3 Phase VFD Feeding a 2 Phase Capacitor Run Motor

Your post is very confusing.

Is the 1/4HP motor a 115VAC or 230VAC rated unit?

Is it 3 phase or single phase?

If it is a 115VAC rated motor;

Applying more than 20% higher voltage (138VAC) will cause severe motor damage and most likely instant failure of the motor windings and/or the capacitor.

Applying 10% higher voltage (127VAC) usually will not damage the motor windings.

Applying voltage at less than 85% of rating will cause excessive current, motor winding heating, possible locked rotor, and subsequent failure of the windings.

The same percentages of "over" and "under" voltage apply to a 230VAC rated motor.

You must use a 3 phase VFD on a 3 phase motor and a single phase VFD on a single phase motor.

I sounds like you are trying to over engineer this thing.

In a response to your question. I say "NO"

this is a single phase, capacitor run motor.

as such, it is a two phase motor. phase shift is provided by a capacitor.

the question is, can i run this from two phases of a 3 phase vfd (removing the capacitor of course!), and how much of a performance hit will the 120 degree phase shift be, when it is supposed to be a 90 degree phase shift.

I can wind a scott T transformer, but that requires two transformers instead of one.. i will need a transformer to provide 250 volts for one coil and 120 for the other.

If I wind a scott T transformer it will force proper 90 degree phase shift and a more stable voltage for the second winding... which is an improvement.

the two windings are different voltages, which is why i'll need a transformer to excite the second winding to a higher voltage than the first.

the vfd would be programed for 60 hz 115 volts as standard.. a small transformer would double the voltage for the second winding. but the phase shift would be 120 degrees instead of 90 which means there will still be a rotating field, but it won't provide as much torque as it should.

I still think this will increase efficiency, but not by nearly as much as providing a proper two phase (90 degree phase shift) would.

Again, why do you want to do this? You will likely end up destroying an expensive VFD or burning up the motor, they are NOT designed for this. I cannot recommend this.

The motor's only 1/4 HP, just buy one with the right voltage winding or buy a single phase step down transformer. It will probably be cheaper and easier (and will work) than buying a VFD.

What exactly is the application?

There are lots of ways to skin a cat, but some folks seem determined to do it the hard way.

If you look for it, there are off-the-shelf solutions to this problem that don't involve so much unnecessary complication and expense.

How about one of these : http://www.kbelectronics.com/Fan_Speed_Controls_Triac/Wall_Mounted_Fan_Controls.html? Works OK on PSC motors and is 115VAC.

You apparently:

1. Have a well engineered idea laid out (which may or may not work)

2. Have a definite need to keep the motor in question

So, why not try this and let us know what the outcome is. It sounds fascinating.