Really need inverter duty motors?

[OEMsparky]

I have an application where we need to put a few ac motors on a small machine. The motors are 3/4hp and will be speed controlled with a vfd. The size of the inverter duty motors is a big concern as the machine is pretty small. Is there a chance that inverter duty motors are really not needed? On these machines, the motors are speed scaled with an analog signal but the speed is constant per product - much like controlling the speed of a conveyor. Anyone have experience with this? The only literature I can find on the need for inverter duty motors is put out there by the manufacturers of these same motors...

 

[AFab]

Inverter duty motors a wound with inverter duty wire.
Standard duty motors may not be and therefor may fail
due to voltage spikes generated by the VFD.

 

[jOmega]

Shaun,

Suggest contacting several motor vendors: Marathon Electric, Leeson Electric, USEM, Baldor, etc... and discuss application with them. That way you will have their recommendation, and the confidence that the motor they recommend will serve your application well.

Other than that, and without a lot more specific detail, we'd all be just guessing.

jO

 

[Ahr35181]

shaun,

My understanding of the subject:-

For EX Hazardous Area Applications the motors must be Inverter Rated, as well as EX certified of course.

For Non-Hazardous Area applications, motors simply require derating (as jO suggests, motor manufacturer will advise), more so with constant torque (e.g conveyor belt) than variable torque.

Alan

 

[Baldor]

The smaller the motor, the more of the "voltage doubling" effect will occur. Also true for cable length. At 3/4 Hp, you should be able to find an off-the-shelf output filter to reduce voltage spikes at the motor.

 

[vcoul]

Input voltage to the VFD has a lot to do with highest possible spikes onto motor windings (as does lead length).
What voltage is the application?

 

[OEMsparky]

this particular application is 120v. Occasionally 230v or 480v applications come up as well.

 

[avigodner]

PWM inverter operation may create the following problems - motor winding increased insulation stress and overeating due to motor additional PWM loss.

If I understand correctly, stress voltage build-up may happen because of possible cable length. This seems not to be your application case.

Anyway, you are supposed to face additional PWM inverter operation caused motor loss that via potential overheating may lead to 20-25% motor power derating.

For some more info on machine PWM loss see PWM loss article overview at

http://www.drbrushless.com/cgi-bin/articles.phtml

 

[Barry1961]

This would be a good question for the motor people. They will want to know speed range ratio along with the voltage. I have run a lot of "standard" motors with VFD's up to a 10:1 ratio and never had a problem when the leads were 50' or less. Over 50' I have had problems while running with 480V over 50' and had to put in filters. I think all motors now are wound with ISR or Inverter Duty Wire but do not know for a fact.
Of course you have to oversize the motor so you can trade torque for speed when you go over the base frequency(60hz).

Barry