How to keep VFDs and motors running

February 10th, 2015, Published in Articles: Energize, Articles: Vector

 

Applied to blowers and pumps, VFDs offer energy savings and, in mechanical applications, allow fine adjustments which are impossible with other methods. Yet they are not simple plug-in-and-forget devices.

Fig. 1: Changing parts until the problem goes aways is an expensive way to troubleshoot.

Fig. 1: Changing parts until the problem goes away is an expensive way to troubleshoot.

Variable frequency drives (VFDs) are full of electronics and are susceptible to many problems, from incoming power disturbances to environmental hazards and incorrect use – as well as a few unexpected factors.

The motors they drive present their own challenges. Following here are a few things to bear in mind and some errors to avoid.

Start with baseline motor readings

Don’t just take the motor out of its box, put it in place and hope for the best. Take insulation resistance readings from phase to phase and phase to ground before putting a motor into full operation. Measure the insulation resistance of the windings using an insulation multimeter to determine favourable readings.

Measure the starting and running amperage, the running voltage and the leg-to-leg balance, as well as 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 running temperature, it is difficult to determine the actual cause of the problem.

One often sees no problems until heat builds up because of high ambient temperatures in some plant. Compare the results of an infrared thermometer or a thermocouple connected to your multimeter.

Take periodic measurements

Depending on your preventive maintenance schedule and the cost of unscheduled downtime, take additional amperage, resistance and insulation resistance readings. Compare these readings to previous readings. If the measurements deviate by more than between 5 and 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 whether the motor matches the application and was specified for the system, or whether upgrades are needed.

Check the protection

Consult the protection systems, overload contactors and fusing to determine whether the overload is set for full load amperes or whether it is set too high or too low. Ensure that the fusing is 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?

The temptation exists to replace fuses with higher-rated ones if they blow repeatedly, but if the overload shorts across the fuse, it no longer trips and the motor could burn out. This requires a lot of back-checking, consulting the manuals and studying nameplate data.

Don’t change parts; troubleshoot

Some technicians will change parts until the trouble goes away. This is an expensive way to troubleshoot, since the cost of most motors and exceeds $500. It is not unusual for motors or drives which fail in one application to function in another.

This makes the job of finding the original problem difficult since the failure was only temporary. Was the application or a combination of factors the cause of the failure?

Check cabling

Check the line at the motor, not just at the wall panel which may be meters away. Power lines in hostile (e.g. high temperature) areas may fail, even when protected by conduit. Checking the voltage at the panel and not at the motor may result in replacing a perfectly good motor when the problem is located in the wiring.

Look at the drive’s setup and parameters, check the acceleration and deceleration times and whether the motor is running at line frequency.

Choose the correct motor

Motors are sometimes used in applications for which they are not designed. Inverter-rated motors make a big difference in terms of the longevity of the system. Running a standard-duty motor at 50 Hz, for example, often leads to overheating. Similarly, running it at 90 or 120 Hz may work for a while only. The duty cycle of the motors will help determine which application they are suited for. A motor designed to run eight hours a day, five days a week will fail prematurely if it is run 24/7.

Nameplate data is an important troubleshooting tool. It will provide the motor’s service factor, duty cycle and more. It will also provide useful information about the protection circuits and fusing.

Check for power problems

Many drive failures are caused by power spikes, phase loss or undervoltages. After a power issue, it’s important to measure the power to see if the problem has been corrected: the drives will pay the price. The machine operators may restart automatically and try to run again when the power returns, leading to total motor failure because of the single-phase conditions. Most newer drives, however, have settings that will not let the system restart after a fault.

Contact Leanne Cole, Comtest, Tel 010 595-1821, info@comtest.co.za

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