Earth Leakage on a 3 phase Motor

[Andrew TinsleyParticipant @andrewtinsley63637]

I have been checking out my stock of 3 phase motors using an Altivar VFD.

All ran fine, except one dinky little AEI 3 phase motor. It must be at least 50 years old and has never been out of its original box.

The Altivar would barely get it moving before switching off with an earth leakage message. Tried it a couple of times and the second time it tripped the supply RCD for the VFD.

OK a leaky winding to earth, not too surprising. The motor is wired in delta, Before binning it, I tried meggering the windings to earth. At 500 volts DC, the earth resistance was 100 megohms, at 1000 volts DC it was 50 megohms.

Now those figures suggest that the motor is actually good! All I can think of is that the spikey VFD output is causing the insulation breakdown. Maybe on a true 3 phase supply it would be OK?

Any Thoughts?

Andrew.

[Andrew TinsleyParticipant @andrewtinsley63637]

[noel shelleyParticipant @noelshelley55608]

With resistance readings like that I would keep the motor and try to run it on true 3 phase. It might be worth putting the VFD on a scope when under load and see what you get. It could be that the motor was too small to suit the VFD.. At this time of year dampness can be the cause of much trouble. Noel.

[Simon Williams 3Participant @simonwilliams3]

This may be a load of bunkum, but for what it's worth –

I don't think this is earth leakage as in insulation resistance at all. I think this is about circulating currents, though I don't yet see the mechanism for an earth leak trip to pop out.

In the days before the idea of VFD's was ever thought of, motor designers didn't need to take into account the effects of high frequency components of the applied voltage. So some early motors (as I have been told), when fed from a VFD suffer from the induction effects of high frequency current internally causing all sorts of strange effects, usually associated with heating effects, bearing wear (likely because of currents induced in the bearings) and general inefficiencies of all kinds. Not all three phase motors are compatible with use on a VFD.

Motors of a more modern construction are better able to cope with the consequences of having edgy waveforms superimposed on the supply voltage and behave as one would expect.

Taking this to more modern again, the cost of fancy semiconductors suitable for mains voltages has reduced significantly, and VFD designs use this to reduce the obnoxiousness of the AC waveform applied to the motor, getting better approximations to a sine wave.

I suspect your audit of motors has thrown up a cross section of the historic timeline of the steady improvements in motor design.

[Phil WhitleyParticipant @philwhitley94135]

Wot Simon said, plus the RCD tripping could have been caused by spikes pushed back at the VFD by the motor, and filtered out by the VFD capacitive filter, which dumps any unwanted charges to system earth. Older motors sometimes do not like VFD created 3 phase!

[Andrew TinsleyParticipant @andrewtinsley63637]

Thank you guys,

As I suspected, the motor is probably fine, so is the VFD. Seems as though 50 year plus motors just do not like VFDs.

I shall keep the motor because it is a museum piece and I like the silver AEI logo on the black background. It really looks smart!

Andrew.

[old martParticipant @oldmart]

When I bought the Alivar VFD for the motor on the Tom Senior mill, I was concerned about earth leakage. The Altivar has EMC filters and the manual mentioned earth leakage if the filter was enabled. As the machine was in a museum, I was not sure if the ELCB's might be 10mA rather than the domestic 30mA. As it happened, there was no sign of a problem. The Altivar can have the filter switched off to eliminate the earth leakage at the cost of loosing the EMC. You could try switching the filter off to see if that made any difference.

[SillyOldDufferModerator @sillyoldduffer]

My understanding is pretty much as Simon describes. Worth remembering that although old motors are solidly made, they're products of their time in terms of design and materials, plus any problems due to old-age. Damp murders electrics.

Insulation is probably the main problem; early enamel was only rated up to 80°C though it slowly improved. After about 1970, 120°C became the norm, and today 180°C is common. I've seen motors rated up to 240°C. The improvement is partly chemistry, and partly application. Back in the day they didn't know how to stop micro-holes appearing, but 100 years of R&D did wonders.

Motor design changed subtly during the 1970's to conduct heat away better, reduce circulating currents and improve insulation. A modern motor will be about 20% more efficient than anything made pre-war. Bearings were altered to stop stray current passing through the balls or rollers after it was realised even tiny sparks cause severe wear. Took a long time to identify why bearings fitted to electric motors clapped out faster than normal because smooth 3-phase is relatively gentle and the damage takes years to develop. However, the extra volts generated by sharp VFD waveforms give old-style bearings a severe thrashing, though I guess obvious damage would still take years to appear in a lightly used hobby motor. The same sharp waveforms are also more likely to blowout through any micropores in the enamel insulation. Old motors and new VFD's aren't made for each other.

If it really mattered I wouldn't choose an old motor for anything, but for hobby use why not, as long as it's safe? Andrew's motor will probably last longer than I will, even if his VFD causes sparks to fly inside!

Dave

[Robert Atkinson 2Participant @robertatkinson2]

Posted by noel shelley on 31/12/2020 13:45:43:

With resistance readings like that I would keep the motor and try to run it on true 3 phase. It might be worth putting the VFD on a scope when under load and see what you get. It could be that the motor was too small to suit the VFD.. At this time of year dampness can be the cause of much trouble. Noel.

DO NOT connect an oscilloscope to a VFD!!!

Unless you know EXACTLY what you are doing you are likely to damage the 'scope, start a fire or electrocute yourself.
Monitoring waveforms on VFDs (and mains generally) requires specalist probes and / or 'scopes.

Simon is probably correct and there may also e issues with capacitance to ground with the old windings. It is unlikely to be a simple insulation issue, given your measurements.

Robert G8RPI.

Edited By Robert Atkinson 2 on 31/12/2020 15:44:35

[Andrew TinsleyParticipant @andrewtinsley63637]

I have just tried running my antique AEI 1/4 horse motor on an open V, 240 Volt 3 phase supply. Surprise, surprise, it runs without a murmer of a complaint.

Just for the record, I supplied the "phantom" phase via a 10 microfarad run capacitor. I didn't bother with a higher value start capacitance as the motor was running under no load conditions.

Out of interest, I did check the VFD output via my storage scope and could see no difference between the antique motor and a modern ones that run correctly. Storage scope necessary because the VFD shuts down PDQ on the antique motor! I do have the correct gear to do this check, please don't try to do it with a normal scope.

Andrew.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Andrew Tinsley on 03/01/2021 11:25:40:

Out of interest, I did check the VFD output via my storage scope and could see no difference between the antique motor and a modern ones that run correctly. Storage scope necessary because the VFD shuts down PDQ on the antique motor! I do have the correct gear to do this check, please don't try to do it with a normal scope.

Andrew.

I wonder what the VFD is detecting that tells it to do an emergency stop? Earth leakage seems likely, but I suppose the electronics could also detect phase to phase leaks, and over and under voltages, perhaps due to resonance effects. Any hint at all on the storage scope? Hours of fun if you have the wherewithal to look at VFD outputs without blowing the scope up. Better than most television!

Dave

[Andrew TinsleyParticipant @andrewtinsley63637]

I suspect that it may be phase to phase leakage from the first few centimetres of the windings. John Cox mentions this somewhere in one of his books on electric motors in the workshop practice series. At least I think he does, but can't find it at the moment.

Andrew.

[MK_ChrisParticipant @mk_chris]

Equivalent problem. I had acquired a Pultra 1590 lathe and a Meddings Pacera drill. Both circa 1950. Both had damaged / missing parts. Both being sent for scrap. Motors are the original 3 Phase units and had not run for at least 40 years. The Pultra has the Mardrive unit and the Meddings a Brook motor. Taking advice I had both motors tested then bought a Drives Direct unit with a Sine Wave Filter (transformer). Avoiding winding insulation failure was given as the reason for the filter. Wanting to stay as original as possible my wallet suffered larger damage than I had hoped. Both motors run well after nearly 6 years: but a heart attack and advancing years has left me with limited progress.

Chris.

[SteviegtrParticipant @steviegtr]

Just to be sure. I assume the motor being very old is not one that has 400/415v windings. Most of them were. It is only in recent decades that motors can be bought with 240v windings. Which, when i was in my early's. Most of the start circuits were star delta start. Star to get it moving & then into delta 415 to run.

Or am i telling you how to suck eggs here. If so except my apologies.

Steve.

[Andrew TinsleyParticipant @andrewtinsley63637]

Hello Steve. and Chris,

The little AEI motor is in fact wound for dual voltage operation all six wires come to the terminal board and there is the modern link system to convert from 240 to 415 volts. I have had it for at least 45 years, but I think it is probably quite a bit older than that. So the layout was probably the precursor to modern motors.

Hello Chris,

Now that is interesting. I had no idea that you could use a transformer between VFD and motor. I have visions of all sorts of nasties happening to the transformer!!!! But obviously not. I am not a great fan of static 3 phase converters. I just rigged up a V type "3 phase" to see if the motor would run (and it did). I suppose that 3 phase transformers are not uncommon (I had some powering continental kit on 3 phase (our voltage was a tad too high for the kit!). However, I have never seen a small 3 phase transformer. I presume you could use 3 single phase ones instead?

I was going to use the AEI motor on a period Delapena honing machine which has some fascinating history. The original 3 phase motor was burnt out, hence the interest in getting the little AEI motor going. I have at least three 240 volt isolation transformers knocking about, so I will give it a try unless someone knows better.

Thanks all,

Andrew.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Andrew Tinsley on 03/01/2021 17:03:51:

…

I had no idea that you could use a transformer between VFD and motor. ….

However, I have never seen a small 3 phase transformer. I presume you could use 3 single phase ones instead?

… I have at least three 240 volt isolation transformers knocking about, so I will give it a try unless someone

…

Although small three-phase transformers exist, I've never seen a 50Hz, though these guys sell them. 400Hz transformers were common government surplus back in the 1970s and maybe still are – ex RN mostly, but I think the RAF used them too?

You can use three single-phase transformers to create the equivalent of a 3-phase transformer no problem. I believe, but could well be wrong, that you can't create true* 3-phase from one phase input and three separate transformers because they don't share their magnetic flux across a common core. Anybody know for sure?

Dave

* Though a similar phase shift machine could be created by inductors in the same way as static converters use capacitors.

[Anonymous]

Posted by Andrew Tinsley on 03/01/2021 17:03:51:

I had no idea that you could use a transformer between VFD and motor.

I suspect that's a misunderstanding on the part of the original poster. While it would be possible to use a transformer on the output of a VFD to get a 415V supply from 240V input, for instance, there are much simpler, and cheaper, ways of doing that. Plus the transformer would have to deal with high frequency noise and fast edges which wouldn't be good for core losses.

A sine wave filter (as mentioned above) fitted between VFD and motor is simply three single phase LC filters that remove the high frequency components, converting the PWM to a sine wave. The three inductors are wound on a three leg core that superficially looks like a 3-phase transformer, which may be where the confusion arises.

Andrew

[Andrew TinsleyParticipant @andrewtinsley63637]

Thank you Andrew.

When a sine wave filter was mentioned (and also referred to as a transformer), I certainly got confused! I can well understand an LC filter on each leg of the supply looking like a 3 phase transformer. Very easy to get the wrong end of the stick, which I most certainly did!

Andrew.

[MK_ChrisParticipant @mk_chris]

Hello again Andrew Tinsley.

Initial test set up. The black box is 180H x 120W x 65D (Maplin). Has tapered sides so the unit is sat on a Vero board with two tapered bits of wood under. The cables pass through holes in the front cover.
Slide out of the box if required.

The inverter shows the underside with the output cable not connected (T1 T2 T3 on the right).
Now mounted vertically to show screen and the set up buttons to the front.
The Filter notes just say : Sine Wave Filter. Input on U1 V1 W1 Output on U2 V2 W2.
The invoice includes Sine wave Filter 3 Amp. All bought in 2015 and "just works".

Not shown is the connected Control Pendant and the 3 phase socket. Accepts either the lathe or the drill.
I admit that all else is a bit above my pay grade. Hope this is of some assistance.

Chris.

[Andrew TinsleyParticipant @andrewtinsley63637]

Thanks Chris, now I understand!

Andrew.

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