Is a VFD Good or Bad for Old 3 Phase Motors?

There are several issues with this.

First off is the winding insulation. VFDs work by “tricking” a motor into reacting to a string of DC pulses as if it is AC power. In creating those DC pulses, the wiring between the drive and motor can act like capacitors and great higher voltage pulses that are superimposed on the circuits, then those bounce back and forth between the drive and motor in waves (called “reflected waves”) that build up to being spikes that are as much as 2.5 - 4x the normal voltage. In 480V drives then, it’s not uncommon to see these spikes reach over 1400V, sometimes as much as 2000V. Older motors used magnet wire in the windings that had 1000V insulation, later 1200V, both of which are inadequate when subjected to these stresses. The thing is that on older motors, you have no way of knowing what was used unless the mfr is still available to ask, and good luck with that even if. But on 230V motors, that 1200V insulation is fine and if the distance from drive to motor is short, even 1000V is OK.

The other aspect is with the bearings. The same effects that cause the reflected waves also cause a static voltage to build up between the stator and rotor and once that builds up to being higher than the dielectric resistance of the grease, it will jump across the bearings and races. In doing so, it causes little microscopic pits, called EDM (Electric Discharge Machining; like welders and plasma cutters) and over time these pits become frost-like and then eventually grooved flutes that destroy the bearings and races. Inverter duty motors take measures to prevent this, older motors didn’t because that issue didn’t exist on a widespread basis. There are retrofit “shaft grounding rings” that you can add to existing motors to deal with this risk

If you can keep the VFD within 25ft (linear) of the motor, the voltage and bearing issues are unlikely to affect you, so long as you resist the temptation to get rid of the “whine” that the motors make on a VFD by increasing what’s called the “carrier frequency”, which is the rate at which the DC pulses are fired. Increasing the CF will move the sound out of the range of human ears, but makes the motor problems worse, especially on older motors.

A third aspect is cooling of the motor. ODP and TEFC motors rely on shaft mounted fans to move air across them to keep them cool. The fans then are slowed down with the motor speed and the way these fans work means that the air flow drops faster than the speed change. So at slow speeds, the motor receives less cooling effect and can burn up without being overloaded. If you add a separately powered blower to move air across the motor when operating slower, you can solve the heating issue too.

Or, as many people do, you use your older motor until it dies, then replace it with a newer inverter duty motor. It depends on what the down-time will cost you.

Excellent point, Dhayanandhan. Also, might I add that the earthing (grounding) wire to the motor should go directly to the VFD's earthing terminal, because it does carry some electronic noise that is best minimized by sending it back to the source. This is something that is instructed by at least some of the VFD manufacturers (and therefore is a code requirement in the USA or other locations under the NEC).

--JMM

Rockwell in their Powerflex series has a proprietary control method and components that reduce the risk of stator end turn insulation failure, the most common problem with applying modern inverter designs to old motors.

Starting about 2003, I've applied those drives to many retrofit jobs in paper mills, large & small NEMA T frame and some U frame motors, and never had the typical end turn insulation failure, over a decade of service in plants up and down the East Coast.

Others probably have this same feature or protection by now, because the customer demands it, rather than try to retrofit an IEEE841 motor to an existing mechanical installation.

I have worked with VFD on old motors from circa 1980 and if you have 220v 3 Phase supply then the DC in the VFD will peak out at 312V which is the chopped into either a pulse position or pulse width waveform to create a 3 phase AC signal.

I have worked with VFD drives on 3 phase motors where the turn down value was 10% and the top frequency was 120%. Yes the 10% drive speed was an issue with frame heating due to lack of air flow because the motors were totally enclosed fan cooled with the down side being an errant hand making contact with the frame. The motors had no failures as the insulation class was sufficient for the heat generated.

We have VFDs on a couple of lathes and they are controlling Chinese motors of unknown parentage but they show no sign of problems and they are driven from 240V single phase 50HZ supply.

For windings with a 103Um insulation thickness, then by the IEC standard the breakdown voltage of the insulation will be in the order of 7000V so with two wires in contact without any damage will be able to withstand 14000v between.

The end turn failure is more likely to be from installation damage during the installation process. So I say give it a go using the old power tools and I suspect your physical duty cycle will be a lot less than the duty cycle of the motors.

Late in this thread,

As usual Jraef always has domain experience to learn from.

My personal experience in a home shop with 3 phase 220v motors from the 1930s to 1960s is your not pushing them to max HP, and you will have a short cord from the VFD to the machine. My oldest machine is a Rockford horizontal milling machine, with a LIMA overhead belt conversion motor and 4 speed transmission. I did open and clean the internals of the motor to inspect the windings.

I do use the variable speed on all my machines. I've even replace the single phase motors on the other spindle machines (drill press, band saw, and bench lathe) to use 3phase. The VFDs allow you to program in a conversion constant so the LED display in my use is showing spindle RPM, or FPM on the band saw. I set the belts to the mid range, and this has always given enough torque. As the motors are all 4 pole, I set the max Hz of the VFD to 120 giving a wide RPM spindle operating range.

On drives of 3HP and smaller, that are designed for single phase input, there is no derating needed per their manuals. For larger HPs, 50% derating is needed to protect the input rectifier diodes. There may still be issues if you use the motor at its max HP with the DC internal supply drooping with single phase power source, which will trip the safety of the drive from undervoltage. I've never experienced this issue, as I have a 7.5HP lathe with a 10HP VFD, and even with .5 inch depth of cuts (low feed rate), have never had an issue.

Also note that for very low RPM output with high torque, the VFD needs to be of the vector type. My first drives (Teco FM100) were not of this type, and low RPM is choppy. There is a term for that, but I can't recall its name.