Consider this scenario: You go to a flea market, and spot an electric motor that looks similar to the one on the wash-down pump in your barn that is starting to make noise. You know that a new motor would cost way more than what this guy is asking for his so-called slightly-used-but-not-abused motor, so you buy it. You get it home, and in the process of installing it, you find out that it is a three-phase motor, and you only have single-phase power on the farm. You call the utility company, and are told that it will be glad to bring in three-phase power from the road to your barn for about $15,000, but you will have to wait six months because it is a little backed up. What to do?
This is the perfect place to use a phase converter, a device that converts single-phase power to three-phase power. Phase converters come in various shapes and sizes, costing from a few hundred dollars to many thousands, depending on the HP rating and type. The phase converter will artificially generate the third leg of a three-phase system from the two legs of a single-phase system.
There are three basic types – static phase converters, rotary phase converters and solid-state phase converters. Plus, there is a forth option that can be used for motor applications – a variable-frequency drive (VFD).
Static Phase Converter
The static phase converters consist of a several capacitors, and often a transformer, sized to the amp draw of your pump motor, to generate the third leg of three-phase power. They have been around for a long time, and are the least expensive of the three types. Their advantage is price, and their disadvantage is that they do not provide as closely balanced power as do the more expensive units.
Rotary Phase Converter
The next least expensive units look like an electric motor with a large electrical box attached to the side that houses the capacitors. The capacitors generate the third-phase, and the motor, called an idler motor, cleans up and balances the power (see Figure 2, above). With a rotary phase converter, you can run more than one pump at a time, as long as you do not exceed the maximum capacity of the unit. The power is balanced better than with a static phase converter.
Solid-state Phase Converter
For applications that require stricter control on the power, solid-state phase converters are the best (see Figure 3, above). They have the advantage of providing very clean power with no harmonics or distortion. Solid-state phase converters are very efficient, and operate in conjunction with the load motor at nearly unity power factor. They can power multiple loads, and can be located remotely from the load. Some pump installers use them in situations where there already is three-phase power available, but it is so far out of balance, it will not safely run a pump. In this case, a solid-state phase converter is hooked to two legs of the three-phase power, and the pump gets perfect three-phase power from the converter.
VFDs as Phase Converters
Although variable-frequency drives (VFDs) are designed primarily to control the speed of AC motors, they can be adapted to function as phase converters (see Figure 4, right). They have the added advantage of allowing you to control the speed of the pump to match the load.
Problems can arise when using VFDs as phase converters if:
- They are used to power loads other than motors – not an issue here.
- There are multiple loads on the VFD – one pump per VFD.
- The motor needs to provide braking action – not a problem for pump applications.
- The distance between the motor and the VFD is appreciable – can be an issue with submersible applications, but with proper filtering, usually not a deal killer.
- The current drawn by the VFD is large compared to the rating of the utility step-down transformer – usually not an issue.
When VFDs are used as phase converters, the drive must be de-rated. Typically, it is necessary to double the size of the drive to the load.