[WesM]

As the title states, my CNC knee mill has a 1987, 3HP, 3ph, Taiwan sourced spindle motor. This motor has E class insulation. Will my motor self destruct since its not F rated for inverter duty if I run it on a VFD?

[sploo]

I appreciate this post's a couple of months old; but if it's of any use - I'm looking into this myself at the moment.

From what I've been able to understand, the issue is that a VFD results in high voltage spikes; which can break down the motor insulation. As such, using a non-VFD rated motor with a VFD may reduce its lifespan. However, from what I've been able to discover, dual voltage motors (380/400/415V in star and 220-240V in delta) are highly unlikely to have problems if run at the lower voltage (i.e. 230V).

Another issue is long cable runs between the VFD and motor. But "long" in that context seems to be 25 feet (~8m) or more; which is highly unlikely in a home workshop, where the VFD will probably be just a few feet from the motor.

I'm not an electrician, but the gist seems to be that it'll probably by OK.

[joeavaerage]

Hi,
the problem is that the very rapid switching waveforms of a VFD induces very VERY high voltage peaks inside the motor, and I mean kilovolt type spikes.
The windings of inverter ready motor tend to be vacuum impregnated with epoxy/polyurethane and what have you whereas induction motors are just dipped in varnish.

Cheaply made induction motors seem to fail quickly, whereas some of the old school English and US made induction motors, back when US and English meant quality, will often
perform really well on an inverter. I've been running an English made motor on a VFD for years as a grinder.

Cheap Chinese and Indian motors...don't bother. They are likely to blow up within a month or so with just regular power let alone on a VFD.

One thing to bear in mind is that even if the motor survives electrically if you run it at high speed, much higher than its rated speed you'll blow the bearings.....especially if you are silly enough
to direct connect the motor shaft to a tool. The deep groove ball bearings will not tolerate that sort of abuse, no matter where its made. If you want a motor to last then do not over tax the bearings.
The tool or spindle needs its own bearings....don't rely on the motor bearings.

Craig

[joeavaerage]

Hi,
A two-pole-pair-per phase motor running on 50Hz will have a synchronous speed of 1500rpm, and a normal running speed of about 1425rpm.

The same motor running on 60Hz will have a synchronous speed of 1800rpm and a normal running speed of about 1720rpm

Were this motor wound as a single-pole-pair-per phase those synchronous speeds would be 3000rpm for 50Hz input, and 3600rpm for 60Hz input.

The manufacturers do not shag around and design and build a new rotor for each speed combination. Thus most, if not all induction motors are designed to run at 3600rpm,
the max that market would expect. They will use the same rotor in a 1435rpm motor....that's fine, but no need to have another specialist part.

As a consequence just any induction motor you could run at 3600rpm. In fact you could probably push that to 4000rpm or even 4500rpm (with a VFD) if you were feeling lucky.
I do not think I would try this with a three-pole-pair-per phase or a four pole-pair-per phase motor as you might find in big industrial motors, but most of the motors we hobbyists would use
are not in that category.

Craig