Can you test any motor using a VFD?

[thejohnson]

We sell many skids using a beefed up 208(only)/3/60 motor ranging from 1/2 HP to 20 HP. However we don't have 208/3/60 in the building. Our testers have been using a VFD to get the 208 but they are not inverter duty motors. I don't believe this is OK. Can someone prove this for me?
Also, we do have 240/3/60. What will this do on a 208 only motor? Thanks, Jeff

 

[Skogsgurra]

It shouldn't be a problem for testing. The PWM has an adverse influence on insulation and bearings - that's true. But a 208 V motor is not a problem at all. And for short test runs, I wouldn't bother.

I have had short term effects on small 1/4 HP motors at 500 V. They fried after a couple of days on VFD. We added du/dt filters and that saved them.

So, if you want to be absolutely sure; use sine filters or du/dt filters. Or try to get one of those sine-wave inverters. (Not supposed to say their name. I own shares in that company).

Gunnar Englund

http://www.gke.org

 

[CJCPE]

In what sense are the motors "beefed up?"

Inverter duty motors have insulation rated for for VFD output waveforms with fast rising switched voltages that can ring to high peak voltages. If the cable from the VFD output to the motor is less than 25 feet or so, the peak ringing voltages may not be too high for a standard motor.

Inverter duty motors are also designed to cool themselves at reduced speeds. If they are not operated at full torque for long periods of time at low speed, standard motors can tolerate VFD operation without overheating. Motors that are "beefed up" by using a frames size that is larger than the standard frame for the rated horsepower are generally better able to tolerate the reduced cooling air flow that comes with low speed operation.

If the motors are designed for 208 volt operation, operating at 240 volts is only 15% overvoltage. That will increase current by 7-1/5% or so and increase losses by 2-1/2 percent or so causing the motors to run hotter than normal. The motors can probably tolerate that also.

 

[ScottyUK]

A three phase autotransformer is surely cheaper than a 15kW VFD? It certainly is in the UK. That being the case, why don't you derive 208/3/60 from your 240/3/60 supply?



----------------------------------

I don't suffer from insanity. I enjoy it...

 

[aolalde]

Thejohnson

If the test is for mechanical purpose and last a few minutes no harm will be done to those motors by a VFD adjusted to deliver 208 V 60 HZ.
240 volts means 15.3% over-voltage, most of the manufacturing standards allow 10% max over-voltage. The magnetic circuit could saturate increasing the line current and overheating the winding and core laminations. If you can monitor the motor current during the test and it is no larger than the current stamped on the motor nameplate a short time test will not harm the motor.
The safe way is a 240/208 Volts transformer as ScottyUK recommends.

 

[waross]

A pair of 3 KVA transformers will buck your 240 volts down to 208 or close to it, and supply enought current to run a 20 HP motor.
Assumption;
Full load current for 20 Hp. at 208 Volts, 59.4 A
Add 25% = 74.25, USE 75 AMPS.
Transformer; 240V to 32V, "buck boost" rated.
240 volts minus 208 volts = 32 volts.
With 240 volts on the 240 volt winding, and the 32 volt winding in "Bucking" configuration you will have 208 volts.
With two transformers in open delta you will be able to transform 240V three phase to 208 Volt three phase.
Needed transformer capacity. 75 amps x 32 volts = 2400 VA.
A 3KVA transformer will provide 3000/32 = 93.75 amps.
Solution; Two 3 KVA 240V-32V, "Buck Boost" rated transformers connected open Delta.
The transformers can be connected in autotransformer configuration and the resulting voltage will be 211.8 Volts.

Years ago, our firm serviced a factory with a mix of 600 volt branch circuits and 480 volt branch circuits. They had a mix of 575 volt motors (600 volt circuits) and 460 volt motors (480 volt circuits). As changes were made we were continually faced with running 460 volt motors on 600 volt branch circuits and running 575 volt motors on 480 volt circuits. A pair of small "Buck Boost" rated transformers was used in each case, in auto transformer configuration.
Two transformers with 480 volt primarys, and 120 volt secondarys, will give a perfect transformation from 480V to 600V or 600V to 480V.
This may be the most economical solution to provide 208 volts from 240 volts for a motor testing circuit.
yours

 

[LionelHutz]

Without knowing your test conditions it is hard to say what is correct. ie, load, duration etc.

I will say that you are unlikely to have a problem running a 208V motor on a VFD being fed 230V. Most motors are built using insulation systems that are good up to at least 480VAC so your 208VAC motor should have insulation suitable for use with a VFD. Also, there are many non-VFD rated motors being controlled with a VFD which are sucessfully operating in the world.

If your tests are just short duration or not at full-load then you'd also be fine operating the motor directly from 240VAC.