Electrical insulation to ground testing, also known as MegOhm testing, has been one of the primary methods used to for evaluating the condition of electric motor insulation for a century (or thereabout). The primary reason for its use has been to perform a safety check of the barrier between the current carrying conductors and the case of the electric motor.
A 1983 EPRI project determined that 37% of motor failures, across all size ranges, were detected as winding problems with 17% of that being insulation to ground. That relates to 6.3% of faults detected with insulation to ground readings FOLLOWING motor failure. In a 2000 study performed by PG&E (the Electric Motors Performance Analysis Tool project) on 480 Volt motors from 5 to 250 horsepower, 4% of the motors surveyed, that were determined to be in poor condition, had insulation readings below 100 MegOhms. In both cases, only 1 in 20 motors with problems were determined through insulation to ground testing, with 1 in 3 (or, 1 in 2 with the PG&E study) having winding shorts and/or insulation to ground faults. In effect, winding shorts are 6 times more likely to be the cause of a motor fault.
Now, when looking at the complete motor system, including the incoming power, controls, cables, motors, couplings and driven equipment, electric motors fail even less frequently than expected. In most cases, the cause of failure is the electric motor acting as a fuse. With motor faults being a conservative 15% of motor system faults (some surveys show higher, and others lower, numbers), with loose connections, poor contacts, cable faults, driven equipment failure, coupling faults, belt and sheave failure, being more prevalent, that leaves insulation to ground tests detecting a rate of failure of 1 for every 120+ motor system faults.
Conclusion: With insulation resistance measurements detecting a conservative 0.8% of motor system faults, the following considerations should be made:
A motor system diagnostic program should be implemented that views the complete system and includes the ability to detect winding shorts, power quality, loose connections, cable faults and mechanical condition of the system. A combination of tools including MCA, MCSA, Vibration Analysis, Infrared Analysis and other technologies should be considered for the greatest possible return on your motor system program.
Insulation resistance is found to be lacking as a predictive maintenance tool, but should be included in any program for safetly issues. The values determined should be at least the IEEE 43-2000 recommended values.
One of the key issues when performing reliability testing on electric machines is how to determine time to failure. Reliability determination and test accuracy becomes even more critical when applied to the military. Now, not only is production and cost (and mission) an issue, but lives may be at stake.
In this case, a 475 kW, 460 Volt alternator on a military vessel was tripping on high temperature. As a Ships Service Diesel Generator (SSDG) it is a critical system. Military reliability specialists quickly eliminated the cooling system and isolated the problem to the SSDG circuit. The SSDG had been installed the previous year and had been in storage for about 18 years prior to that. Some repairs had been performed and the windings re-insulated prior to installation. To access and remove the alternator for repair, a hole would have to be cut in the ship’s hull, making test reliability and accuracy a critical factor.
The motor diagnostic techniques of Motor Circuit Analysis (MCA) and Motor Current Signature Analysis (MCSA) were ordered. The first MCA test identified a winding short from the switchboard, a second test at the connections identified that the short was actually two shorts, one in the cable and one in the alternator. A ‘heat run’ was then performed with the alternator at 50% load for fourty minutes. A number of issues were identified relating the rotor and windings when trending four samples taken every ten minutes. A final MCA test was performed following the fourty minute run and the short was still identified along with an insulation to ground reading change that resulted from a temperature increase calculated at 140 degrees C that had occurred somewhere in the stator. Overall temperature was monitored and appeared steady.
It was determined, based upon the data collected and the presented history, that the alternator would have to be removed and repaired. However, the alternator could run under partial load conditions until the next dry-dock period several months away.
How was this determined? All of the information, including the severity of the shorts, MCSA data and temperature rise during the ‘heat-run,’ as well as the past history and experience of the analyst, assisted in the determination. Observation limits were given on operation, including current and temperature rise monitoring. Total time for testing and evaluation was less than 90 minutes.
One of my objectives in the upcoming Blogs is to assist you, the reliability and maintenance professional, in being able to make similar judgements with confidence.
Electric motor energy retrofits offer facilities both energy and reliability opportunities. Unfortunately, in many cases, most companies will not consider the opportunities unless a two-year simple payback is identified.
The costs which are considered, that make the decisions challenging, include the full cost of the replacement motor, the labor involved in removing and re-installing and other associated costs. When reviewing the cost benefits of a few percent efficiency, the load and the actual annual operating hours, the payback may show as a great many years.
Instead, when considering a repair versus replace decision, the costs involved are only the difference between the replacement cost of the new motor and the repair costs of the old motor. In this case, the increase in efficiency will normally provide a generous return on investment. However, if left until it is time to make the repair versus replace decision, getting equipment up and running takes precedence over energy improvements.
Energy and premium efficient motors provide not just an improvement to energy costs, but are normally more reliable than standard efficient motors. Therefore, it is a benefit to both energy and reliability, and to the resulting bottom line, to use energy and premium efficient motors where possible.
Through all of this, another opportunity presents itself: Determine what motors are operating in poor condition and replace them before it becomes urgent. In 1989 through 1991, such a program was developed to consider electric motor and condition analysis. In the Performance Analysis Tool project, by Pacific Gas and Electric, electric motors were evaluated using datalogging instruments, motor circuit analysis, vibration analysis and the US Department of Energy’s MotorMaster Plus software.
Datalogging times varied depending upon the operation of the equipment. Vibration analysis, which required access to each electric motors, took longer. Motor circuit analysis time was quick, as testing was performed from the motor control center or disconnect. For example, testing 20 motors required less than 90 minutes to test with motor circuit analysis, another 90 minutes for basic electrical measurements and a full day using vibration analysis. It should be noted, however, that numerous maintenance opportunities were noted during the walk-through necessary to perform vibration.
In 1990, as part of the project, ALL-TEST Pro, A Division of BJM Corp, Dreisilker Electric Motors, Inc. and Pruftechnik co-funded modifications to MotorMaster Plus as the first US DOE Bestpractices (www.oit.doe.gov/bestpractices) Market Transfer success. Coordinated by Dr. Howard Penrose of ALL-TEST Pro, the modification included the ability to enter MCA, insulation to ground and vibration data directly into MotorMaster Plus, then to be able to sort the motors by condition.
The program was found to be very successful with the following comments:
1) While MCA was very successful, detecting 64% of the problems found within 1 of 6 motors tested, it required equipment to be de-energized.
2) Vibration analysis took a significant amount of time
3) Potential return was significant enough to make any inconveniences insignificant
Now, with the application and ability of motor current signature analysis to detect electrical and mechanical faults, a combination of motor circuit analysis and motor current signature analysis will allow a technician or analyst to take data and analyze it quickly. Coupled with MotorMaster Plus, an electric motor energy and condition analysis can be performed quickly and with great success.
There are several basic steps to such a program:
1) Select candidate motors to be evaluated
2) Collect motor nameplate information and note anything unusual
3) Collect MCSA data
4) Collect MCA data
5) Enter the findings into MotorMaster Plus
6) Run repair versus replace reports and return on investment reports
7) Make changes and enter data into MotorMaster Plus
8) Verify energy savings using MotorMaster Plus reporting features.
Selecting the electric motors for review will be the most challenging part of the project. The most successful programs select motors from 5 to 250 horsepower from one department, as a pilot project. Otherwise, full surveys of a large plant usually become overwhelming and are either not pursued or only partially successful. In addition, complete motor nameplate information is required for an accurate MotorMaster Plus survey. Ensure annual operating hours are noted.
A motor current signature analysis instrument which provide automated information on condition as well as reporting voltage, current, power factor, RPM and kW is recommended. If necessary, for variable loads, an MCSA device which allows datalogging should be selected. Motor circuit analysis data should be collected when the motor can be de-energized and a device selected which provides automated analysis.
Enter the nameplate information and findings into the MotorMaster Plus software. MCA information can be entered into the MotorMaster Plus maintenance fields and MCSA information can be entered into the vibration fields.
Repair versus replace reports can be run through MotorMaster Plus including condition analysis. Once the repair versus replace scenario has been run and printed, a cost analysis can then be performed.
For example: A 50 horsepower, 1775 RPM, 64.5 Amp, 440 Vac motor was tested on a Glycol pump in a critical application. It was determined that there were motor and driven equipment related mechanical problems including: Soft foot, stator faults, unbalance or misalignment, and looseness. A MotorMaster Plus study identified that the motor was operating at 75% of load and was 90.6% efficient. A premium efficient motor rated 94.9% efficient motor could be selected with a simple return on investment of 0.9 years. This equated to an after tax ROI of 366% and a benefit to cost ratio of 5.55. Simple decision to make.