VFD for Single-Phase Motor

Here is one possible scheme for powering a capacitor-start/capacitor-run single-phase motor from a VFD.

This scheme utilizes the well-known "Scott T".

The ideal transformation ratios are 0.5, 0.5 and 0.867, but others may be practical as well.

If trouble (i.e., saturation, or excessive current) in the start winding is observed, a buck/boost transformer in buck mode could be tried.

Obviously, the capacitors and the centrifugal switch, and the thermal overload protector (if fitted), must be bypassed.

The use of the transformers, as shown, also allows for driving single-phase motors which have one lead in common between the run and start windings, which is often the case. (If so, then T3 and T4 would likely be in common).

Clearly, this approach isn't going to be good from zero Hz to 120 Hz, because of transformer limitations.

You wrote:
"Clearly, this approach isn't going to be good from zero Hz to 120 Hz, because of transformer limitations."

In the interest of learning something, why is this and what could happen?

"In the interest of learning something, why is this and what could happen?"

Transformers, as with most magnetic components, are designed for a specific frequency (if resonant) or for a frequency range (if non-resonant).

As there are no capacitors in the above schematic, resonance is not being used (it is, however, in many rotary phase converters).

So, the lower limit is a function of core saturation, while the upper limit is a function of leakage reactances (primarily parasitic capacitances).

For variable speed over a wide range (10:1, at least), VFD and a three-phase motor is the best choice.

For variable speed over a very wide range (100:1, at least), a dc motor and a "field weakening" drive is the best choice.

For variable speed over a more restricted range, especially when using a motor for a machine which cannot economically be upgraded to three-phase, the existing single-phase motor, and a VFD connected to the motor using a "Scott T" may be a good choice.

Risks are primarily transformer overheating, and inefficient energy conversion.

The VFD can usually be programmed to be self-protective.

Here's the basic conversion scheme:





Note that 0.5-0-0.5 and 0-0.867 are the preferred ratios.

However, as the start winding of a single-phase motor is often driven at above normal line voltage, using a capacitor for phase shifting, and as phase shifting is not needed in this application, the applied voltage using the specified 0.867:1 transformation (a step-up, which is consistent with the general Scott-T conversion scheme) could lead to start winding saturation.

Hence, the suggestion to use a 1:1 transformer, in this case.