Converting single-phase power to three-phase VFD

Convert Single Phase VFD to Three Phase VFD
A common question we get asked about is phase conversion: Can a variable frequency drive (VFD) convert my single-phase power supply to run a three-phase motor?

Single-phase AC power is common in many residential and agricultural settings, but can also be seen in some industrial locations. It usually has only two phases (L1 and L2) and possibly a neutral. Single-phase power for 120, 240, and occasionally 480 VAC systems is common. A three-phase power supply has three phases (L1, L2, and L3). Three-phase power in the United States is typically 240 and 480 VAC systems. In some cases, systems up to 600 VAC are also used.

Many people have commutation issues when buying a new or used motor and find that a three-phase motor doesn't work well with a single-phase power source.

If all you need is an off-the-shelf VFD drive that can convert single phase to three phase, this is usually a great option for you. The ratings and dimensions of these VFDs are based on the motor's output three-phase current rating, making them the correct size and easy to install.

One disadvantage of VFDs set up this way is that they usually only run smaller motors.

Another question is whether the site was ever updated to three-phase power. Although the cost of switching the entire system to three-phase power makes it unlikely to happen very often, if it did, then these VFDs would not be able to operate on a three-phase system. These VFDs tend to be cheaper than most, but it's still a shame to throw them away if they're outdated.

Phase conversion using standard VFD
If your motor is too large for a VFD built for phase shifting, you can use a standard VFD for your single phase power supply. This is done by placing two hot wires for single phase on the AC input of the VFD and leaving one input terminal open and unused. This does raise some questions that you have to consider.

Because you are now concentrating the same amperage on two phases instead of three, failure of the VFD's input diodes is likely to occur. To solve this problem, you must enlarge the VFD to accommodate the higher ampacity. A conservative rule of thumb is to double the size of the VFD you need.

For example, if your motor full load amperage (FLA) is listed as 15, double that and resize the VFD as if you needed to power a 30 amp motor. If this is the case for you, we recommend calling one of our experts who can help you through this sizing process and find the right VFD for you.

This process of derating standard VFDs does have some drawbacks. You're buying a larger drive, which means more money and space than if you were using a three-phase input to power a three-phase motor. We always recommend trying to power the motor with three-phase power if possible, but sometimes this is not an option.

Other Phase Conversion Options
In some cases, a VFD is not the best choice for switching phases. One of the most common problems we see with VFD phase conversion is someone trying to convert single phase to three phase, not just the motor. While a VFD does a good job of switching phases for an AC motor, it doesn't do a good job of switching power for the peripherals you're trying to run, which typically includes relays, lights, control power transformers, and other electronic equipment.

If you also don't want to control the speed of the motor, you end up paying for a lot of features in the VFD that you won't use. VFDs are primarily used to control the speed of the motor, so if you want the motor to run at full speed all the time, you may be over-engineering your system.

In this case you should look into phase converters. There are several types, each with its positive and negative sides. Static phase converters are a very economical option, but typically do not run the motor at full rating. Rotary phase converters do a good job of converting power, but have moving parts and create a lot of noise. Digital phase converters tend to be the best at getting the full rating of the motor when switching phases, but it is a more expensive option.

Every electrical system has many factors to consider when you start to need phase shifting. The most important thing is that you identify what you need and what is important to you, and then build a system around it. If you need help with this, you can contact us.