# experience with VFD damaging old motors?



## caspaincmonster (Aug 12, 2014)

Hi folks, I have had an interesting discussion with an engineer at Dietz-motoren. Anyway due to the output by a VFD and the lack of special winding insulation running my motor on a VFD will causing the windings to fail. This is an old motor that was not designed for the switched type output of a VFD.  

"...dieser Motor darf nicht an einem Frequenzumrichter betrieben werden, weil sonst mit allergrößter Wahrscheinlichkeit die Wicklung kaputt geht!
Der Motor darf nur am Drehstromnetz betrieben werden ohne Frequenzumrichter (VFD)."

Even if you don't read German "kaputt" should be clear enough. This is a 1969 vintage 2 speed three phase 230V motor on my recently acquired Rumag milling machine. I am currently running my Schaublin 102 of similar vintage on a VFD and would rather not destroy that motor either.

Has anyone killed a 3-phase motor while it was powered by a VFD?

I guess this means I need to get a rotary phase converter or some sort of balanced true sinewave digital device.

Any advice on building or buying a RPC in Norway (or elsewhere in the 240V 50Hz land) Both my machines are only 1kw and only one at a time would be running so I have modest power requirements. I figure a 2kw motor for a RPC would be sufficent. 

Thanks


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## John Hasler (Aug 12, 2014)

caspaincmonster said:


> Hi folks, I have had an interesting discussion with an engineer at Dietz-motoren. Anyway due to the output by a VFD and the lack of special winding insulation running my motor on a VFD will causing the windings to fail. This is an old motor that was not designed for the switched type output of a VFD.
> 
> "...dieser Motor darf nicht an einem Frequenzumrichter betrieben werden, weil sonst mit allergrößter Wahrscheinlichkeit die Wicklung kaputt geht!
> Der Motor darf nur am Drehstromnetz betrieben werden ohne Frequenzumrichter (VFD)."
> ...



Heat is what kills old motors when run on a VFD.    Slowing a motor down also slows the cooling fan which is part of the motor.   Modern "inverter-rated" motors have class F or better insulation which can take the heat (there are other differences as well but that's the most important one).  If you intend to run the motor under load at reduced speed you should add external cooling.   If you were to use the VFD strictly as a phase converter you would probably be ok.


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## countryguy (Aug 12, 2014)

It is an honest to goodness valid threat to your environment.    I think I posted here sometime back w/ a webpage from someone who sells VFD rated motors and showed numerous photos of motors where the windings or motor itself failed when not of sufficient VFD rating.    Again, all depends on what your are doing, speed, Load & Current factors, plus heat dissipation.    If you are going to run it 20-30mins on .2HP and a few amps.... I dunno?  Probably nothing will happen.   It's a  defacto statement at this point that non-VFD rated motors present a much more significant potential (maybe probable) failure point over VFD rated motors.     And you really do NOT want to breath in Winding fumes anyway.      there are some great surplus motor vendors out there.  shout back if you want some places.  I bought a Metric frame 2HP 3.2K RPM VFD rated w/ good VT and CT ratings for I think like 300$  that was delivered to my house.  Shipping & all.


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## JohnG (Aug 12, 2014)

As John says below, heat is what kills motors.  The cooling fans on motors have to be designed so they don't blow apart at the motors highest speed, so they may not cool the motor adequately at lower speed.  In the woodworking industry, high speed router motors designed to run at 10,000 rpm or higher on high frequency AC will overheat if you try to run them off 60 cycle (in the US) line current.  The thermal overload kicks in, and the motor stops.  I've had to add forced air cooling to several spindle motors I wanted to run at 3600 rpm.  The fans just don't move enough air at lower speed.

I've also seen motors designed for variable speed use with coaxial fan motors built onto them, mostly on conveyor drives.  The fan motor always runs off line frequency and provides constant cooling no matter what the speed of the drive motor.


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## Alan Douglas (Aug 12, 2014)

I was just reading a discussion on this topic, but I can't find where (and even if I could, I probably couldn't post a link to the discussion anyway). The problem is insufficient insulation between phases or between coils to withstand the voltage spikes produced by a VFD.  Heating from low fan speed was not mentioned.


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## ironmonger (Aug 12, 2014)

I have three old motors running on VFD's. Their survival depends on why you have the VFD. I use mine to supply 60 cycle 3 phase to the motors, the variable frequency part is more incidental for my use.

 One is a 3 HP on a 2x72  belt grinder, a 1/2 HP Rockwell tool grinder and a 2 HP on my lathe. The VFD provides a nice soft start, and while it is running below line frequency for 10 seconds or so the nice soft start keep the starting current low. While I occasionally run the grinder around 30 cps, if I have to slow down for a long period of grinding, I use the step pulleys to accomplish that. The waveform of a VFD is an approximation of a sine wave, but my grinder has been in use since 2008 without a motor problem. Motors are pretty tolerant, but I wouldn't use them to supply any transformers that may be in your equipment.

I have used RPC's, and have no problem with them... but even if you build them yourself they can get very expensive if you need to buy many of the parts. I like to build electronics and electrical controls into cabinets or nema boxes. They can end up costing more than the parts if purchased new. Not a big deal as the VFD should be in an enclosure itself.


paul


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## John Hasler (Aug 12, 2014)

Alan Douglas said:


> I was just reading a discussion on this topic, but I can't find where (and even if I could, I probably couldn't post a link to the discussion anyway). The problem is insufficient insulation between phases or between coils to withstand the voltage spikes produced by a VFD.  Heating from low fan speed was not mentioned.



You shouldn't get significant spikes unless you have a very long cable between the VFD and the motor.


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## hvontres (Aug 13, 2014)

I run a VFD on my bridgeport as an Inverter (no variable speed). I was a little concerened about spikes damaging a $800 motor, so i added a three phase reactor on the output of the drive. This is basically three coils on a common frame that adds about 3% impedance to the system. i figured that the added inductance would help smooth out the PWM from the drive before going down to the motor. i am not sure if that was realy necessary, but it seemed like cheap insurance at the time.


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## ianmoss (Aug 15, 2014)

My 1964 14x60 lathe has a two speed motor that I have been running on a VFD for at least 8 years. It has never had a problem. I never drive it on the higher of the two speeds, and use the four speed gearbox to keep the motor speed up when turning slow. I expect it will keep running for many more years. The VDF has an option for a remote stop/start/speed which I have mounted on my lathe carriage. Works great.


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## Jim Nunn (Sep 1, 2014)

The main issue with older motors and VFD's is that the motors insulation my only be rated for 800V.  400/480VAC inverters typically generate up to 1400 V peek to peek.  this is the reason for the insulation breaking down.  On a 230 VAC network the drive only produces 650 to 700 V peek to peek.  I cannot speak for IEC motors built for use in Europe but in the US running on 230 VAC older motor insulation break down is a non issue.  One approach you could consider is purchasing a load reactor which is wired between the drive and motor.  the reactor tends to block the carrier frequency which also allows the motor to run cooler.  a comment on carrier frequency all small HP drives are shipped with a carrier frequency of 4 Khz large drives 200 Hp and above are shipped set for 2 Khz.  setting the carrier frequency to higher than 4  Khz the motor must be derated by a minimum of 20 to 40%

Heat is always an issue about the best turn down you can expect on non Inverter duty motors is 4:1 which is to say you should not run the motor below 1/4 speed without derateing the motor.  In the old days we would remove the fan,  I'm assuming that it is not a TEFC motor, and bolt a large muffin fan on the inside of the end bell on the motor.  just pick up  the largest computer fan you can find most run on 12VDC and use a wall power supply from you junk box and run the fan continuously.

Jim Nunn


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## lens42 (Sep 1, 2014)

I have done a ton of web digging on this topic. While lots of people will helpfully quote scientific sounding reasons why VFD-rated motors are desired (most of which can traced back to motor vendors) I challenge anyone to find even ONE first-hand case of a home shop motor failure on a VFD. I couldn't, and I think that is pretty telling because there are lot of people on a lot of forums doing this. I've put VFDs on four machines and have never had any problem. I think the truth is that for heavy duty industrial environments with high temp and humidity and 440V it's probably a good idea to have a VFD rated motor, but for the rest of use, use the motor you have. If it dies, THEN get a new one. Remember also that any contemporary VFD has protections so in the event of a motor problem, you most likely will not get smoke, just a fault code.


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## Buzsaw (Sep 1, 2014)

Korona Effect is what happens to a motor with a drive. I am a HVAC service tech and have seen this occur. As was mentioned it is the shorting out of the windings between phases not to ground. I have a number of drives in my shop all using single phase input, my mind set is not to worry about this issue with such intermittent use. If I were using the machines everyday for production I would change my opinion only if the motor was a specialty motor that could not be easily or economically be replaced.


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## British Steel (Sep 17, 2014)

Buzsaw said:
			
		

> If I were using the machines everyday for production I would change my opinion only if the motor was a specialty motor that could not be easily or economically be replaced.



Like a dynamically-balanced[1], constant-horsepower, 3-speed Dahlander wound motor (9-terminal), made in the 50's and the size of a dustbin, perhaps... Load reactors are compulsory, and an interlocked microswitch on the lever for the Frankenstein speed-switch to drop the VFD output when you move it!

[1] Can balance a penny on edge on the motor at full speed, or on the toolpost taking a heavy cut... Ah, Big Iron's wonderful!


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## stupoty (Sep 17, 2014)

Ive just had my lathe motor rewound, vintage 1940's US made(3hp). They have dipped the coils in a heat cured varnish.  When i looked inside it before it was rewound the coils all seemed to be tied up with string and fabric wraped, maybe the individual windings can resonate with the the pwm ac simulation from the vfd.

so far it's very happy and its on a very economy vfd too.

Stuart


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## jgedde (Sep 17, 2014)

In theory, a VFD _could _damage an older motor designed to run off a sinusoidal AC line source.  

A VFD generates sinusoidal currents in the motor by rapidly turning on and off an internally generated DC supply using a process called pulse width modulation (PWM).  Keeping it simple, the on and off times are varied sinusoidally at a high frequency making the output voltage look like a digital signal.  The motor's inductance effectively "filters" out the voltage into a more or less clean sinusoidal current and the motor does what it's supposed to do - generate torque.  The problem is, the high frequency voltage changes leak across any internal motor capacitance (mainly between windings).  So, the insulation on the windings not only acts as an insulator, but as a dielectric.  These leakage currents can, over time, degrade the older style insulation (basically varnish).

Modern quality motors usually use a double insulated type magnet wire using synthetic materials: both a varnish and a polymer.  As a result, these wires can take a much higher voltage before breakdown (and resultant arcing or corona) occurs.  The leakage still occurs, but causes no harm.

One could make a VFD that provides a truly sinusoidal voltage output in one of two ways.  The first and most efficient would be use to use a PWM output stage feeding a filter to eliminate the high frequency voltage modulation.  The second could be considered mostly like a three phase audio amplifier.  This technique would produce low distortion sinusoidal output indistinguishable from that received from the power company.  That said, this would be a purley analog solution and would waste as much as 60% of the power it takes from the line as heat.

That said, I would say if you're at all worried about damage to a vintage motor, a rotary phase converter is your best bet.  Either that or replace the motor with a modern one, and keep the vintage one preserved for posterity.

My qualifications for stating the above come from the fact that I design motor controllers for a living.  My company makes high reliability motors for space and high end military applications.

John


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## Alan Douglas (Sep 17, 2014)

Corona discharge occurs at the surface of a conductor, a function of applied voltage and radius of curvature (it happens first at points and sharp edges as the voltage is raised).  Corona is generally not a problem at under 1000 volts.  A corona discharge produces ozone which degrades organic insulation rapidly.

There should be no sharp edges on magnet wire, but insulation cracks, or voids or impurities in the impregnating varnish might have the same effect.  That's just speculation on my part, but it might account for the occasional breakdowns of older motors on VFDs, and for their rarity with 240V motors.


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## stupoty (Sep 17, 2014)

I do wonder how much of the old motor / vfd problem issues come from the motor being quite old and having some issue due to age or use. (Insulation breakdown etc.)

i have a small precission drill that has a universal motor that goes bang when attached to its motor mount , but runs fine when not on the mount.  It seems to have an issue that the screws have pressed against the windings and warn the varnish through vibration.

Always good to hear experienced voices on the problems as its quite interesting to hear experiences and be explained the complex bits.

stuart


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## Splat (Apr 9, 2015)

I can tell you I'm looking for a new motor for my Heavy 10 with Teco JNEV-101-H1  VFD. It was working fine, though I wasn't using the lathe much, and then last week the VFD errored and locked up when I tried to start up the motor. I didn't smell anything burning....and no magic smoke emitted from the motor or VFD but I knew something wasn't right. I called Teco and spoke to Brian in "drives" department. Good guy. After informing him the age of my motor he said he was surprised it lasted as long as it did with the VFD. The reasons given were what John and Alan wrote above.


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## RJSakowski (Apr 9, 2015)

A concern that I have about using older three phase motors is that the inductance of the windings was optimized for 50 -60 hz operation.  When you vary from that ideal very much, the efficiency will drop.  Transformer cores are different for 50-60 hz than for 400hz and considerably different for audio uses where  a desirable response would be flat  from 20 hz to 20 khz.  I would expect the same for motor iron.


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## John Hasler (Apr 9, 2015)

RJSakowski said:


> A concern that I have about using older three phase motors is that the inductance of the windings was optimized for 50 -60 hz operation.  When you vary from that ideal very much, the efficiency will drop.  Transformer cores are different for 50-60 hz than for 400hz and considerably different for audio uses where  a desirable response would be flat  from 20 hz to 20 khz.  I would expect the same for motor iron.


Inverter-rated motors have the same inductance as older ordinary ones.  The main difference is that they have higher temperature insulation.  The VFD reduces the applied voltage as the frequency decreases to take the decrease in impedance into account.


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## John Hasler (Apr 9, 2015)

stupoty said:


> I do wonder how much of the old motor / vfd problem issues come from the motor being quite old and having some issue due to age or use. (Insulation breakdown etc.)
> 
> i have a small precission drill that has a universal motor that goes bang when attached to its motor mount , but runs fine when not on the mount.  It seems to have an issue that the screws have pressed against the windings and warn the varnish through vibration.
> 
> ...


Please either repair that motor or replace it.  It's both a shock and a fire hazard as it is.


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## RJSakowski (Apr 9, 2015)

John Hasler said:


> Inverter-rated motors have the same inductance as older ordinary ones.  The main difference is that they have higher temperature insulation.  The VFD reduces the applied voltage as the frequency decreases to take the decrease in impedance into account.


Thanks John.

I have a number of three phase motors from 1 to 5 hp and wondered about the feasibility of using them.  What is a reasonable range of rpm for a VFD driven motor?


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## John Hasler (Apr 9, 2015)

RJSakowski said:


> Thanks John.
> 
> I have a number of three phase motors from 1 to 5 hp and wondered about the feasibility of using them.  What is a reasonable range of rpm for a VFD driven motor?


Add fans to provide cooling at lower speeds where the integral fan doesn't do much and you should be fine to just use them.


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## stupoty (Apr 9, 2015)

John Hasler said:


> Please either repair that motor or replace it.  It's both a shock and a fire hazard as it is.


No no its not in use  that was the symptoms, when i had it apart I  cleaned it a bit and thougt that the brushes had arc'd due to poor storage then I re assembled and realised what the issue was.

I'm nearly but not quite that silly, well mostly.

Stuart


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