Art Godfrey (retired)
Birclar Electric & Electronics
Romulus, Michigan
Technical Services Committee Member

With the growing popularity of variable frequency drives (VFDs), it is likely almost every EASA service center has repaired motors powered by one in a customer’s installation. For these motors, it is best that after repair they are tested using a VFD, typically at no-load. This will provide operation mimicking the actual customer applica­tion, at varying speeds, and will help ensure proper mechanical operation throughout the speed range. This will include detecting vibration problems, identifying any resonant speeds within the operating range, and more. Also, if the motor is used above utility line fre­quency (i.e., 50 Hz or 60 Hz), it should be operated up to top speed for vibra­tion measurement and a good break-in of the bearings.

Selection challenges
Selecting a VFD for use in service center testing can present some chal­lenges. For these challenges, many sellers of the VFDs may be at a disad­vantage, because their typical sales are for applications for a single motor or combination of motors, and likely for the life of the VFD. This means one set-up, and it will be left alone for a long time. In a service center environment, theVFD will need to power a range of sizes and types of motors. Our ex­perience was with five different VFDs used for service center testing, four of which were of different types and had different programming. See Figure 1 for a photo of a VFD.

First you will need to determine the type and rating of the VFD you will want to purchase. For type, you will choose between non-regenerative and regenerative options. The default choice will likely be non-regenerative since the testing is no-load. However, it will be found that a motor in test will act like a generator if it exceeds its set speed, such as during a too-rapid acceleration where the desired speed may be exceeded. In this case, the bus voltage may exceed the VFD’s safety factor and fault the drive. This would not be the case with a regenerative drive, which would be more costly for the same rating and require a trans­former. Figure 2 illustrates a speed indication readout that is available with some VFDs.

Voltage, power rating and frequency
After determining the type of VFD, you will need to evaluate the required voltage, power rating and frequency range. You will likely find other fea­tures or parameters of importance, but these are a good start. For voltage, you’ll want the high to be as high as the majority of motors you expect to repair. For this article, it was assumed to be 600V or less. Power rating will be an area where the default reaction will reflect that “bigger is better.”  Big­ger will be more costly. Since the test we’re using is no-load, a motor two or more times the power rating of the VFD should be able to be test-run at no load. Note that running a motor much larger than the rating will require caution as speed is increased — the bus-overvoltage problem mentioned above. Figure 3 illustrates use of a VFD with a power rating less than the motor being tested.

Maximum frequency of modern VFDs should be sufficient and will likely well exceed the requirements for most service centers. Older models will have less capability. Pay attention to minimum frequency that will be more important for starting motors larger in rating than the VFD. Be aware that the VFD may present some limits when used at less than the maximum values. We found that with one modern VFD, programmed for use at less than rated voltage (460V), meant that the param­eter for maximum frequency needed to match the parameter for nominal frequency. For frequencies greater than 60 Hz, this meant operation at a volts-per-hertz ratio lower than customer use. (In the vast majority of test runs, we used frequencies above 60 Hz.) With another modern VFD, use below 20% of its rated power limited the op­erating flexibility of the VFD.

Decide on new or used
The purchase decision may include whether to buy new or used. New VFDs can be annoyingly “smart” com­pared to a service center’s need due to the large number of parameters, pro­gramming and how one may react to motors of varying power ratings. Older VFDs may be easier to set up but less tolerant of unintentional abuse, such as a re-start while coasting without including the provision for this in the drive set-up. Also, a used drive will have aged power components, with no practical way to determine remaining life. Another caution for a used VFD, or any that has not been powered up for some time, is the likely need to “re-form” the bus ca­pacitors. This is required because the insulation between conductor lay­ers in the capacitor loses effectiveness over time if not powered up. The so­lution is to apply voltage in steps which “re-forms” the insulation. For more information, conduct an Internet search for “re­form bus capacitors.”

The second challenge is selecting a drive that has programming that is as user-friendly as pos­sible for the variety of motors that will be tested. In my opinion, determin­ing the ease of set-up will be more difficult than determining the rating of the VFD you choose. The sales information put out by the manufacturer will highlight capabili­ties that have little or no relevance for prospective service center use. For this part of the selection process, it would be most valuable if you can get a recommendation from another user such as a service center with similar application needs. Lacking that, ask vendors to demonstrate the drive set-up for as many different ratings as you can convince them to try. This approach will be lacking insofar as you will not get to see how the VFD runs the sample ratings. 

Another approach is to ask pro­spective vendors to put you in contact with their best product service person, with the intended objective being that this person will have a better feel for potential problems you may have in your many applications. To facilitate this process, it may be helpful for you to make a list of typical ratings you will want to test, emphasizing the ranges expected. Adding to the challenge, you will find that manufacturers may use different terminology for essentially the same parameter. 

Operating parameters
Once a VFD is acquired, someone will need to study the operating pa­rameters to interpret their meaning and to determine which of the hun­dreds available will need to be dealt with on a day-by-day basis. Once the key parameters have been identified, it may be helpful to list them on a form and complete a copy for each job to be tested. Besides normal safety practices, caution in every test set-up and opera­tion of a motor at test should be the foremost rule. 

We experienced three drive fail­ures, all bad enough to have to scrap the VFDs. One failed from a flying restart. The VFD turned off, then back on again while the rotor was turning at a considerable speed. Given new bearings and good lubrication, coast­ing to zero may take some time, and restarting too soon can be tempting. At least one large, high-speed machine we encountered took 20 minutes to coast to zero. One had several failures of power transistors, for no reason we could determine. A third suffered catastrophic failures of most of the power transistors, while running at a set speed but with the drive set for a relatively high switching frequency. (Higher switching frequencies stress the power transistors and may call for de-rating the VFD.) The other two drives, a 150 hp (111 kW) and a 600 hp (450 kW), have functioned well for many years. It should be noted that our service center tested an unusual mix of ratings. Your results should be better. Also, it is my opinion that test­ing powered by the VFD should be limited to a few personnel, given the set-up complexity compared to sine-wave-power testing.

When it comes to installing the VFD for use, a transformer will be required for a regenerative VFD, but is just an option for most six-pulse non-regenerative VFDs. Definitely use the prescribed type of fuses. Cabling from the VFD to the test motor should be as flexible as possible, sized for maximum current expected. Our service center used cables with as many strands as possible. Also, a low-impedence, high-strand count ground cable was always used from the drive to the test motor. For protection of the bearings, a temporary grounding brush was also sometimes used.

The acquisition and use of a VFD for final test-runs will improve the knowledge and capability of the ser­vice center personnel, although there will be challenges along the way.

AVAILABLE IN SPANISH

Categories: AC motors, Drives/controls, Service center equipment, Motor testing

Tags: Variable frequency drives Motor testing

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