Running older induction motors on VFDs

[Guineafowl]

TL;DR

What cheap and practical steps could a hobby workshop take to limit the damage caused by VFDs to old motors not designed for them?

The ‘spiky’ output of variable frequency drives (VFDs) can damage the insulation of older motors. They’re often used to run three phase motors on single-phase supplies in hobby workshops. (Modern motors have upgraded insulation and are designated ‘inverter-duty’.) I wonder what cheap and practical steps one 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 domestic mains wiring regulations, but they are apparently not sensitive enough.

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

 

[Baluncore]

TL;DR: What cheap and practical steps could a hobby workshop take to limit the damage caused by VFDs to old motors not designed for them?

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

Not at the motor terminals, but at the inverter output. The voltage switching outputs need to see an inductive load. Wind three inductors on separate ferrite cores, one for each line, then couple them to a delta of pulse rated capacitors, giving three lines with limited dv/dt. A good VFD should have that circuit in its output to reduce radiated EMI.

You could also neutralise the motor power factor with a delta of capacitors at the motor terminals. That would be OK for fixed frequency operation, but difficult if you used a wide range of VFD frequencies.

 

[Guineafowl]

Not at the motor terminals, but at the inverter output. The voltage switching outputs need to see an inductive load. Wind three inductors on separate ferrite cores, one for each line, then couple them to a delta of pulse rated capacitors, giving three lines with limited dv/dt. A good VFD should have that circuit in its output to reduce radiated EMI.

You could also neutralise the motor power factor with a delta of capacitors at the motor terminals. That would be OK for fixed frequency operation, but difficult if you used a wide range of VFD frequencies.

So this would be a dv/dt, or perhaps LC filter, rather than a snubber?

What sort of values would work? Elsewhere it’s been suggested 5mH and 0.47uF. I’ll need a meter that measures inductance, unless I can just buy suitable inductors. I don’t mind spending a bit, since they’d be protecting essentially irreplaceable motors.

The carrier frequency of one of my VFDs can be adjusted from 1-15 kHz, default 3 kHz, if that’s relevant.

How would correcting the motor pf (which you helped me with before) deal with VFD voltage spikes?

 

[Baluncore]

So this would be a dv/dt, or perhaps LC filter, rather than a snubber?

Snubbers in this case would be RC networks that make the output look like a low resistance at high frequencies. That will attenuate the pulse on the line. The LC circuit does not waste energy in a resistor.

I don’t mind spending a bit, since they’d be protecting essentially irreplaceable motors.

Did you look at the lines with an oscilloscope? How do you know there is a problem, or is this just a fear?

How would correcting the motor pf (which you helped me with before) deal with VFD voltage spikes?

It would better isolate the motor inductance from the VFD. Correctly chosen capacitors across the motor will limit dv/dt, and reduce circulating current between the motor and VFD, that might otherwise saturate the VFD output chokes.

 

[Guineafowl]

Did you look at the lines with an oscilloscope? How do you know there is a problem, or is this just a fear?

I have a couple of old CROs, but not the right probes for connecting up. The VFD/old motor problem is described here: https://www.mtecorp.com/wp-content/uploads/SP-006-E_Web.pdf
Also, when buying new motors, they are often specified as ‘inverter-ready’ as mentioned above, implying that older ones are not.

 

[Baluncore]

If you can hear a buzz when a motor runs on a VFD, then that vibration may chafe the field coil insulation. The buzz is the sound of the higher harmonics that are not being attenuated by the VFD output LPF.
Any voltage spikes from the VFD will be due to there being insufficient LPF at the VFD output.