Single Phase to Three Phase Power

Using a Variable Frequency Controller for this application

Most people will tell you that you can take a three phase VFC and feed it with single-phase power and operate a three-phase motor. Well, here are some facts to consider.

Inputting single phase on a VFC that is designed to have a three-phase power source input is at best risky. Some will tell you that you need only to double the horsepower and it will work. Will it? Lets see.

Lets take a normal 5 horsepower three-phase motor at 230 VAC.
Normal current for a typical squirrel cage induction motor is 15.2 amps at 230 volts.

We know that if we multiply the 15.2 by the square root of 3 (1.732) we should be able to come up with the amperes that will be required on the input of the VFC.

15.2 x 1.732 = 26.3264 amps
This is calculation is only valid considering a good 3-phase system where the power factor is above .92

The rating of a TECO N3 VFC at 230 volts, 10 HP is 35 amps but this is the output amps. This looks to be well over what one would need to drive the motor. The input amps of the VFC are not published in the price book so is the input the same as the output? Not usually.

Due to the power factor on the input of the drive being at .96 or greater, which is created in the VFC by the Capacitor Bank, most manufacturers input is rated lower than the output of the VFC. Also, excessive bus ripple is also created on the DC power supply in conjunction with the capacitor bank causing even more deration needed on the input.

The other thing to consider is that there will be considerably more harmonic current on the input of the drive due to the single-phase source. This is additive to the RMS current of the input demand.

If a VFC comes equipped with a DC link reactor, this can help with the ripple on the bus and one could possible assume that you could use the VFC above to do the job. Does this unit come with a DC Link Reactor? The answer is NO.

A purpose built single-phase unit usually also comes with added bus capacitors. This adds storage capacitance for the output to make up for the lost phase on the input.

This VFC will probably run the load for some time before a failure occurs but be certain the in time the front-end of this unit will suffer from the stress of the demand and the excessive ripple and you will be left with a VFC that has blown input rectifiers and possibly more than that.

Most manufacturers offer a VFC purpose built for single-phase input applications. Most are only three horsepower and below. If you need a unit to do more than this, you better do your homework and seek out a unit that is rated for the horsepower that you need and is purpose built for single-phase input.


Scott Gordon

P.S. We have the answer for the above application!