AC Motors and Phase Converters

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What can you do if you need larger motors but have only the usual single-phase, 120/240-volt service? A phase converter permits three-phase motors to be operated on single-phase lines. The phase converter changes the single-phase power into a sort of modified three-phase power that will operate ordinary three-phase motors and at the same time greatly reduce the number of amperes required while starting. In other words, when operating a three-phase motor with the help of a phase converter, the same single-phase line and transformer that would barely start a 5-hp single-phase motor will start a 7 1/2-hp or possibly even 10-hp three-phase motor, and a line and transformer that would handle a 10-hp single-phase motor (if such a motor could be found) would probably handle a 15-hp or 20-hp three- phase motor. The National Electrical Code (NEC) requirements for phase converters are in Article 455. Phase converters are expensive but their cost is partially offset by the lower cost of three-phase motors and the increased labor efficiency gained when larger machinery can be used.

Two types of converters

The static type of phase converter has no moving parts except relays. It must be matched in size and type with the one particular motor to be used with it; generally, there must be one converter for each motor. The rotating type of converter looks like a motor, but can’t be used as a motor. Two 240-volt, single-phase wires run into the converter; three 3-phase wires run out of it. Usually several motors can be used at the same time. The total horsepower of all the motors in operation at the same time can be at least double the horsepower rating of the converter. Thus, if you buy a converter rated at 15 hp, you can use any number of three-phase motors totaling not over 30 to 40 hp, but the largest may not be more than 15 hp -- the rating of the converter. The converter must be started first, then the motors, starting with the largest and then the smaller ones.

Required horsepower ratings

Some words of caution are in order. A three- phase motor of any given horsepower rating won’t start as heavy a load when operated from a phase converter as it will when operated from a true three-phase line. For that reason, it’s often necessary to use a motor one size larger than is necessary for the running load. This does not significantly increase the power required to run the motor once it’s started. The converter must have a horsepower rating at least as large as that of the largest motor.

The voltage delivered by the converter varies with the load on it. If no motor is connected to the converter, the three-phase voltage supplied by it’s much higher than the input voltage of 240 volts. Don’t run the converter for significant periods without operating motors at the same time or it will be damaged by its own high voltage. Don’t operate only a small motor from a converter rated at a much higher horsepower because the high voltage will damage the motor or reduce its life. It’s good practice to make sure the total horsepower of all the motors operating at one time is at least half the horsepower rating of the converter.

Check with your power supplier before buying a converter; some don’t favor or permit converters. If converters are permitted, the line and the transformer serving your farm must be big enough to handle all the motors you propose to use.

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