208 volt rewire to 220

[Bi11Hudson]

As an "old school" electrician, I would say there is little to no difference for 208, 210, 220, 230, and 240 volt motors. Running at full load where one HP=~750 watts, an older motor (208 V) may run warmer than normal. My standard is to lay an open palm on the motor for 45 seconds.(count to 60) If it's too hot for that, it's running hot and needs to be investigated. The problem there is that each person has a different perspective of "hot".

VFDs generate a synthetic sine wave, older motors may "rebel" at being run at below or above base speed. This is primarily a magnetic charactistic and running a 60 cycle motor at 50 cycles will cause heating. This is a common issue running American made motors in Europe and is largely ignored.

There are more modern motors that are specifically designed to run at variable frequencies. But for a hobbyist running a machine tool, full load is seldom, if ever, reached. When you get below 50 cycles, it may run warm. But as long as you can't cook an egg on it, it shouldn't hurt.

The one big issue is flammables, such as oily rags. Housekeeping is an important issue in a shop anyway, but attention must be paid to motors. Any motor, even the drive motor on a table saw should be protected from sawdust.

.

 

[graham-xrf]

A modern VFD, despite the name, does not "vary the frequency" in any direct way. It rectifies and filters the incoming AC, to make a "DC bus". Then it creates an artificial AC, switching the supply to the windings on and off at a high audio frequency, usually somewhere between 6kHz and 20kHz. This is normally configurable, and the energy is controlled by altering the proportion of ON time compared to OFF time. The method is known as "Pulse Width Modulation". The motor is run with variable phase slip much larger than values traditional for 50/60Hz AC motors.

The input range for voltage is normally very wide, and a VFD can "turn down" the power to whatever you like to set. You should find your motor works just fine if the VFD is given the 220V input. If you figure how to set the parameters, you might have a good time configuring the settings to suit your motor, and having stuff like torque overload limits, and deceleration braking times.

If you hear annoying sounds and squeals, it can often be cured by setting the carrier frequency to something like 12kHz or more. I have used as high as 16kHz, even on a screened 3PH motor cable 30m long. I would not use any setting below 8KHz.

Use the right cable. VFD's have very specific connection instructions to screened cables. Ordinary SY flexible control cable has the very tough factory rated outer, with steel armour which also serves as a screen. Do not put up with high current switching harmonics messing up other electronic kit nearby - such as DROs, and computers. Not expensive, and easy to do. Please forgive if this is stuff you may already know.