air gap of the motor.
5. Rotor Core: The rotor is also made up of many layers of steel, as outlined in the stator core description. It houses the rotor bars.
6. Rotor Bars: Are made of either an aluminum alloy (caste rotors, normally in motors under 600 Volts) or a copper alloy (normally in motors above 600 V). The rotor bars are shorted on either end by end rings, which are part of the casting for aluminum or welded, for copper. The rotor bars can also be referred to as the secondary, as in the secondary of a transformer.
Author’s Note: The fun part of the exercise of writing a Blog is that you have to illustrate everything in writing. In order to make this readable to the average person, I will withold formulae for the operation of a motor until it is either necessary to describe the operation or to calculate for fault analysis.
For the purposes of this initial description, we are going to treat the motor as an ideal motor. This means that it is theoretically perfect, with no defects and perfect tolerance. Also, that the power supply is also perfect. As we start into the different sections of analysis, we will begin to describe the imperfections in the motor, how to analyze them and how to determine pass/fail and trending.
Visualization:
Three phases of voltage are supplied to the motor separated by 120 electrical degrees. We will assume that Phase A (we will refer to the phases as A, B and C) is at 0 Volts. The peak voltage in a 480 Volt motor will be (480 Vrms/0.707 = ) 679 V peak (Vp). As Phase A increases towards 679 Vp, the four coils will become more magnetically North, South, North, South. Once the voltage reaches its peak and starts to decrease, the coils will become less magnetically N, S, N, S until the voltage reaches 0 V, again. It then starts towards negative 679 Vp and the four coils become more S, N, S, N. When viewing all three phases, if you could see the magnetic fields, looking through the stator from the end, you would see a vortex (it would look like a whirlpool) that is very strong close to the core and weaker towards the center. The speed of the vortex is called the synchronous speed and can be calculated as Ns = (120 x f)/p, where 120 is a constant, f is the operating frequency (60 Hz in the USA) and p is the number of poles of the motor. In this case, we are dealing with a 4-pole, 60 Hz motor which would then have a synchronous speed of 1800 RPM.
The rotor bars in the induction motor, sometimes referred to as a ‘squirrel cage’ rotor, have the magnetic fields cutting through the bars at Ns. This generates a voltage and current, with a resulting magnetic field, within the rotor. The initial rotor frequency (fr) will be the same as the stator frequency (fs), resulting in a very high rotor current and resulting strong magnetic field. The fields induced into the rotor will be the opposite of the fields within the stator (S below a N) and will be both attracted and repulsed (opposites attract, likes repulse) placing a magnetic torque on the squirrel cage. As a result, the rotor begins to turn.
In an ideal motor, where there is no load, the rotor can come close to the Ns, but can never reach it as there must always be a magnetic field crossing through the rotor bars in order to produce voltage and current. The difference between the speeds is referred to as the Slip (Nslip), which is normally referred to as a percentage. Therefore: Nslip = ((Ns-Nactual)/Ns) x 100. So, if our ideal motor runs at 1785 RPM and has an Ns of 1800 RPM, the Nslip would be 0.8%. At 1785 RPM, the rotor frequency would be: fr = 60 Hz x 0.8%, or 0.5 Hz, which can also be referred to as the slip frequency (fslip).
Note: As a reference, when we discuss PPF (Pole Pass Frequency), we will be referring to the number of poles of the motor times the slip frequency. In this case, it would be: 0.5 Hz x 4 poles = 2 Hz. This is an extremely important point to remember.
Now, during all of this, work and heat are being generated in terms of losses and torque. We will discuss these in tomorrow’s lecture.
Time to Failure Series Continued
Electrical Signature Analysis – Part 2 Electric Motor Theory (1)
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
As we start into ESA theories and application, we will have to spend a little time on electric motor theory. As we will start out with a basic system, AC motors are the simplest in terms of operation and components, we will start out with AC induction motors. Using this base knowledge, we can later expand it to cover wound rotor motors, synchronous motors, machine tool motors, servos, traction machines, generators, DC motors, transformers, etc.
In fact, in order to open this topic, we will have to briefly discuss some transformer theory. Keep in mind, an AC induction motor is just a transformer with a rotating secondary. As a transformer transforms one level of voltage and current to a second level of voltage and current, an AC induction motor converts electrical energy to mechanical torque.
Trivia: One of the purposes of using high voltages for transmission is to reduce losses in the transmission wires. Voltage does not produce losses, only resistance and current as electrical losses through a conductor are Watts = I2R. Therefore, if you were moving 480 Volts and 1,000 Amps across a conductor with a total resistance of 10 Ohms, you would have 10 million Watts or 10,000 kW (10MW) of losses. However, if you increase the voltage to 13,200 Volts, the current will be 36.4 Amps which would produce losses of only 13,250 Watts or 13kW, a reduction of 99.9%! Therefore, the purpose of a transformer is to increase T&D (Transmission and Distribution) voltages then to reduce them to a useable level. This reduces the overall T&D system losses. Transformers also work to isolate systems from each other.
The primary purpose of a transformer is to increase or reduce voltage and to have the inverse effect on current. This concept took the brilliance of a Croatian immigrant electrical engineer by the name of Nikola Tesla (we will discuss Tesla in more depth in a following part of this lecture series, including the technology battle between Tesla and Thomas Edison, which I briefly covered in an earlier Blog). The concept is simple and has to do with using alternating current, magnetic fields and the use of a transformer ratio. For the purpose of this first part, we will be working with the concepts with an ‘ideal transformer’ (ie: no losses, no connections, theoretical). We will represent transformer ratio as Na and will use a subscript 1 for the primary (high voltage, low current side) and a subscript 2 for the secondary (low voltage, high current side).
The transformer ratio can be determined by comparing the number of conductors (turns – T) on the primary side to the secondary side such that Na = T1/T2. The effect is due to the mutual inductance between the primary and secondary circuits as described in the “Motor Diagnostics and Quantum Mechanics Part 7” lecture. Therefore, if a transformer has 100 turns in the primary (T1 = 100) and 10 turns in the secondary (T2 = 10), then the transformer ratio would be described as a ratio 10:1.
Now, if you have 480 Volts and 100 Amps required at the secondary, and 13,200 Volts available at the primary, you would use the formula: V1I1 = V2I2 in order to calculate the current at the primary. Therefore: I1 = (480V * 100A)/13,200V = 3.6 Amps on the primary. The transformer ratio can be determined as 13,200V/480V = 27.5. For example: The transformer may have 275 turns in the primary and 10 turns in the secondary.
Now, the question is, how does this impact our understanding of a three phase induction motor? Simple: The stator windings are the primary and the rotor bars in the motor are the secondary of a transformer.
How is the voltage and current induced into the transformer?
This is where we fall back onto a basic understanding of physics. If I pass a magnet over a conductor, it causes electrons (classical physics) to move in the inductor, creating a current (electron flow). Now, if I pass a current through a conductor, I will generate a magnetic field. If I create a coil, the magnetic fields add, and the magnetic field increases. If I then place a medium (such as a piece of iron) within the coil, I begin to direct the magnetic field such that the medium has a North and South pole. This is the action in a DC field.
Now, in an AC field, as the voltage and resulting current increase in a coil (ie: the primary), a magnetic field increases. If you have a coil in close proximity, the increasing field will effectively ‘cut through’ the conductors in the second coil, generating a voltage and current in the second coil. Because you will also have a magnetic field in the second coil (secondary), you will generate a torque between the fields. This torque is referred to as Electro-Motive Force (EMF). The currents in both coils will depend upon, not only the impedance of the transformer, but the impedance of any loads attached to the secondary of the circuit. The frequency will also be maintained in both the primary and secondary.
Now, starting tomorrow, we can begin describing the operation of an AC induction motor, using the basic principles of this Blog. However, as we will discover, there are a few complex principles required as we move forward (such as the interaction of fields in a three phase system).
Time to Failure Series Continued
Electrical Signature Analysis – Part 1 Introduction and definitions
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
In this lecture series, we will be discussing Electrical Signature Analysis (ESA), which is a method for evaluating electrical machinery while energized. The topic will be quite broad and is to include an analysis of supply power through the driven load.
While we will rely upon some of our previous discussions to provide information and definitions for some of our new information, we will start this series by providing some definitions unique to ESA:
• Voltage: Electrical pressure, is also termed as electromotive force. Voltage is generated.
• Current: Defined in classical physics as electron flow. Current is demanded in order to produce work and is a result of the load.
• Upstream/Downstream: Upstream refers to the electrical system in the direction of generation or distribution from the point of test. Downstream is towards the motor and load from the point of test.
• FFT: Fast Fourier Transform (FFT) is a mathematical method of separating the frequencies of a ‘sine wave’ and presenting them as frequencies and amplitude.
• Spectra: Is the graph of frequencies and amplitudes resulting from an FFT.
• Voltage and Current FFT: Spectra of voltage and current.
• Motor Current Signature Analysis (MCSA): A method of viewing demodulated current and current FFT’s to evaluate the condition of machinery downstream of the point being tested.
• Voltage Signature Analysis (VSA): A method of viewing voltage FFT’s to evaluate the condition of machinery upstream of the point being tested.
• Torsional Analysis (TA): A method of viewing the current resulting from the load and its torsional effect (pulsating loads, etc.).
• Inrush Analysis: A method of viewing the inrush effects on voltage and current when electrical machinery is started.
• Power Quality: The industry has defined this as reviewing voltage and current. Voltage unbalance, over/under voltage, voltage and current harmonics and current unbalance.
• Power Analysis: This is defined as viewing power quality as well as surges, swells, transients, interruption, etc. and requires datalogging capabilities.
• Electrical Signature Analysis (ESA): A method of evaluating the motor system, which includes supply, control, motor, coupling, load and process, utilizing MCSA, VSA, TA, Inrush Analysis and Power Analysis.
The purpose of ESA is to obtain enough information, concerning the circuit being tested, to evaluate the health of the electrical system from supply through load.
ESA has been successfully applied in these applications:
• AC induction motors
• Variable Frequency Drives (VFD’s)
• Wound Rotor Motors
• Synchronous Machines
• DC Motors
• Alternators and Generators
• Machine Tool Motors and Servos, including robotics
• Driven equipment including Belted, Direct Drive and Geared
• Transformers
• Traction Equipment
• And numerous other applications
What it comes down to is the ability to evaluate the information provided by ESA. That is the purpose of this lecture series.
Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 17 Conclusion
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Note: A detailed paper covering this lecture series can be found on ReliabilityWeb: http://www.reliabilityweb.com/art04/mca_concept.htm
We have covered this particular subject as far as we can go within this concept, and without introducing any significant math. The purpose has been to cover the philosophy behind analyzing time to failure using MCA technology. Our next lecture series, starting tomorrow morning, will cover the other half of Motor Diagnostics: Electrical Signature Analysis. This will allow us to include energized testing as part of our estimating time to failure concept, as the dynamic impact of the motor will effect the overall motor ETF.
In the Motor Diagnostics and Quantum Mechanics portion of our overall series, we discussed the difference between classical physics and quantum physics, how it describes the application of MCA, what tests are performed with MCA and how each describe the common insulation failure methods.
The concept of ETF is simple: Present the probability that an insulation system will fail within a stated period of time with a technology that will not accelerate the failure. The tables presented in the Estimating Time to Failure papers, Part 1 and Part 2, were developed based upon the probability distribution following the history of a population of electric motors tested and trended in the field. The probability of failure is calculated simply as: Pa = Na/N where Pa is the probability, Na is the number of failures and N is the number of failures. This can be applied as part of a simple reliability curve e(-t/M) where e is the natural log, t is the time being determined and M is the mean time to failure. This allows us to determine the probability that the insulation system will survive over a specific time period based upon observations from the original evaluated population. This also allows us to assemble an ETF calculator based upon the probability curve, basic reliability algorithms and operating conditions.
Combined, we have been able use ETF to determine insulation failures out to 4,000 + hours.
In the Electrical Signature Analysis series, starting Friday, August 13, 2004 (just because I like Friday the 13th), we will start with definitions covering both MCA and ESA, the properties that make up a full ESA capability and the systems that both have been evaluated on.
Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 16 Wound Rotor and Servos
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Note: A detailed paper covering this lecture series can be found on ReliabilityWeb: http://www.reliabilityweb.com/art04/mca_concept.htm
Recently, a question was posed about how we handle our research: The theoretical part of our MCA research FOLLOWS the detection of faults in real-world environments by MCA users and our laboratory consists of industrial and commercial locations, motor repair facilities and utilities. Therefore, the materials presented in this series of lectures do not discuss the potential capability based upon theory, but the results of actual applications with the theories representing the best explanation for real-world results. The theoretical part of our work involves verifying repeatability for new findings, keeping up to date in the latest theories, definitions of physical laws and research in both engineering and physics. The real-world part of our work involves continued direct contact with MCA users, of all technologies, users of other technologies and interaction with manufacturers of other condition based monitoring equipment (such as infrared, ultrasonics, etc.).
Now for our next lecture:
As previously discussed, one of the abilities of MCA equipment is the ability to test across the airgap for mutual inductance. In fact, MCA induces a voltage and frequency equal to the frequency applied to the stator and the transformer ratio of the stator windings to rotor windings. The total effect depends upon the airgap of the motor between the top of the stator teeth and rotor core. This produces the effect of the measured AC properties of MCA to be able to read across the airgap.
Wound Rotor Motors and Synchronous Motors
In a wound rotor motor or synchronous motor, the transformer properties allow the detection of winding shorts across the airgap. In the case of machine tool motors that have permanent magnet rotors, the magnetic domains of the ferromagnetic material will interact with the airgap frequency – can be audibly heard as a tone that varies by the applied frequency. Following will be short descriptions on the effect and how to evaluate the condition.
Wound rotor motors have, normally, wye connected windings which are connected to a variable impedance source (usually a resistor bank). By increasing resistance, the starting torque increases while the breakdown and full load torques decrease. This allows for either starting high inertia loads or speed control by increasing slip. By decreasing resistance, the starting torque is reduced while the breakdown and full load torque increases, allowing for speed control by decreasing slip. The resistor bank is both used to change the rotor circuit resistance and dissipate heat, in effect acting as a ‘heat sink’ for the rotor. The rotor is AC. Winding shorts occur in a wound rotor the same as in a stator.
Synchronous motors have wound rotor fields which are used to carry DC voltage. The DC voltage is used to lock the rotor with the rotating magnetic fields so that the rotor turns at the same speed as the rotating field. A second winding, called the amortisseur winding is an induction winding with the bars mounted in the field poles and shorting rings. This is used to produce starting torque and to act as a torsional absorber to reduce faults in the DC fields due to induced AC. Rotor fields normally fail due to heat generated from the AC magnetic field and contamination. The faults, just as in a DC motor field, occur from the inside out.
When a fault is detected in the Fi and I/F test results, this may indicate that a fault exists in the stator or rotor. The quickest and simplest way to determine whether the fault is in the stator or rotor is to move the shaft and re-test. If the test results change position, then the fault is in the rotor windings, if the results remain in the same position, then the fault is in the stator windings.
Servo and Machine Tool Motors
Most servo and machine tool motors have permanent magnets in the rotors. The magnetic domains (‘permanently’ aligned magnetic dipoles) will have a direct effect on the inductance of the windings. If a rotor is held in one place and a set of readings taken on a permanent magnet machine, the inductance in one or two phases will be very high, which causes the impedance to follow. Therefore, there is, normally, an extreme inductance and impedance unbalance. In a few cases, the extreme can have some effect on Fi or I/F (but not both).
Winding contamination is still measured by comparing the impedance and inductance of each phase. Verifying the Fi and I/F, in a few designs that cause variance, will require an extra step, unless it is being trended (the difference between phases will be maintained regardless of rotor position). To verify condition, view real-time inductance until it is at the minimum reading for that phase, then perform winding tests. Repeat this action for each phase. The tolerance for Fi and I/F is then +/- 1 from the average between phases.
Conclusion
Extensive work and field (not lab) confirmation has gone in to the analysis of winding fault detection in the application of:
• AC machines through 13.2 kV
• AC generators through 250 MW
• DC machines to over 8,000 hp
• Synchronous machines to over 10,000 hp
• All types and models of machine tool and servo motors
• Locomotive and Automotive AC and DC traction motors
• AC traction transmissions
• Transformers, dried and oil filled, through 135 kV
• Magnetic coils
• Capacitors – including power factor correction
• Cables
In each case, MCA has performed as expected.
If you wish to have an Adobe Acrobat copy of the lecture series, so far, please email hpenrose@alltestpro.com with your name, email and company affiliation. If you wish to have additional information on Motor Diagnostic equipment, please include your mailing address.
Howard W Penrose, Ph.D.
Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 15 DC Windings
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Note: A detailed paper covering this lecture series can be found on ReliabilityWeb: http://www.reliabilityweb.com/art04/mca_concept.htm
As most may have realized, quite a bit of time has been spent on AC induction motors. For some reason, most research on testing has been focused on this subject and little on DC motor analysis. However, you are in luck, as we have spent time researching DC motor analysis.
For winding analysis, there are two components that we will focus on for DC machines. The first is the armature and the other wound component are the fields (series and shunt) and interpoles. So, we will break them down, covering first the fields, then the armature.
Series fields, labeled S1 and S2, are connected in-line with the armature and are made with a few turns of large wires. The good news is that the series fields rarely fail, nor do the interpoles. The shunt coils, on the other hand, generate a great amount of heat (high resistance and resulting I2R losses). The main cause of failure in fields and interpoles is thermal degradation, as they carry dominant DC voltage. However, there is a component of AC that is carried on the DC voltage that results from SCR switching, called the form factor. This is the remaining AC ripple that results when the AC voltage is chopped with the SCR circuit of a DC drive. Therefore, winding contamination will have some limited impact with capacitive faults. A majority of shunt coil faults are the result of poor ventilation in which the coils degrade from the near center of the coil out.
When trending the condition of DC motor shunt fields, the Fi and I/F are compared from test to test with a tolerance of +/- 1 digit change in each over time. The MCA method of test allows for the very early detection of faults that are detected later as a loss of speed and torque control in the DC motor. If possible, splitting the coils into two groups and comparing the Fi and I/F will allow for immediate fault detection.
The armature circuit is actually an AC circuit as current flows in one direction then the other as the commutator bars pass the brushes. Therefore, faults such as contamination will have a greater impact, as in AC motors. In the case of a DC motor, brush wear generates carbon dust, which is a dielectric. When the carbon dust accumulates within the armature, breakdown will occur in the same way as contamination in an AC motor. Therefore, there are several methods of reviewing the condition of the armature:
• The first method is used for trending and performing an evaluation of the condition of the insulation system. Testing is performed through the armature circuit twice, and the results are compared. Due to the change in circuit capacitance from carbon contamination between conductors, or on the surface of the coils, carbon contamination generates non-repeatable results in impedance, Fi and I/F. If caught early enough, low pressure, dry air, can be used to clean out the armature and correct the problem.
• If a commutator or armature short is suspected, all but two brushes 90 degrees apart from each other should be lifted. Using circuit impedance, line up the edge of a brush along the edge of a commutator bar and take a reading. Move through each brush and record the circuit impedance for the circumference of the armature. The impedance results should follow the same pattern, or a repeating pattern. A drop in impedance indicates a winding fault.
MCA is capable of evaluating the condition of DC motors. Early detection of a fault will allow for trending of the insulation failure over time.
Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 14 Winding Shorts
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Note: A detailed paper covering this lecture series can be found on ReliabilityWeb: http://www.reliabilityweb.com/art04/mca_concept.htm
In last month’s series we discussed dipolar action in both electrical (insulation) and magnetic (steel) materials during operation. We shall briefly provide an overview then provide a little more detail in this lecture.
One of the most difficult faults to detect, in the past, has been a developing winding short. This requires several short definitions:
1. Winding Short: The point where a fault has become conductive between turns or windings.
2. Developing Short: Insulation defect(s) that are in the process of degrading towards a winding short.
This is an important distinction, as there are those who try to muddy the waters discussing time to failure in a winding short. It is necessary to understand that once a winding short has occurred, the failure has occurred and there is no time to estimate.
Developing shorts are often the result of a manufacturing defect, winding contamination, overloading, insulation degradation or other conditions. The short, itself, will occur once the insulation system has become weak enough that an event, such as a surge, sag, swell or start, causes the fault to arc. Because of the amount and type of insulation between turns, as opposed to the amount of insulation between conductors and ground, the fault will most likely occur between conductors before the fault arcs to ground.
When a defect occurs in a winding due to a developing short, it effects the electrical properties of the insulation system. Changes to either the capacitive or resistive properties of the insulation system will cause a reactive problem due to changes in the makeup of the system. For instance, in a developing short, the changes to the insulation system cause changes to capacitance due to changes in how the dipoles are excited (dipole spin). As a result, there are changes to how the insulation reacts in that area, causing a leakage reactance variance and heating due to forcing the insulation to polarize with higher applied potential.
At design voltage, most defects do not become apparent until a distinct change occurs, which may be represented by a severe current unbalance, nuisance tripping or a direct short (smoke). As a result, as faults occur due to thermal deterioration, contamination, moisture or other reactive faults, the circuit impedance will change, slightly, at first, then more dramatic as the fault progresses. It is important to note, at this point, that inductance will not change until the fault has progressed to a winding short.
One of the keys to proper MCA testing is that inductance is not used as a primary method of detection for developing shorts. Instead, two specific measurements are used in combination to determine the type and severity of the the defect. These measurements are the circuit phase angle (Fi) and a current/frequency response (I/F) method.
When a defect occurs in the winding, it changes the effective capacitance of the complete circuit. The change to capacitance will directly effect how the low level current lags behind voltage with the usual result being an increase to capacitance and a reduction of the phase angle in the effected phase. Once the fault becomes more severe, it will begin to effect the surrounding phases. This normally occurs when the defect exists in one coil or between coils in the same phase. A very small change to capacitance within the circuit can be detected, allowing the detection of single turn faults and pinhole shorts when using very low frequencies relative to the applied test voltage.
A second method of fault detection uses a current ratio, similar in method to the frequency response method used for transformer testing. However, the low voltage current is measured, then the frequency is exactly doubled and a percentage reduction in the low-level current is produced. When the frequency is doubled, small changes in capacitance between turns or between phases are amplified, causing a change to the percentage reduction when compared between phases.
The most effective method for analysis is to compare phases of the winding circuit. When testing, using the integer results of Fi and I/F:
• Without a rotor installed in the motor: The circuit impedance effects of the transformer ratio of each phase is discounted and there is no tolerance between phases. Severe core damage, such as groupings of shorted laminations, may have some impact on the results.
• With a rotor installed in the motor: The circuit impedance effects of the transformer ratio of each phase is accounted for. There are two effects of the rotor: a) The rotor circuit acts as an amplifier for the circuit fault, allowing earlier fault detection in assembled motors; and, b) A tolerance must be provided for Fi and I/F. By experiment, this has been found to be mostly +/- 1 digit from the average for any size motor or transformer.
• The type of fault can be estimated with good accuracy: a) Both Fi and I/F are out of tolerance is a short within a coil; b) Fi is out of tolerance is a coil to coil short; and, c) I/F is out of tolerance is a phase to phase short. This distinction is important when analyzing trended results to estimate time to failure.
The combination of Fi and I/F allow for the early detection of winding shorts and type of short being detected in any size machine. Also, due to the use of low voltage and the result that only a small change to a circuit capacitance is required to detect the faults, early developing shorts can be detected quickly and trended to failure.
Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 13 Winding Storage Degradation(3)
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Note: A detailed paper covering this lecture series can be found on ReliabilityWeb: http://www.reliabilityweb.com/art04/mca_concept.htm
One type of insulation degradation that is often overlooked relates to storage of electric motors for an extended period of time. Following are two examples:
1. A 480 V generator is stored in an uncontrolled environment for 18 years. It is then installed and runs, satisfactorily, for several months, then starts tripping on overtemperature at a more and more frequent rate. When tested, it is determined that the insulation resistance drops significantly during operation and MCA tests identify that Z and L do not follow the same pattern.
2. Spare coils for a large DC machine are stored for a period of 28 years in an uncontrolled environment. A majority are found to fail when comparing MCA results. Another set of coils is stored for over 10 years sealed and packed with dessicant. All are found to pass testing.
Electrical insulation will degrade in storage. The rate depends on the makeup of the components of the insulation system and the storage environment. Moisture, temperature and chemicals will react with the insulation system in a variety of ways. The result is similar to ‘tire rot’ if you leave a car or tire sit in one spot for a long period of time – you will note that it becomes brittle and fractured.
Some insulation systems will become brittle and small ‘treeing’ may occur in the insulation system. These tiny fractures adsorb moisture, which changes the capacitance of the circuit in operation. This will cause the fault point to fail in operation.
Chemical vapors and pollution will chemically react with most insulation system materials, causing embrittlement. Moisture or chemical adsorption may result, which will either cause a conductive path or change to the circuit capacitance in operation.
In the next lecture, on Monday, we will discuss winding shorts, how they develop and how long it takes for them to develop from a defect to a full short. We will then spend the rest of next week discussing how MCA tests track this degradation, including case studies. Following next week, we will discuss:
1) The application of MCA in wound rotor equipment;
2) The application of MCA in DC machines;
3) The application of MCA in Machine Tools and Servos
Following these topics, we will begin our lecture series on Electrical Signature Analysis.
Please email me with any comments. I enjoy receiving feedback on my efforts, in particular to know that the time being invested is helping you. My email address is: hpenrose@alltestpro.com. Include any particular areas that you would like to see information concerning the topics under discussion.
Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 12 Winding Thermal Degradation(3)
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Note: A detailed paper covering this lecture series can be found on ReliabilityWeb: http://www.reliabilityweb.com/art04/mca_concept.htm
Electrical insulation systems are rated by voltage rise time and thermal limits. It is generally understood that for every 10C temperature rise, the insulation life is reduced by half (as per the Arrheneous Chemical Equation). The causes are related to electro-chemical reactions due to the excitation of the atomic structure and resulting entropy (per classical laws of thermodynamics). In this lecture, we will discuss the failure of insulation due to thermal degradation.
First, we will define a few insulation thermal terms:
• Ambient Temperature: The environmental temperature near the electric motor. This value may also be displayed on the motor nameplate and represents the maximum ambient temperature, at sea level (<3000 ft), when the motor is operating at full load.
• Temperature Rise: The temperature resulting from motor losses during operation. This value varies depending upon the load of the motor.
• Operating Temperature: The total temperature when ambient and temperature rise add. The operating temperature is measured in the windings.
• Insulation Classes: Class A: 105C; Class B: 130C; Class F: 155C; Class H: 180C; and, Class N: 200C. The actual temperature will be above the temperature class and below the next class up. Most motors are designed in such a way that the motor thermal rating is 10C below the insulation class rating.
• Thermal Limit: The insulation temperature where a chemical change (Oxidation) will rapidly occur in the insulation and degradation occurs.
Insulation Action in Insulation Breakdown
The stray load (leakage) losses of an electric motor result directly from the excitation and polarization (dipolar spin) of the atomic structure, and are observed as heat. Within the insulation system, itself, a chemical reaction continuously occurs (even when the motor sits idle, over time). A motor that survives through its complete thermal life (an unusual situation) will degrade slowly based upon thermal cycling and electro-mechanical stresses. The end result will be a shorted or grounded winding.
If the motor is operating and reaches its thermal limit, several reactions occur. As insulation temperature increases, normally, the atomic structure expands, leaving more room for excitation and dipolar action, reducing the insulation resistance. Depending on the makeup of the insulation system, a thermal limit is achieved where the atomic/molecular structure crystalizes and begins to shrink. At this point, fractures can occur, allowing contamination to also effect the condition. As the system crystalizes, dipolar action becomes more restricted, additional losses occur due to an electical forcing function of insulation dipoles, and the fault point degradation accelerates. Once the action starts, capacitance decreases, causing low voltage circuit impedance to increase. Finally, the insulation goes through a chemical change as the insulation structure breaks down, resulting in winding shorts and possible insulation to ground faults. As the insulation system approaches this point, the effect between conductors and conductors and ground become more resistive and result in a drop in circuit impedance.
The area of fault is effected by the electrical and operating environment. For instance, a single-phased motor will have thermal breakdown in one or two phases of the winding system, an overloaded motor will have fairly even breakdown of the insulation system. With the exception of single-phasing and overload, insulation breakdown occurs over a long period of time. We will cover this in more detail in a following lecture.
Conclusion:
There are several steps involved in thermal breakdown of an electric motor insulation system. The rate of degradation depends on the types of conditions that the motor is operating in.
In the next lecture, we will discuss the reaction of insulation systems in storage.
Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 11 Winding Contamination (2)
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Note: A detailed paper covering this lecture series can be found on ReliabilityWeb: http://www.reliabilityweb.com/art04/mca_concept.htm
Winding contamination in electric machine windings has an impact on circuit capacitance. In fact, winding contamination that effects the insulation system will cause an increase in circuit capacitance. The value of change is relatively small, requiring a low voltage test in order to detect the change.
The effect is similar to the dipolar effect that we discussed in July. However, the winding contamination, such as water, oil, gasses and chemicals, has a looser atomic structure such that the molecules polarize versus the atoms and they are also able to move due to the spacing between molecules. The ability to rapidly polarize causes an increase in circuit capacitance.
In addition to the change in capacitance, there is a second, more potentially damaging quantum effect. Water vapor or condensate (as well as other contaminants) have a peculiar effect when they polarize in a magnetic field: The polarization causes increased spacing between the molecules of vapor resulting in an expansion of the vapor or condensate. If the contaminants are absorbed into fractures or fissures in the insulation system, not only will there be a capacitive change, but also the expansion will cause increased damage to the insulation itself.
The Insulation System Effect
The contaminants build up on the insulation system. In operation, the extra layer of contamination both causes a change in the circuit capacitance and creates uneven increased temperature through the winding system. This causes a reaction within the insulation system causing breakdown and resulting changes to circuit capacitance. The effect becomes a progressive fault as the change to the insulation system causes an increased change in circuit capacitance, and so on. The reaction of contaminants with the insulation system, depending on the volume and type of contaminant (ie: a submerged motor will have a significant change and may fail rapidly) with most common contaminant volumes allowing for long term trending.
Motor Circuit Analysis
As previously discussed, increased capacitance causes a negative impact on impedance such that the relationship between the pattern of inductance between phases is different from the impedance. The impedance value collapses towards the inductance values. Once the value changes, the insulation system has started to degrade and corrective action would be necessary to correct the problem. This can be done by either cleaning the windings and re-insulation, referred to clean, dip and baking the winding, or rewindng the motor. If the degradation is detected and acted upon early enough, correction action is possible, if left too long, rewinding is necessary.
In the next part of this lecture, later this week, we will discuss insulation breakdown due to thermal degradation.
Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 10 Winding Faults (1)
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Note: A detailed paper covering this lecture series can be found on ReliabilityWeb: http://www.reliabilityweb.com/art04/mca_concept.htm
Many times I have had to demonstrate or test equipment ‘set up’ with so-called realistic faults by individuals evaluating MCA technolgy. These have consisted of resistors placed in the winding, scratched wire, cut wires and other man-made faults. Unfortunately, none of these demonstrate the true faults that occur within electric motors nor demonstrate the ability of MCA technology to trend winding faults. While MCA easily detects these problems, a methodology used to demonstrate actual motor failures goes a lot further.
So, before we start discussing evaluating the time to failure in different fault situations, we will first review the actual process of motor failure. For your homework, if you have not reviewed the July lectures, you may wish to. Also, a brief review of the MCA Concept paper.
Tomorrow, we will start the lectures with a discussion of winding contamination.