Variable frequency drives (VFDs) continue to gain popularity. Applied to blowers
and pumps, they offer energy savings, and in mechanical applications they allow fine adjustments that wouldn’t be possible by other methods. Yet they are not simple plug-in-and-forget devices. With electronic features, they’re susceptible to all sorts of problems, from incoming power disturbances to environmental hazards to improper operation. The motors they drive present their own challenges.
Dan Orchard has been in maintenance almost 24 years, starting in the Navy. He’s now a senior technician at INTIGRAL, Inc., a maker of sealed insulating glass window panels in Walton Hills, Ohio, where he deals with motors—from 300 hp down to 1/16 hp — and variable speed drives on a daily basis. All those years of practical experience have taught him a great deal about how to keep motors running and what to do when they stop.
Here’s Dan’s list of things to do and errors to avoid.
Start with Baseline Readings of the Motor.
Don’t just take a motor out of the box, install it, and hope for the best. Before
putting a motor into full operation, take insulation resistance readings from phase to phase and phase to ground. Measure the insulation resistance of the windings using an insulation multimeter (Orchard uses a Fluke 1587 Insulation Multimeter) to determine what a good reading is.
Measure the starting and running amperage, the running voltage and the leg-to-leg balance. Measure the temperature at first startup, unloaded, loaded
and after a period of use. A motor may run hot because it’s been used hard, is in a high temperature area, or has a problem. Without knowing its normal temperature, it’s difficult to tell which is the case. “It’s nice to know whether they are running hot or whether that’s normal for them,” says Orchard. “A lot of times we won’t see any problems until the heat really builds up, because the inside temperature near the glass tempering ovens in my plant in the summertime is normally about 130 °F.”
Orchard measures temperature using a Fluke 61 Infrared Thermometer or a thermocouple connected to his 1587 and will often compare results between the various methods.
Make other measurements periodically.
Depending on your preventive maintenance schedule, and the cost of unscheduled downtime, take additional amperage, electrical resistance and insulation resistance readings. Compare these readings to previous readings. If the measurements deviate by more than 5% to 10%, start looking for bad electrical connections or loose- or ill-fitting mechanical connections. Has the load increased, the frequency of use changed or have ambient temperatures increased or decreased?
Find out if the motor matches the application and was specified for the system, or if upgrades are needed.
Check the protection.
Look at the protection systems, the overload contactors and fusing. Is the overload set for full load amperes or set too high or low?
Is the fusing correct for the application? Overload contactors are designed to take care of overloads, while fuses and circuit breakers are intended for short circuit protection. Are they sized according to the load? Do the fuses blow without tripping the overload? Are the fuses rated properly? If the fuses blow repeatedly, there’s a temptation to replace them with higher-rated fuses. But if some time later the overload decides to short across itself and doesn’t trip any more, suddenly those fuses that are too high may make the motor cook. That means a lot of back-checking, pulling out the manuals (if they’re available) or looking at the nameplate data.
Don’t change parts instead of troubleshooting the actual problem.
Some technicians will change out parts until the trouble goes away. This is an expensive way to troubleshoot, since most motors and drives can be expensive. It’s not unusual to find that the same motor/drive that failed may start working in another application.
This makes the job of finding the original problem harder, since the failure was only temporary. Was it the loading, application or a combination of things that led up to the failure?
Cabling can be an issue, too.
Check the line at the motor, not just at the panel on the wall, which may be a hundred feet away. Power lines in high-temperature areas, even when protected by conduit, may fail. Checking the voltage at the panel and not at the motor may result in replacing a perfectly good motor when the problem is in the wiring.
Look at the drive’s setup and parameters. Check the accel/decel times. Are you running at line frequency, higher or lower?
Make sure it’s the right motor.
Sometimes motors are put into applications for which they are not designed. Inverter rated motors make a big difference in the longevity of the system. Running a standard-duty motor at 50 Hz, for example, often leads to overheating. Similarly, running it at 90 Hz or 120 Hz may work for a while, but the motor can’t accept that as a steady diet.
The duty cycle of the motors will help determine where and what application they are suited for. A motor designed to run eight hours a day, five days a week, will fail prematurely if it has to run 24/7.
Nameplate data is an important troubleshooting tool. It will tell the motor’s service factor, the duty cycle and more. It will also provide useful information about the protection circuits and fusing.
Check for power problems
Many drive failures come from power spikes, phase loss or undervoltages. After one of these occurrences, it’s important to measure the power to see if the problem has been corrected or is still happening. If you don’t check the power after an outage, the drives will pay the price. When the power comes back after an outage, the machine operators may simply automatically restart and try to run again. “Suddenly you start popping drives, burning up motors, because of single phase conditions, because a lot of the machines will try to start,” says Orchard.
Most newer drives have settings that will not let the system restart after a fault has occurred. Orchard sets his up so that something like a missing leg of 480 V ac is not overlooked, using phase loss indicators to help the maintenance staff look for problems.
Source: Fluke Corporation
The information contained in this article is intended for general information purposes only and is based on information available as of the initial date of publication. No representation is made that the information or references are complete or remain current. This article is not a substitute for review of current applicable government regulations, industry standards, or other standards specific to your business and/or activities and should not be construed as legal advice or opinion. Readers with specific questions should refer to the applicable standards or consult with an attorney.
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