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 horsepower rating and type. The phase converter will artificially generate the third leg of a three-phase system from the two leads of a single-phase system.
There are three basic types – static phase converters, rotary phase converters, and solid-state phase converters. There is a forth option which can be used for motor applications – a VFD.
Static Phase ConverterThe static units 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 the more expensive units do.
Rotary Phase ConverterThe next least expensive units look like an electric motor with a large electrical box attached to the side, which houses the capacitors. The capacitors generate the third phase, and the motor, called an idler motor, cleans up and balances the power. 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, and the power is balanced better than with a static phase converter.
Solid-state Phase ConverterFor applications that require stricter control on the power, solid-state phase converters are the best. They have the advantage of providing very clean power with no harmonics or distortion. They 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 that 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.
Variable Frequency Drives as Phase ConverterAlthough variable frequency drives (VFDs) are designed primarily to control the speed of AC motors, they can be adapted to function as phase converters. They have the added advantage of allowing you to control the speed of the pump to match the load (see Figure 4).
Problems can arise when using VFDs as phase converters if:
1. They are used to power loads other than motors – we’re fine here.
2. There are multiple loads on the VFD – not a problem for pump applications.
3. The motor needs to provide braking action – not a problem for pump applications.
4. 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.
5. 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.
A precaution: Regardless of what type of phase converter you choose, static, rotary, solid-state or VFD, make sure the unit has enough capacity to start your motor. Remember that the starting current drawn by an electric induction motor can be six or more times full load amps (FLA). Some 10-HP phase converters only will power a 10-HP motor if it is started without a load, and it may take a 15-HP, or even 20-HP, unit to start your motor under load. Consult the pump, motor and/or phase converter manufacturer for information on starting recommendations.
If you have access to the Internet, there is a ton of technical and supplier information on phase converters. Also, check with your local pump distributor. He may well have a source of phase converters, and offers the advantage of being there for you if there is a problem.
Next month, we will continue our series on the electrical side of pumped water systems with a look at three-phase control panels. ’Til then…