”The Fundamentals of Pumped Water Systems” takes a look at the benefits these devices provide.

Figure 1 - Static-phase converter.
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 run hot. You know that a new motor would cost way more than what this guy is asking for his “slightly used and in good shape” 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 and are told that they will be glad to bring in three-phase power from the road for about $15,000, but you will have to wait six months because they are 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 poles of a single-phase system.

There are three basic types - static-phase converters, rotary-phase converters and solid-state phase converters. The 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 (see Figure 1).

Figure 2 - Rotary-phase converter.
The next lesser 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 (see Figure 2). 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.

For applications that require stricter control on the power, solid-state phase converters are the best (see Figure 3). 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 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.

Figure 3 - Solid-state phase converter.
A precaution: Regardless of which type of phase converter you choose - static, rotary or solid-state - 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 to 10 times the normal running current. Some 10 HP phase converters only will power a 10 HP motor if it is started without a load. It may take a 15-HP or even a 20-HP unit to start your motor if it is hard to start. 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 ….