VFD Motor Requirements

[Jnox]

We have several large motors with possible VFD applications, but I have some concerns. The motors are 1960's vintage. There is a good possibility they have been revarnished sometime in the past.

My question is, where can I find a good list of requirements which have to be met for successful VFD application?

I do not want to burn up any motors. I understand that insulation type is the key, but what else?

Thank you in advance!

 

[itsmoked]

list of requirements which have to be met for successful VFD application

1) You need a speed other than full speed for the equipement to run correctly.

2) If the process can run more effieceintly at a lower speed for reasonable amounts of time.

3)

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DickDV]

jnox, it would help if you would give us a few more details. Volumes have been written and are still being written on this subject.

For example, give us the horsepower range, the working voltage, the expected motor speed range, and the ambient temperature.

We may be able to save you a bunch of money compared with rewinding or replacing your existing motors if we knew more about what you are planning.

 

[Jnox]

Well, lets see. I will try to provide more details.

This is just a general question. I need a list of requirements for a VFD application.

I'm not sure if this is the right question to ask.

I know if you have an old motor, if you want to put a VFD on it, you need to meet specific insulation specifications.

My question is, what are the other requirements? Is it motor temperature rating?

I understand that you might run into temperatures issues in older motors at lower speeds. Is there any specific information regarding to this subject, or do I need to contact the VFD manufacturer?

My situation is, I have a whole bunch of OLD motors (different voltages/HP/Speed) with possible VFD application. What do I need to look at to see if the motor can handle a VFD?

I hope this helps!

 

[dpc]

I think the two biggest concerns are winding insulation failures and bearing damage.

VFD outputs have a lot of high frequency components that put additional stress on winding insulation compared to normal 50 or 60 Hz sine wave. As the distance between the VFD and the motor increases, the problem can become a lot worse. There are output filters that can be used at the VFD to help somewhat, but it is reasonable to assume that you will see an increased rate of motor winding failures if you connect a bunch of old motors to VFDs. Inverter-duty motors have additional insulation in the windings to help reduce these failures.

The bearings can be damaged due to circulating current through the motor shaft and bearings to ground due to common mode current in the VFD output. This is hard to predict. But inverter duty motors generally have one bearing insulated from the motor frame to help reduce this problem.

The low speed heating issues are probably not the biggest concern, unless you are planning to run at very low speeds.

But your question is hard to answer because it is not very specific. Can the motor "handle" a VFD? It probably can, at least for a while. But you will likely be shortening the life of the motors. There is no way to predict the increased risk of winding failure for any one motor, but taken as as group, the failure rate will almost certainly increase. So if these are large expensive motors, you might want to take a hard look at the cost/benefit tradeoffs of retrofitting VFDs. If things are working now without the VFD, what benefits while you derive from adding the VFD?

 

[DickDV]

jnox, I don't mean to be hard-headed on this but the answer to your question is not nearly as simple as you assume it to be.

For example, motor insulation takes a heavy battering from the drive output pulses but this is highly variable depending upon operating voltage and especially the motor lead lengths back to the drive.

Also, motor thermal issues are very troublesome. They vary based upon hp, motor enclosure style (ODP, TEFC, etc.etc.), drive carrier frequencies, speed operating ranges including overspeed, and sometimes service factor.

Having said that, maybe your only course is to contact motor manufacturers for their inverter-duty motor specs and study them. Plan on doing that for a long time, tho. You will find that even the reputable motor manufacturers have no standards as to what constitutes an "inverter duty", an "inverter ready", and an "inverter rated" motor.

I've been doing drive/motor application work for over 19 years now and, still, I get a surprise several times a year. Never too late to learn something new, I guess!