How to Use a Three Phase Motor with Single Phase Power

We receive inquiries all the time from customers who have three phase equipmentand need to operate the equipment from single phase power. The main question we get is “How do I size a three phase drive for single phase input supply?”

For those of you who are not already aware, we offer variable frequency drive products that are designed for single phase input, three phase output. The products that are available however max out at 3hp or 5hp.

If you have an application that exceeds the 3-5hp range you can de-rate a three phase variable frequency drive for use with single phase input power. The formula we use for de-rating variable frequency drives for use with single phase power is as follows: Westart withthe motor current and multiply it by 1.73, then we take the revised current and size a variable frequency drive with the corresponding amps.

For example. Lets say I have a 7.5hp motor that pullsapproximately 23 amps; I want to use a vfd, however I only have single phase 230V line voltage. I need to multiply the 23 amps by 1.73 which gives me the new revised current (39.73 amps) Now I know I need a three phase 230V drive that is rated at40 amps or higher.Avfd that is rated at approximately 40 amps is about 15hp,and for this particular application would run a 7.5hp 240V driveperfect on 240V single phase power.

Precision Electric, Inc highly recommends contacting us or someone with experience to safely install variable frequency drives and/or electric motors.