Motor duty cycle and VFD

[ischgl99]

Is there a rule of thumb or calculation for the duty cycle of a motor run at speeds other then it's nameplate? For example, if a motor is controled by a VFD on a machine tool that runs at all different speeds intermittantly, maybe as low as 5-10 hz, does the duty cycle of the motor need to be adjusted for the slower speed? I know for continuous operation cooling is an issue, but is it still an issue for intermittant use and can the duty cycle be calculated based on the speed the motor is run to determine how long the machine can be run at the lower speed?
Mike Bensema

http://www.dutchmenservices.com

 

[electricuwe]

It's difficult to give rule of thumb for that case because in the case of VFD operation the duty cycle doesn't describe the operation properly.

Look at the following example:

Case 1: intermittent operation with low speed and no operation in between

Case 2: intermittent operation with low speed and operation with full speed but lower than nominal torque in between

The motor will be able to operate without overheating in both cases:

In case 1 because the duty cycle is low and in case 2 because the operation with full speed will allow to remove more heat than is generated during that time

Unless you can't predict operation schedule and perform a suitable thermal calculation the best thing is to do a rough guess and provide suitable protection.

From my point of view for machine tool application you should distinguish the following type of application:

spindle drive:
Low speed operation for extended period of time with high torque possible. I would provide an independent cooling fan in that case.

axis drive:
Low speed operation only for short periods of time and with low torque, operation without independent fan might be possible.

 

[ischgl99]

Thanks electricuwe, I thought it would be something like that, but it's nice to hear another point of view. Mike Bensema

http://www.dutchmenservices.com

 

[jbartos]

Suggestion: The motors can be designed various ways. It is probably most appropriate to contact the motor manufacturer tech support regarding any irregular motor operating conditions.