Question about motors for VFDs

[mickeyf]

Here or somewhere recently I saw mention that a motor for a VFD should be rated for variable speed, and there was even a photo of a motor name plate showing speeds from xxx to yyy RPM. I had been under the impression the pretty much everyone who used VFDs just used whatever 3 phase motor they found surplus or cheaply, or whatever - motors that were expected to run at only a single speed. I have not used a VFD, but I have several "plain old, not variable speed rated" 3 phase motors in my motor collection that I hoped to use with a VFD some day.

What do people actually use, and how much difference does it make?

[J Tiers]

ANY 3 phase motor "can be used" with a VFD, assuming basic voltage etc ratings are observed.

SOME 3 phase motors are specifically made to retain cooling over a wide range of speeds. Those usually also have somewhat enhanced wire insulation, which can help withstand the spikes typical of PWM-generated sine waves.

Older motors tend to have better coil-to-coil insulation, but may not have good turn-to-turn insulation within each coil, as the wire relies on old-style varnish alone. This can be offset by putting relatively simple suppression inductors in the VFD output.

Doing that will also reduce the effect of pulses in harming bearings through capacitive currents. A grounding contact on the shaft can also deal with that issue. The matter is usually not a big problem at 230VAC. It can be at 415/480 VAC.

Older motors have cooling designed to work at full speed. They generally are not good for full current (as with vector mode) down at low rpms, below perhaps 33% of nominal. For V/Hz operation, that is much less of a problem, but when using older motors not "VFD rated", it is wise to limit the speed reduction and/or the torque required at low speeds. The fans simply do not move enough air. This can be counteracted with external fans.

Some claim that the high frequency pulses from the VFD will cause heating in the iron. Since the actual current variations at high frequency are limited, and the resulting magnetic field changes are small, this is not a large issue, and is usually ignored.

So, in general, yes you can use older motors with VFDs. People do it all the time. If you use an inductor (even a crude one) in the VFD output lines to reduce the "spikes" , and either limit the slow speed power, or use an external fan, you should not generally run into issues.

People who are not careful about those matters often get away with it also. But if you "push the matter", you may get bitten. Simple precautions will go a long way toward eliminating issues.

[rdfeil]

Jerry has pretty much said what is needed about this. I will add that a common failure of older motors on a VFD is an effect called "First Turn Failure". This is an insulation failure in the turn to turn class. It is a breakdown of the winding insulation in the first turn of the winding. It is caused by the spikes Jerry mentioned. The spikes have a rapid rise time and therefore appear as a high frequency to the winding. The first turn appears as a higher impedance and therefore causes a high voltage to be caused at the first turn or bend in the coil wire. This voltage can breakdown the insulation at that point causing failure. This is another thing addressed with VFD rated motors. I have seen this and everything Jerry mentioned as failures in VFD powered motors. None of them are real common in general, but they do happen. If you plan to use a VFD in vector mode and load the motor heavily invest in the load reactor and most issues will be solved. The capacitive current bearing failure is a different issue and takes a lot of time to happen. It is not an issue for the home shop. It is also primarily a function of the carrier frequency of the VFD. With that said, the biggest cause of failure is overheating due to lack of cooling because the internal fan is moving to slow and/or the motor is heavily loaded. If you plan to run slow and cut heavy an external cooling fan would be a good thing.

[PStechPaul]

The other issue is maximum speed if you overclock the drive. Most motors, even 4 pole rated 1700 RPM, will safely run at 3500 and perhaps 4000 RPM, but beyond that can be risky. It's better to use a four pole or even a six or eight pole motor, so it can run safely at its rated 800 or 1100 RPM as well as (probably) up to 3x to 5x with reduced torque. So it's better to use a larger motor if you have room for it.

[Noitoen]

At the moment I'm running a standard 160Kw 400v screw compressor with a 400Kw VFD at 35 Hz to lower the air output to our nitrogen generator with no problems. The compressor is pulling around 120Kw at this speed.

[Doozer]

The motor tag is propaganda, designed to make you think that motor is special and you need it above others.
Nope.

-D

[Noitoen]

The motor tag is propaganda, designed to make you think that motor is special and you need it above others.
Nope.

-D

400V VFD's can destroy older motors due to the way the dc voltage is modulated to generate the AC wave.

[J Tiers]

The motor tag is propaganda, designed to make you think that motor is special and you need it above others.
Nope.

-D

While that is intended as a bit of a joke, there is also some truth to it.

"Inverter rated" motors have a rating for speed range. That is both for cooling issues, and for ability to withstand higher speeds.

They have enhanced insulation. The wire insulation is intended mostly to take care of the "first turn" issue. There is also bearing insulation or other means specifically to take care of the bearing current problem.

And there are some other changes dependent on manufacturer, which address various other considerations which are of generally lesser inportance.

But it is not as if an older motor should be expected to quickly fail.

There is a tendency in the last couple or three decades, for everything to be a "specialty", for everything to require a "certification" covering that particular narrow usage, or occupational niche. And there are "certification programs", certification "inspections", etc to perform the certification. Naturally these all have fees, it has become something of a hidden industry.

Along with that is the belief (or in some cases legal requirement) that only people with the certification can do that particular work, or that only equipment carrying the certification can be used. Yes, in some cases (UL etc recognition, Mine safety recognition, etc) that is useful and even necessary. But in many cases it is somewhat silly.

We have not (yet) got "telephone sanitizers, first class", but that's potentially only because there are no public telephones anymore.

400V VFD's can destroy older motors due to the way the dc voltage is modulated to generate the AC wave.

That is exactly what Robin and I have mentioned. The fast "edge rates" of the pulsed (PWM) output, which can cause capacitive coupling, and the buildup of larger "spike" voltages. Use of an inductor on the VFD output will reduce that to where it is no longer an issue.

[wmgeorge]

I've installed many VFD's on all sorts of motors big, small old and new and including my own machines. Never had an issue on any of them, but did not try to run any at near zero speeds or 2x the rated RPM.

[Noitoen]

I've installed many VFD's on all sorts of motors big, small old and new and including my own machines. Never had an issue on any of them, but did not try to run any at near zero speeds or 2x the rated RPM.

My lathe has a 4 pole motor that I push to double speed without problems. The VFD is 400V and the motor is configured for 230V 50Hz and only after 87Hz does the torque drop. At that frequency the motor is being fed with 400V and there is no more voltage to raise to maintain the torque. The correct voltage for 100Hz would be 460V and the motor would have double the horsepower (the VFD is rated for double the power). I use higher speeds for softer materials so, no issues. I haven't changed speed on the belts for a very long time.

[Doozer]

While that is intended as a bit of a joke, there is also some truth to it. .

Not a joke sir.
Those are my beliefs based on my experience.
If you buy into the propaganda, -sucker-.

-D

[wmgeorge]

My lathe has a 4 pole motor that I push to double speed without problems. The VFD is 400V and the motor is configured for 230V 50Hz and only after 87Hz does the torque drop. At that frequency the motor is being fed with 400V and there is no more voltage to raise to maintain the torque. The correct voltage for 100Hz would be 460V and the motor would have double the horsepower (the VFD is rated for double the power). I use higher speeds for softer materials so, no issues. I haven't changed speed on the belts for a very long time.

Sure I have also ran the same way, But the bearings in the motor are another story.

[mickeyf]

Thanks all for a much more comprehensive response than I was expecting - especially J. Tiers. So, if you are taking a "play it safe" approach - a) don't push the limits of max or min speed, b) consider additional cooling, and c) use inductive spike protection on the output lines. Toriods, perhaps?

[J Tiers]

Toroids are fine. But there are (naturally) details which you want to avoid trouble from.

It does not take much inductance to drastically cut the risk. Almost all the overvoltage is within the first 5 or 10 turns in the motor, and the worst is taken by the first turn or two.

You do not want a lot of turns in an added inductor, because you may run into "saturation" of the core, which would reduce the effectiveness. You are best off to use a commercial unit meant for the purpose, but it is certainly possible to DIY. Individual coils rated by inductance and current capacity are also available from sources like Digikey, and Mouser.

You can ask here about any specific DIY solution, several of us can give you a good answer.

[wmgeorge]

Modern well designed, not Chinese VFDs have no issues.