VFDs Application

[JimmyJunior]

Hello:

Can VFDs work with regular motors? What parameters should we observe?

At my agency opinions are divided on this issue: VFD rated versus non-VFD rated motors.

Thanks,

Jimmy Junior

 

[davidbeach]

How long do you want the motors to last?

 

[catserveng]

David hits the main issue in a nutshell, non-VFD rated motors usually don't last as long as VFD or inverter rated motors.

I have seen a lot of motors on VFD's with acceptable service life, but power lead length between VFD and motor, load factor, voltage supply stability, start and stop frequency, all effect overall motor service life.

I think the main problem with VFD's on motors from what I have seen is the impact of fairly high amplitude voltage spikes as the VFD is working, this tends to stress the motor winding insulation, especially at the stator end turns, and causes insulation failures.

Also VFD's can be a cause of motor shaft currents, and most motors do not address this problem with the result being shortened motor shaft bearing life.

This is a pretty simple explanation of what can be a pretty complex issue. Hope that helps.

 

[CJCPE]

Look at these:

NEMA Application Guide for AC Adjustable Speed Drive Systems (free download)

Information Guide for General Purpose Industrial AC Small and Medium Squirrel-Cage Induction Motor Standards (A condensed, free download version of NEMA standard MG 1)

The term “VFD rated motor” is not defined by a standard. It can mean whatever any given motor manufacturer says that it means. Be sure to look carefully at the literature provided by prospective motor manufacturers to determine the stated capabilities of each product. If the manufacturer does not provide written recommendations regarding the use of a particular model with a VFD, it is probably not a good idea to use it with a VFD.

 

[sberbece]

See attached a good paper from WEG Australia. It covers the subject quite well.

 

[alehman]

I would discuss it with your drive manufacturer if possible. Most publish application guides that may help.

 

[LionelHutz]

The voltage has a large effect too. I would say that a 480V motor has a better chance of a long life on a VFD than say a 600V or 660V motor on a VFD.

These days, I don't see manufacturers doing much extra for a "VFD rated motor" compared to their standard offerings. Most manufacturers seem to use the same wire for all motor builds because it's cheap enough by volume so the insulation in the motor is no better VFD rated or not. They may install insulated bearings in their VFD motors and not in their normal motors which can help with bearing currents.

Despite what the big blue company says about their great insulation I see nothing special about their motors compared to others. They can fail prematurely without explanation just the same as everyone else.

Honestly, a good rewind shop can make the trouble a new motor is giving everyone just disappear. When a new motor keeps having winding failures and I'm dragged into solving the problem I'm usually pretty happy when the decision is made to rewind one of the failed motors instead of just replacing it with another new one. That means the problems will likely go away.

The most common problem I see causing winding failures is that the motor manufacturers do not observe care maintaining the turn to turn and coil to coil spacing/clearance. They machine mush wind the motor which ends up with turns from one coil up against another coil putting a large voltage across the enamel. Sometimes the voltage between 2 wires is close to line to line levels. Other times they use a connection scheme which puts a high voltage on each coil, sometimes up to 200V per coil. Sure, theoretically, the wire can handle this no problem but in practice not so much so.