Using VFDs on old motors

[Andrew SkinnerParticipant @andrewskinner94774]

As mentioned in a recent thread, the ‘spiky’ output of VFDs can damage older motors whose insulation was never designed for them. I wonder what cheap and practical steps you can take to mitigate this, given that dedicated sine wave filters are quite expensive. I think most of us quietly ignore the problem and hope for the best.

Apparently, the motor windings themselves act as quite good low-pass filters for the current, and that it’s actually the voltage rise time that causes the problem. The high dv/dt pulses result in reflected waves sloshing back and forth due to impedance mismatch. These waves add together in the cable between VFD and motor, leading to high voltage transients that damage old motor insulation.

^^ I’m sure this is a poor summary, and also sure that you could write a whole monograph on the subject. But, down to brass tacks, what practical steps could we take in a hobby workshop with several old motors on VFDs?

1. In-line chokes, such as old fluorescent light ballasts, providing they could take the rated current?

2. Keep the VFD-motor cable short, and manipulate the VFD’s carrier frequency based on this length?

3. MOVs, or something, to clip the transients? I looked into the surge protection devices (SPDs) now becoming more readily available because they’re mentioned in the new UK wiring regs, but they are apparently not sensitive enough.

4. Some sort of snubber network at the motor terminals?

[Pete RimmerParticipant @peterimmer30576]

Here’s my solution. I just put a VFD on it and stop worrying. I have VFD’s on two pre-70’s gear hobbers, 1960 milling machine, Mk1 Clarkson grinder, Mg9 cylindrical grinder and several machines no longer owned. So far I’ve had exactly no motors fail on any of them.

My 1942 lathe which is on it’s original DC motor uses a parker electronic drive (like a VFD but for DC motors) and I have spend the last 12 years switching the power on and off with the drive run-enabled – something else I was assured would kill the drive in short order, and that’s still going strong too. I bought a spare drive just in case and it’s still in it’s box.

I’m not saying that it’s not a problem, but it sure as heck isn’t so much of one as the people who keep parroting about it would have you believe.

[renardiere7Participant @renardiere7]

I’m with Pete, same experience here. Probably jinxed myself now!

[Andrew TinsleyParticipant @andrewtinsley63637]

In a previous thread, the Steinmetz connection was referred to as a bodge. Looking at the original Steinmetz papers, I think that this is somewhat unfair! At least the Steinmetz solution produces sinusoidal voltages which are what motors were intended to run on.

The hugely popular VFD solution, utilises a high frequency switching solution which is far from the ideal sinusoidal voltage. An oscilloscope shot of the current “waveform” is just a square block of high frequency pulses. I would venture to suggest that this is far more of a bodge than that of the Steinmetz connection!

However I am reluctant to push this further as the topic has been well aired recently.

As to running older motors on VFDs, I am not aware of any methods of reducing the high frequency voltage spikes associated with VFD outputs. Running motors from the 1980s such as the Crompton Parkinson and Brook Gryphon, is probably relatively safe in an amateur workshop setting (relatively low usage). Motors from say the 1960’s and earlier are the ones that are problematical. I have some pre war kit fitted with integral motors that cannot be easily replaced. I would not dream of running them on a VFD. Have you seen the prices that rewinds command?

If you do your homework, you will see that some motors are advertised as suitable for VFD operation. Mainly TEFC motors. I am not sure if this is a marketing ploy, but definitely there are some motors that are not suitable for VFD use!

Sorry if this is not more useful,

Andrew.

PS If you have an older motor that you intend to run on a VFD, I would urge you to check the insulation resistance  with a Megger, preferably with a 1000V output, but 500 V is better than none. If the motor has been out of use for some considerable time or is new to you, then put it in a warm environment (airing cupboard if you have one, for a week or two). If the insulation resistance test is not looking too good, this treatment can dramatically improve the insulation resistance. Always run the motor with an RCD protected supply. If the leakage current goes up then the RCD will cut and save the motor. Without this protection the motor will probably burn out a set of coils.

PPS I note a couple of contributors say just fit a VFD and forget it. I certainly have some sympathy with that view, motors of say up to 60 years old are relatively immune and failures are unlikely. The older motors are a different kettle of fish. I have had a couple of failures with old BTH motors on a VFD, that is why I used the Steinmetz connection for many years.

 

[Andrew SkinnerParticipant @andrewskinner94774]

Pete – yes, that’s what I do at the moment. I’ve got a few machines on a rotary, mainly woodworking, a coolant pump on a Steinmetz and Tom Senior mill with a new, inverter-duty TEC motor on a VFD.

But, the Boxford AUD/VSL, Beaver VBRP and an old morticer have their original motors on VFDs. These motors are either old imperial or custom-fitted ones so would be a nuisance to replace, and uneconomical to rewind. If I could knock up a simple snubber circuit for them, that would be nice. I could even get a run of PCBs made by one of those online companies, and sell ‘peace of mind’ kits for a reasonable fee!

Andrew I do megger newly-acquired motors, mainly with a Fluke MFT but I have a collection of old meggers, including a wind-up Bakelite one. I quite like them, for some reason. Usually, I change the bearings and while in there, pull out the star point.

[duncan webster 1Participant @duncanwebster1]

When I converter a Ferret grinder using Steinmetz, the motor manufacturer strongly advised against using a VFD. I didn’t pursue it as I wasn’t intending to, but the motor man had no axe to grind, so is worth listening to. Of course loads of people will get away with it, but a few won’t.

The grinder worked a treat by the way, slight loss of power didn’t matter as the motor was well up to light domestic use.

[Pete RimmerParticipant @peterimmer30576]

I have a Steinmetz connection on my Christen drill sharpener, and had one on my Union Tool & Cutter grinder and both worked just fine. I did once have a Mk1 Clarkson grinder which was a bit slow to spin up but otherwise worked just fine. I just choose a capacitor that gives the best 3rd-phase voltage and doesn’t make the motor run too hot after half an hour of use.

I once put together a control box for a lady who was building a ring roller powered by a 3 phase geared motor using a Steinmetz setup. It used a timed relay to add in an extra capacitor for 1 second to give a startup boost. It was required because if you stopped the roller under load it would struggle to start off again. Worked great.

Steinmetz is used in industry too. I had to remove a rising traffic barrier from a building access road one time. It featured a 415v 3 phase motor (wired in star), a capacitor and a 2-way switch (for manual operation, plus control stuff for automatic). It was very simple, they put the capacitor across 2 of the phases, neutral to the remaining phase. The switch applied 240v single phase to one end of the capacitor for up and the other end for down.

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