Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 9 MCA (3) Rotor Testing
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
For those of you following along, I missed Friday. Spent the day with the family at six flags in Springfield, Mass. at the waterpark. I look like a lobster! Finally, I can move my fingers enough to type.
In this final part of the July series, we will briefly discuss rotor testing. Note that each of these topics will be going even further in-depth during August.
When you set up a coil, you have loops of turns which generate an inductance. As you pass an alternating current through this coil, you will generate a magnetic field that varies directly with the current within the coil (as current increases one direction, a magnetic field with a particular north-south polarity exists, as it changes direction, the magnetic field collapses then changes polarity). When the coil is wound around a material, such as steel, the lines of magnetic flux condense closer to the coils. The material, itself, will react by having magnetic dipoles polarize in the direction of the magnetic field. In an AC circuit, these dipoles resist the change in direction of the field, resulting in hysterisis.
In a three phase induction motor, the phases are seperated by 120 electrical degrees. The applied voltage and resulting current, in a good electric motor, will be separated by 120 electrical degrees. As the currents increase and decrease, the result is a whirlpool effect in the airgap between the stator and rotor. The fields ‘cut through’ the rotor bars, generating a voltage, and resulting current, just as in a transformer (as a matter of fact, the principles are the same).
Now, the effect of being able to evaluate the condition of the motor rotor is “based upon Faraday’s law of electromagnetic induction, according to which a time-varying flux linking a coil induces an emf [voltage] in it.” (‘Electric Machines and Electromagnetics,’ Syed A Nasar). Such that the ratio of impedances are: (e1/e2) = (N1/N2).
The motor circuit analyzer excites the core steel based upon the amount of current available to the circuit and reacts across the airgap. The direct relationship to the ability to detect the rotor across the airgap depends upon the distance across the airgap, the area of the steel magnetized and the length of the rotor core. In longer cores, the effect will carry across the airgap and excite the rotor core and induce the instrument frequency into the rotor circuit. In very short cores, the firnding effect of the magnetic field from the stator has a similar effect. In large machines, the amount of energy available from an MCA device allows for the detection of rotor defects only above the area immediately surrounding each coil side.
This produces multiple effects:
1. The mutual inductance changes as the rotor position changes as a direct result of the change to the transformer ratio between the primary (stator) and secondary (rotor). A good rotor will show as a repeating pattern, a bad rotor will change the transformer ratio and a defect will appear as a non-repeating pattern.
2. Fractures will be readily detected as the induced energy is relatively low and the oxides on the surface of the defect will change the transformer ratio. Whereas, in higher voltage rotor tests, the energy may be significant enough to pass through the defect.
3. In rare instances, the airgap may be too significant and very little to no variation of the mutual inductance occurs. In this case, larger defects, such as multiple fractures or a broken bar, will show as a variation in the straight line.
4. MCA technology has the ability to detect wound rotor, synchronous rotor field and other wound-rotor defects across the airgap. Because of the impedance ratio between the primary and secondary, rotor winding defects will show as a change in the Fi and I/F response and will vary based upon rotor position.
Evaluating findings in rotor testing. Note that all methods and effects have been proven in both experiment and in application across fractional horsepower to large, 13.2 kV motor sizes.
1. Rotor Casting Voids and Partial Fractures: Due to the re-direction of the circuit test current around the fault, casting voids and partial fractures will cause an effective change to the dimension of the coil dimension of the rotor circuit. This will cause a small fluctuation in the mutual (and resulting circuit inductance) as the effected part of the rotor circuit passes over the stator circuit. Due to the small variation, the effect can only be seen at points where mutual inductance is not at a peak. This means that small (partial) fractures and casting voids that do not emulate broken rotor bars will show as small defects in sine waves on the slopes, but not at the peaks (or valley – negative peak). As a result, the full torque capability of the motor design will still be available.
2. Broken Rotor Bars and Fractures: As above, except that a full portion of the secondary will not be in the circuit. This means that, as the effected portion of the rotor circuit passes over the stator circuit, a significant change in the mutual inductance will occur. Due to the significant variation, the effect will be seen in at least one peak and/or valley of the pattern. As full inductance is not available at the fault point, and current cannot flow through this point, a direct effect on the full torque capability of the motor design can be observed.
3. Wound Rotor and Rotor Field Faults: In any style of wound rotor, the Fi and/or I/F will be effected when the winding breaks down. Because of the dampening effect of the transformer ratio, and the airgap, these faults are noted at a later stage. Confirmation of a fault is determined by rotating the rotor by some amount and remeasuring. If the results change phase, the fault is in the rotor, otherwise, the fault is in the stator.
Conclusion:
MCA inductance techniques for evaluating rotors is extremely effective. It still requires a sinusiodal voltage output of the instrument being used. Several styles of instruments are available with some requiring multiple positions and recording the inductance and others viewing the inductance (visually) real-time.
In our August series of lectures, we are first going to start by reviewing different types of faults and their effects.
If you have been following along with this series, please send me an email and let me know your level of interest in this subject and the upcoming topic of estimating time to failure. Also, if you have any questions or comments, please pass them along to:
Sincerely,
Howard W Penrose, Ph.D.
Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 8 Motor Circuit Analysis (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
In yesterday’s lecture, we discussed how low voltage values of impedance and inductance can be used to detect insulation system degradation through comparing a motor phase to phase and without the need of an index number. This allows for the analysis regardless of actual insulation resistance value as with polarization index limits of 5000 MegOhms per IEEE Std 43-2000. In today’s lecture, we will explore the phase angle (Fi) and current/frequency response (I/F) tests.
In 1994, P Hammond and JK Sykulski published a book entitled “Engineering Electromagnetism: Physical Processes and Computation,” in which they outlined a simple method for analyzing complex systems. The concept was simple, and common sense, in that they described how a complex system such as electromagnetics and electrical fields could be described by dividing the system into tubes and slices. In order to visualize this concept, think of a loaf of bread: Slices would involve cutting the bread much the way we do for sandwiches (assuming you are not on a low carb diet) by slicing along the width of the loaf. Tubes would be described as slicing down and across the loaf length-wise. If you make these slices of the right size, you could mathematically describe the entire loaf of bread based upon its properties accurately and with a single formula.
In the case of the motor, we can perform the same analysis on the insulation system, which is expressly outlined by Hammond and Sykulski for determining the properties of Capacitance and electrical and magnetic dipoles (which we will discuss along with rotor testing). For the case of what we are describing, here, I am not going to include the magnetic dipoles of the core or rotor steel. Through experiment, we have identified that there is an impact on Fi and I/F based upon severly shorted laminations (eddy-currents – smeared laminations), core steels in which the properties have changed and severe rotor winding faults. However, the severity requirements allow us to determine the condition of the stator or rotor core through a simple confirmation should a fault be detected. This will be explored along with the rotor test. This effect, on the other hand, allows MCA to detect the condition of rotor windings (wound rotor and rotor fields) through the stator windings.
In a winding insulation defect, between conductors, the dipole moment of that part of the insulation system changes. This directly effects the capacitance of that part of the circuit. In an operating motor, the higher impressed voltage forces dipolar excitation (rotation) generating heat proportional to the change in capacitance and in relation to the rest of the insulation system. This is termed as leakage losses, or a reactive fault, within the insulation system. As the temperature increases at the fault point, the capacitance become inversely proportional to the applied temperature, causing further changes to the capacitance at the fault point. However, these changes are minute when compared to the impressed voltage, leaving them virtually undetectible until the fault has progressed to the point where: The conductors become ‘welded’ together; The motor starts nuisance tripping; or, An unexplained current unbalance occurs.
By using the low voltage, AC sine wave of MCA, the dipoles become excited, without a forcing function, between conductors and coils. This causes capacitance to become dominant over voltage allowing very small changes to have a significant impact on the circuit. When viewed using tubes and slices, the effect is figured as the tube sum of the inverse sum of all of the capacitive slices (see “Motor Circuit Analysis Concept and Principle” p.5 for the formula). Both based upon the physics of the circuit and experiment, this demonstrates how MCA is able to detect even individual conductor shorts throughout the complete motor winding based upon the sinusoidal voltage frequency applied. It also demonstrates how MCA can be used to trend the development of an insulation defect that will develop into a short. (We will be discussing this in-depth in the next lecture series). However, impedance will not detect these inter-turn shorts, early enough, directly. Therefore, other effects of the change of capacitance and resulting impedance in the circuit must be measured.
Fi is the timed peak voltage of the output of the instrument and the peak current resulting from the circuit being tested and is represented as an angle. In a circuit with relatively balanced capacitances, the integer comparison of Fi from phase to phase should be balanced. A tolerance of +/- 1 degree is included in assembled motors because of the mutual capacitive circuit effect of the rotor (stator coils, air gap, rotor windings make up a separate capacitance circuit). A small change to the capacitance of the turns within a coil, or between coils in the same phase, will cause a change to the circuit capacitive reactance. This, in turn, shifts the phase angle downwards. For example, in a good insulation system, a phase angle of 76, 77, 78 degrees from phase to phase would be considered good. A phase angle of 75, 77, 78 would be poor, with a developing short across the first set of windings. A phase angle of 72, 78, 78 would indicate a severe condition involving only a few turns within a coils or between coils of the same phase.
I/F is the measurement of the resulting phase current, at low voltage, before and after the applied frequency is doubled. This value is represented as a percentage reduction in current. As with phase angle, the circuit capacitances directly effect the response in shorted turns within a single coil, or coils shorted across phases. A tolerance is supplied as +/- 2 of the average for pass/fail and +/- 1 for trending. In a developing short, the values will first increase, then will rapidly decrease as the developing short becomes more resistive. A result of -44%, -45%, -46% (% reduction of current) would be considered good, a result of -44, -44, -47 would be considered a developing fault.
Therefore, when utilized together, measurements of Fi and I/F both allow for:
1. The detection of winding shorts through the complete winding;
2. The relative identification of the type of developing short within the insulation system;
3. The ability to provide a simple set of rules for detecting developing faults in any size or voltage rating winding system;
4. The ability to detect and trend the developing fault; and
5. This ability requires low voltage AC sinusoidal voltage testing techniques.
In the next lecture, we will discuss the detection of rotor faults using MCA and the quantum mechanics of how to determine the severity of the fault.
In next months primary lecture series, we will discuss how these tests are used to track and trend developing winding shorts and the probabilities involved (reliability).
Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 7 Motor Circuit Analysis (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
The physical laws governing classical and quantum physics are different, as are the laws of aerodynamics for supersonic versus sub-sonic flight, as are the laws that govern high voltage winding and insulation analysis over low voltage winding and insulation analysis. For example, the laws governing high voltage testing for determining winding shorts utilizes the ionization of materials and gasses requiring an impulse voltage; whereas motor circuit analysis utilizes minute changes in circuit capacitance that can only be detected at low voltages.
For motor circuit analysis of a complete circuit to work:
1. A DC resistance (in milli-Ohms - R) and insulation to ground test must exist (meg-ohm meter - MegOhm) to detect loose connections and insulation breakdown between conductors and ground.
2. Impedance and inductance readings measured with a low (<7 volts, optimum), sinusoidal AC voltage through individual bridges. This is important, as will be demonstrated, as some commercially available instrumentation uses pulsed voltage outputs and calculated values of impedance. In these cases, the relationship between impedance and inductance are meaningless. Impedance (from here on shown as Z) and Inductance (from here on shown as L) are used solely as a means to determine insulation condition such as winding contamination or overheated insulation.
3. Phase angle (Fi) and I/F readings measured under the same conditions as Z and L. These two readings are measurements of the circuit response to variances in capacitance (C) of the circuit. These are used to detect insulation breakdown between conductors, coils and/or phases, and can be used in combination to determine the type of fault, whether it is between turns in a coil (turn to turn short), conductors between coils in the same phase (coil to coil), or coils between phases (phase to phase).
Resistance and MegOhm testing have been previously described, so we will start with the relationship of Z and L.
The Relationship of Impedance and Inductance
Impedance can be broken down into resistive, inductive, capacitive and frequency components. It is refered to as the complex AC resistance of a circuit where:
Z = sqrt (Square root) of (R2 * (XL – XC)2)
XL is the reactive component of inductance (inductive reactance) and is represented as:
XL= 2*pi*f*L
XC is the reactive component of capacitance (capacitive reactance) and is represented as:
XC = 1/(2*pi*f*C)
When measuring the motor circuit, the R, L and C are the sum of all of these components of the circuit.
The L component of the circuit represents the sum of all of the self and mutual inductances of the circuit. Self inductance is the L of an individual coil and how it relates to itself between turns (loops) of the conductor. Mutual inductance is the relationship of the L of one coil to the L of another coil in relatively close proximity. For instance, if I place a coil on a table, isolated from any other influence, I will get one measurment of total L on the coil. If I then bring another coil in close proximity (or even a piece of steel), I will get a second measurement of total L from the coil. Inductance, itself, is a measurement of the magnetic field capability of the coil per unit of current. It is independent of frequency or voltage and is based upon the relationship of the number of turns and dimensions of the coil as well as the relationship to another coil, if one exists, and the distance to that coil.
The C component of the circuit represents the sum of all of the self and mutual capacitances of the circuit. Capacitance, as discussed earlier in this lecture series, is directly related to how the insulation system between conductors and between conductors and ground polarize. It is also directly related to ‘leakage’ currents, or current flow between conductors and conductors and ground (at a very low level, in a good insulation system) and is inversely proportional to temperature in a dielectric. In an operating motor, the circuit capacitance changes as the electrical field potential changes due to dipolar action (the polarization of the system can be visualized as dipoles spinning). This is where quantum mechanics comes into play, as the dipoles are, literally, atoms that have a shift in position of the ‘orbit[1]’ of electrons around the nucleus. Thus, there is a positive and negative pole to the atom, itself.
Note: To put the size of an atom in perspective, if you were to expand an apple to the size of the earth, an atom would be related as being the same size as the original apple.
In an insulation system, the dielectric is a solid, therefore the polarization occurs atomically. In the case of fluid or gassious contaminants, the polarization occurs molecularly. For example: Water vapor (humidity) or water, which contains two hydrogen atoms and one oxygen atom, will polarize, as a unit. The relationship is such that it has a polarization and capacitance different than that of insulation. When contaminants are present in an insulation system, and they effect the insulation system, the capacitance of the circuit increases.
In an insulation system that is overheated (ie: the insulation is breaking down thermally), fewer atoms can polarize, and the capacitance changes, as well.
While a motor is operating, the voltage level is such that early changes to the capacitance of the circuit cannot be accurately measured (to be discussed in depth tomorrow) unless a large area of the insulation system is effected.
How Z and L Are Used In MCA
In an assembled electric motor, there is not only mutual inductance between the coils of the stator, but also mutual inductance between the stator coils and rotor winding. When a rotor is placed in one position, the relationship between each phase of the motor and the rotor will be different based upon the number of turns of conductors within each circuit and the rotor bars related to the coils. In smaller motors, this relationship will be all of the rotor bars above each coil group, in larger motors, the relationship is with the rotor bars above each coil side. Therefore, there is a difference of L between each phase.
When an insulation system is in good condition, the sum of the C for each phase will be similar. When contamination or insulation degradation occurs, the sum of C for each phase will be dissimilar. The result is a change in phase capacitance. In Polarization Index testing, using DC power, this effect is measured only across the insulation barrier between the conductors and ground. By using low voltage sinusoidal impedance, the dipoles of the insulation system become excited (dipolar spin) allowing a view of the complete insulation system. A frequency is normally selected below 1 kHz, as there is a point of diminishing return in higher frequencies, as you, again, enter an area of a different set of physical laws.
As a result of this, Z for each phase can be compared to L, then the relationship can be compared between phases. In a good insulation system, the pattern should remain the same from phase to phase, in a poor insulation system, the pattern will change with impedance normally dropping towards inductance (in severe conditions, the Z will be less than inductance and the circuit will be more capacitive than inductive). For example, if tested with the rotor in place, an L relationship of a low, medium and high reading from phase to phase is measured, it would be expected that a Z relationship of a low, medium and high reading would also be observed. If not, then a defect such as winding contamination or degradation has occurred. This effect eliminates the need for an index value.
In the next lecture, we will discuss how phase angle and I/F are used to detect early defects between conductors. Friday, we will discuss how these measurements can be used to detect rotor defects. And, in August, we will focus on a series on fault analysis and how what we have learned, to date, allows us to estimate time to failure in an insulation system.
Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 6 AC Hi-Pot and Surge
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
High Voltage Breakdown as described by Richard Feynman, Nobel Prize Winner for Physics, “The Feynman Lectures on Physics, Volume 2” California Institute of Technology:
“This result is technically very important, because air will break down if the electric field is too great. What happens is that a loose charge (electron or ion) somewhere in the air is accelerated by the field [electric field], and if the field is very great, the charge can pick up enough speed before it hits another atom to be able to knock an electron off that atom. As a result, more and more ions are produced. Their motion constitutes a discharge, or spark.” (P. 6-13)[1]
In this lecture, we will discuss AC high potential testing and surge comparison testing. Both methods utilize the method of applying twice the voltage plus 1,000 volts to a winding in order to proof the strength of the insulation system. The AC high potential test is used to detect insulation to ground faults and the surge comparison test is used to detect turn to turn defects early in the winding system.
AC high potential tests impress a high voltage sine wave (normally) across the insulation system, although also excites the dipole spin that we have described in earlier parts of this lecture series. A good insulation system, that is clean and dry, should pass without incident or degradation between insulation and ground. However, in the case of missing or damaged insulation, unlike DC high potential testing, it does not have to be in direct contact through the defect. As described in the beginning of this lecture [1], the defect point can ionize and generate a spark. This will also happen through weakened insulation. Once the spark is generated, an impulse occurs within the insulation system which can cause other sparks to occur in the next weakest parts of the insulation system generating tracking (carbon paths through and across the insulation). The fault is characterized by snapping sounds, an odor of ozone and visible arcs. Once this occurs, direct paths to ground exist through the carbonized insulation.
Surge comparison testing is a second high voltage test method that has been utilized to detect winding shorts, or faults, between conductors. The most basic circuit of a surge tester is an RLC (Resistance-Inductance-Capacitance) circuit that uses the windings being tested as inductance (L), and a switch, which is known as a damped oscillator used for generating transients (impulses). When connected to an oscilloscope, you get a rapid, damped (fades continuously through the cycles) sine wave that depends upon the electrical properties of the circuit. In order to view the waveform, the transient must be supplied rapidly. When comparing two like coils, the wavforms should be similar. When they are not, a defect exists. At the same time, the transients also have four other effects:
1) They generate a high charge as described at the beginning of this lecture[1]. If a defect exists, ionization occurs and an arc is formed, further damaging the defect or completing the fault.
2) Moisture and other contaminants ionize quickly and at a different rate than air and far faster than the insulation system. When testing a winding that is in service, when contaminants exist, tracking can occur across the points of the insulation where contaminants exist.
3) The dampening effect occurs rapidly, based upon the rise time of the impulse (transient), which occurs within the first 2-3 turns into the first coils of each phase of the windings being tested.
4) There is little to no dipolar spin in the insulation system making the surge test ok for evaluating pass/fail for impulses, but not the actual condition of the insulation system (ie: dry/brittle insulation, age, etc.).
While AC high potential and surge testing have been effective, in the past, for motor manufacturers and motor repair centers, they remain a potentially dangerous method for evaluating the insulation system of motors in the field. Most applied motor systems, that have been in use, have aged insulation and winding contamination. Mixed with the required testing conditions (clean and dry environment at the winding), make these ineffective tools for field testing. In the case of motor repair shops, the windings should be clean and dry prior to the application of either test, otherwise a good insulation system may be damaged.
In our next lecture, we will begin our discussion on Motor Circuit Analysis.
Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 5 DC Testing
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Last week we explored the dielectric properties of insulating materials and the polarization effect. Polarization, of course, being defined as the alignment of atomic dipoles. If you have any questions, at this point, it is recommended that you go back and review the previous materials, or contact Howard W Penrose, Ph.D.
This week we will use the materials that we have developed to view what is actually happening when we perform DC to ground tests, such as meg-ohm tests, polarization index, high potential testing (we will cover AC high potential tests seperately) and similar tests. In principle, these tests can only evaluate the insulation system between the conductors and ground.
When you place a DC potential across from the conductors (say +) and ground (say -) of any value, the dipoles will begin to align with negative towards the conductors and positive towards ground (opposite charges attract). As the dipoles polarize, the effective capacitance of the circuit changes and the resulting current (leakage) across the dielectric boundary decreases. The instrument takes the leakage current and converts it from milli or micro-amps to meg-ohms (or gig-ohms, or terra-ohms). The greater the current leakage, the worse the condition.
Meg-Ohm meters (MOM) come in two primary varieties, low and high voltage. In all MOM’s, the current is limited so that there is less potential for damage to the insulation system. The applied voltage is selected in such a way as to create an even energy level across the insulation surface of the object being tested in order to detect defects between insulation and ground. This will happen in areas that do not properly polarize, the material condition has changed in the insulation system, or there is continuity between the conductors and ground (direct short). Low voltage MOM’s place a lower potential across the insulation system and are, therefore, not potentially destructive to the insulation system. Incorrectly applied high voltage MOM’s can be potentially destructive, in particular if a voltage equal to or higher than the motor nameplate voltage is applied.
Polarization happens slowly, when a voltage less than the operating voltage of the motor is applied. This results in a curve that is a direct measure of the time for the insulation system to polarize. When performing an MOM test, a standard of one minute can be applied (or once the energy level within the instrument and the system being tested has stabilized) in order to produce a test result. Also, because of this curve, a ratio of a ten minute to one minute test can be observed and presented. This is known as the polarization index, or the average rate of polarization.
DC high potential testing is very similar to a meg-ohm test with the exception that you will also, normally, measure the actual leakage current. In the case of the high potential test, you will apply up to twice the voltage plus 1,000 Volts times the square root of three (1.7) in order to stress the insulation system. The potential harm from this test, as it is potentially destructive, comes when it is incorrectly applied and the full potential (voltage) is applied at once. This can cause violent polarization of the insulation system which may damage it at some small point in the system.
Limits of DC Tests
There are several primary issues with DC testing. We shall explore these in limited detail.
The first relates to our subject of quantum mechanics: You are relying upon less than 50% of dipole ‘rotation’ (actually, change in orbit of the electrons vs protons). This does not simulate the actual insulation conditions of the motor in operation. This means that the detection of brittle or damaged insulation requires that a significant area of insulation must be damaged. In addition, you are only polarizing the area of insulation that acts as a boundary between the coils and ground, limiting the test to a small portion of the insulation system that represents less than 6% of the potential motor faults (17% of potential winding faults).
The second issue relates to the test environment: Moisture (water) has a dielectric constant, as well. Damp windings or even humidity will have a direct effect on the measurements. This is of particular concern when the temperature of the insulation being tested is less than the dew point. It will cause an abnormally low test result that will not represent the actual conditions of the insulation system during operation.
The third issue relates to the insulation temperature at the time of testing: Dielectric insulation materials in electric motors are chemicals. They even follow the arrhenious chemical equation for chemical changes due to temperature. The actual temperature will effect the leakage across the insulation boundary because, as it is a dielectric, as the temperature increases, the amount of potential leakage across the insulation system increases, causing a reduced insulation test result. This requires temperature correction if you are trying to use insulation test results for trending. The temperature correction must utilize a highly accurate temperature reading of the insulation system, itself. This also means that the insulation temperature must also stabilize, requiring some time following de-energizing the electric motor (depends upon the size of the system – from a few minutes to 30 minutes, or more).
The final issue that we will cover is contact: The insulation resistance tests will only be effective for the boundary surface area between the coils and ground. This means that winding shorts, insulation defects on the end turns of the coils and insulation breakdown or contamination within insulation components not directly in contact with the stator frame, will not effect the test results.
In our next part of this lecture, we will cover AC high potential testing and surge comparison testing.
Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 4
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
If you are just starting to follow this Blog, at this point, I highly recommend that you go to the Blog history and follow along from the beginning ‘Time to Failure.’
So far we have discussed how quantum mechanics and reliability relate. We have also discussed polarization, heating and the insulation system. At this point, we will now discuss the next key component of quantum mechanics in motor operation – magnetic di-poles. This will increase our understanding as we begin to discuss the abilities and effects of test instrumentation on the motor system with Quantum Mechanics.
The same effect as the insulation dipole effect occurs in a magnetic field. The magnetic dipoles of the backiron and teeth of the stator core polarize in the direction of the magnetic field. This helps direct the magnetic flux and adds to the strength of the fields within the airgap. The reluctance of the steel to change polarity shows up as hysterisis losses from the steel. Once the field is removed, the magnetic dipoles of the steel quickly randomize.
The descriptions for the polarization of electrical insulation and core steel represent the steady-state application of an applied voltage potential. In an operating three phase system, the effects get far more exciting. As each sinusoidal phase of voltage is impressed across the windings:
�� As the voltage starts from zero, the beginning of the coil energizes, the insulating dipoles between the insulation to ground and the conductors within the coil are forced to polarize.
�� As the voltage continues to rise, the potential at the beginning of the coil is higher than the end of the coil, insulating dipoles continue to polarize and the magnetic dipoles begin to polarize in the direction of magnetic flux generated by the coils.
�� As the voltage hits its peak at the beginning of the coil, a majority of the magnetic and insulating dipoles associated with the start of the coil have aligned and the ones at the end of the coil continue to align. There is a lag in the fields between the beginning and end of the coil, which causes a potential between conductors to exist.
�� As the voltage begins to decrease, the insulating and magnetic dipoles begin to randomize at the beginning of the coil and release energy back into the system as the fields collapse. The fields at the end of the coil hit their peak then start to decrease.
�� The voltage approaches zero, then passes into the negative sequence of the sine wave. The dipoles and fields continue to react, but align in the opposite direction. The result is a ‘dipolar spin’ of both the electrical insulation and magnetic steel dipoles.
The rest of the system:
Now, when considering the relationship of the rest of the electrical system, you have circuit inductance which is defined as the magnetic strength per unit of current. The relationship between conductors within a coil result in internal inductance, the relationship between coils within the stator and the coils within the stator and rotor are referred to as mutual inductance. Inductance is effected by the dimension of the coil(s), the number of turns and other nearby coils. This means that inductance will be one of the final measurements that will change in a faulty winding.
The insulation system between conductors and conductors and ground are modeled as parallel resistances and capacitances. The capacitance value will vary based upon the position and volume of dipoles that have polarized within the insulation medium. Winding contamination and overheating will generally cause an increase in circuit capacitance, with a resulting change in the overall circuit impedance. Early in winding degradation, the relative capacitance, in relation to voltage and current, is relatively minor.
Now, we have a basic understanding of the motor circuit. In the following Blogs, we will explore different testing practices in order to determine exactly what their effects are and the resulting accuracy.
Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 3
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
To conclude the previous paper (Part 2):
The heat that we see is actually the entropy, or random energy lost from the system as it is working.
Note: At this point, if you are just joining in this lecture, you may want to go back into the Blogs to view the previous discussions on Time to Failure. This will assist in helping you understand the philosophy of where we are going. For my purposes, I am using this Blog lecture series to keep me on track as I am working on a physics theory that stems from my work on estimating time to failure. Whether it turns out to be a good or bad theory we will discover together, or, at least we will have discussed some areas that will be of interest, I hope.
Now, onto Part 3:
Understanding what is going on in the insulation system.
The insulation system in your electric motor is not actually a static system, nor is it truly an insulator. Instead, it is a dielectric.
A dielectric system is simplified in a model where you have parallel resistors and capacitors. The insulation medium, itself, is of a chemical makeup which allows some current to flow between conductors and conductors to ground. This value, when a DC voltage is placed across it, starts as a high value of current (referred to as leakage) that drops as the insulation system reacts to the potential. This can be observed using a meg-ohm-meter. When you place your potential, say 500 volts dc, between the conductors and ground of an electric motor (in a good insulation system), you will see a lower insulation resistance that slowly increases until it levels off at some higher resistance value. What you are actually seeing is based upon Ohms law, a meg-ohm-meter actually reads the low level leakage current between the insulation and ground, then converts that value to a resistance, normally measured in the millions of ohms (and higher). You are also directly observing quantum mechanics in action.
Within the insulation, at the atomic scale, the atoms that make up the insulation system begin to polarize. If you were to picture the atom as an oval object (it actually would look far different), the proton would be at one end of the oval, creating a positive charge and the opposite side of the oval would be a negative charge. If you have the positive on the conductors of the motor and the negative on the stator, the negative charge will line up (polarize) in the direction of the conductors (opposite charges attract) and the positive will polarize in the direction of the stator along the insulating boundary (the conductors closest to the stator). As the insulation polarizes, there is less leakage to ground because as the insulation polarizes, the capacitance of the circuit between conductors and ground changes.
Now comes the fun part.
When you place an AC voltage on the windings of the motor, the capacitance of the circuit continuously changes, as the voltage proceeds through its cycle, between conductors and conductors and ground. Because we are dealing with time, the value of voltage and current between the beginning and end of a coil, or phase, will be different, causing a difference of potential between conductors as well as conductors and ground, at any point in the voltage sine wave. As this occurs, the dipoles of the electrical insulation ‘spin’ as a result of the positive to negative potential across them (think of the oval rotating). This causes the continuously changing capacitance within the circuit. The dipolar spin results in energetic action and entropy which is seen as heat from the windings (coupled with the I2R losses of the conductors) as the dipoles are being forced to spin. The polarization of the insulation system causes the dipoles to arrange in such a way that they oppose each other and they randomize rapidly as soon as the potential is removed (they are lined up when a potential is across them so that the positives are next to each other and the negatives are next to each other, forcing them apart and stressing the insulation. As soon as force is removed, the charges move away from each other as quickly as possible in the insulation medium.).
When a fault occurs
Winding shorts and faults to ground do not occur instantaneously, but progress relatively slowly (see Part 1- Superposition principle). An insulation defect caused by any number of reasons, such as contamination, stress, spikes, mechanical movement, etc., starts by changing the properties of the insulation, and, therefore, the reaction of the dipoles in the area of the fault. Changes to the dielectric cause differences in the circuit capacitance which are, in the beginning, very small, then continue to increase as the fault progresses. This is the result of the increased resistance of the insulation medium for dipolar spin, or the ability of the dielectric to polarize. In a system where you are impressing the full motor voltage, the potential forces the insulation to polarize, causing more work and greater entropy (heat) which is defined as a reactive fault within the insulation system. The point of the reactive fault becomes ‘hotter’ due to the entropy, which increases the problem with dielectric polarization, and the cycle repeats, feeding on itself. Finally, the fault approaches a point where it is conductive and a high enough current passes the point to cause a current spike which will either produce enough energy to trip the protection or will generate enough entropy to vaporize the windings.
This is the basis that we will be working with to discuss insulation test methods later in this Blog lecture.
In the next Blog, we will be discussing the magnetic dipoles of the motor core steel.
Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 2
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
OK, so now we have defined reliability in terms of quantum mechanics (Part 1). What about the rest of what we are doing, such as using test equipment? And, better, can it be represented in terms that can be easily understood by people like the CFO or other office or non-maintenance type? Well, in this presentation, we will start discussing it.
One of our first steps is to discuss and define ‘heat.’ As you know, heat is the enemy of any insulation system and for most parts of the electrical system. To understand heating, we have to consider the atom and molecules of matter that we deal with. In classical physics, such as in high school and many college text books, we see atoms represented as round circles and may, depending on if you took physics or not, also have learned about protons, neutrons and electrons. In classical physics, an atom is represented just like our solar system with atoms orbiting around the protons and neutrons. This is still taught this way as it is easy to grasp for most beginners, or people who just need a broad understanding of atomic structure. In Quantum high-energy physics, we now know, using particle accelerators, such as the one at Fermi-Lab in Batavia, Illinois, that the atom is far more complex, having many, many smaller and smaller parts. The good news is that we don’t need to worry about that. What we need to consider is that our classical view of the atom and how it works has changed.
In quantum mechanics, we know that the shape of an atom is constantly changing and that it is not, or at least is rarely, spherical. Instead it is a constantly changing shape that receives and gives off energy. Based upon its charge, it will bond with other atoms, making up a molecule, or group of atoms. We also know that as we add energy, the atom becomes more energetic, ‘vibrating,’ for want of a better term, and giving off energy in terms of photons (as presently described in the most accepted theories). As we cool the system, or remove energy, the atoms vibrate less. In the first case, if enough energy is provided, the atomic structure will loosen, causing a solid to become a liquid, then a liquid will turn into a gas. The heat we feel in an object comes directly from this vibration of atoms and molecules. Lucky for us, these physical properties occur due to very specific natural laws. [Definition of a physical law: Generalizations of how system occur in nature that hold true through experimentation.]
So, what does all of this have to do with us?
Well, let us look at what happens if we pass a current through a loose connection. The classical view, which works well for our large-view observation, is that the current passes through the point of high resistance, and we get heating due to I2R losses at that point. For this example, we will stick with the fact that we know heat is being generated due to current flow (how this occurs is covered by classical physics which is being viewed differently by several existing quantum theories, so we will avoid that issue here). At the point of the resistance, energy is being injected into the molecular structure of the material. The atoms that make up the material vibrate and give off energy. The materials immediately surrounding those molecules and atoms begin to vibrate, due to the energy given off by the atoms close to them from the point of high resistance. Present theory states that the shared energy is photons being given off by one atom and being accepted by another (this was actually put forth by a number of early physicists, including Einstein and later discussed and supported by others, such as Richard Feynman). If enough energy is given off, the fault point will begin to ‘glow’ do to the number of photons that escape. As you move away from the fault point, the excitation of the molecules lessens, and the amount of energy decreases, causing a drop in what we measure as temperature. Thermal insulators accept less energy while other materials (thermal conductors) will accept more of the energy. You will find that most thermal conductors are also the same materials that can conduct electricity and most thermal insulators are the same materials that act as electrical insulators.
To understand the scale of what is going on, if you were to scale an apple up to the size of the earth, an atom would be the size of the original apple.
Kind of interesting, isn’t it.
So, when you go out to do your next infrared survey, remember that what you are looking for, in terms of a fault, is the heat radiation that is due to escaping energy from vibrating molecules and atoms. Quantum mechanics at its best.
In our next part, we will discuss what is happening in electrical insulation while a machine is running. By knowing this, we can then begin to discuss how our electrical test technologies work. In this way, we can understand the strengths and weaknesses of the technologies and how they can, or cannot, be used to trend condition or estimate time to failure.
Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Did you know that you were using modern physics in your motor diagnostics program? Each time you use your motor circuit analyzer, electrical signature analyzer, infrared camera or other technology, including when you work on determining the probability that a system will operate (reliability), you are performing a Quantum Physics experiment (application).
In school, most of us, including engineers, are primarily taught the building blocks of physics including Euclidean geometry and classical physics. All the way back to Aristotle and the birth of logic, we have been taught, in Western thought, yes and no, black and white, right and wrong and true or false. There is no in between, and we are taught that way in school (‘Church of Reason,’ Pirsig, “Zen and the Art of Motorcycle Maintenance,” see previous Blog). In this case, a motor is either working, or it is not, a winding short has occurred, or it has not. In this century, these illusions are obliterated within the ‘new physics’ of quantum mechanics.
The concept of quantum theory was initiated by Einstein in his Theory of Relativity, in which we leave the Classical Physics (Euclid) of three dimensions and include a fourth, extremely important, dimension, time. In the form of astro-physics and, relatively, large systems, classical physics and geometry work along with Relativity. However, in the realm of the very small (atomic scale), these theories and laws collapse. Einstein, to his dying day (literally), worked towards a ‘Unified Field Theory,’ which would bridge the gap between classical physics and quantum mechanics. This has not been accomplished to this day.
There exists, within quantum mechanics, the superposition principle, which describes that a systems state may not just be X or Y, but may also be in a combination of them (somewhere in between). In the case of a bearing, in classical physics, it is either good or bad. In quantum mechanics, it is most likely in some condition in between with a probability of X (good) and a probability of Y (bad). In reliability engineering, we work towards estimating the probability of X and present it as the reliability (chance that it will work). Virtually all systems are in between, hence the need for PdM and condition-based monitoring programs to assist in estimating where the condition of they system is in between X and Y.
(to be continued…)
Developing Your Motor Diagnostics Program by Dr. Howard Penrose, ALL-TEST Pro, A division of BJM Corp.
In Part 6 of the Motor Diagnostics iPresentation Tutorial series, Dr Penrose takes us through the seven steps to developing a successful Motor Diagnostics program. The presentation starts with a review of the importance of the application of such a program for electricians and reliability technicians, then leads into the seven steps that have proven successful as outlined in the 2003 Motor Diagnostic and Motor Health Study. This includes setting up partnerships with other departments and vendors, selecting personnel, communication, training, selecting motor diagnostic equipment and more. Following the presentation, contact Dr Penrose for information on how to download the series in Adobe Acrobat.
This tutorial runs for 16 minutes. Windows or REAL Media Player required for narration playback.
http://www.rcm-1.com/forms/bjm_reg.htm