Three Phase Motors

Most industrial motors are three phase. The main reason for this is that there is very little maintenance of a three phase. Industrial motors do not have the starting devices that single-phase motos have. The three phases of alternating current that supply power for the motor produce the phase shift needed to get the motor started and to keep it running once it is started. All commercial power generated in the United States is generated as three phase. It is converted to single-phase because the three phases can be divided and sent into three different subdivisions or locations. It is cheaper to distribute single phase AC than three-phase AC. Three-phase power requires at least three, and sometimes four, wires for proper distribution.

Uses of Three-Phase Motors

Three phase motors (3ø) are perfect for machine-tool and general uses where dust and dirt are prevalent. Polyphase motors have operating characteristics that enable them to operate any device that may be powered by equivalently rated single-phase motors. Three-phase motors are available in sizes of ¼, 1/3, ½, and ¾ horsepower. They may be used for pumps, compressors, fans, blowers, conveyors, farm machinery, saws and machine tools.

Motors that are totally enclosed are built to handle heavy thrust loads. The grease used in the ball bearings is resistant to oxidation and moisture so these motors should last 10 years under normal operating conditions so they are rated for continuous duty in temperatures up to 108°F. These motors are made by General Electric in1/4, 1/3, ½, ans ¾ horsepower sixes. They have a speed of 1725 rpm. They weigh from 15 pounds for the ¼-horsepower motor to 33-lb for the ¾-horsepower motor.

Explosion-proof, three-phase motors are used in hazardous locations where a spark could start a fire or cause an explosion. The explosion-proof motor is ideal for use in locations requiring motors that are UL listed for hazardous locations.

How Three-Phase Motors work

The stator has windings around it that are placed 120° apart. The rotor is a form-wound type or cage type. The squirrel cage rotor is standard for motors smaller than 1 horsepower, which we are concerned with here.

The rotor will rotate with the rotating field produced by the stator. The stator is nothing more than the primary of a three-phase transformer. The magnetic field produced by the stator revolves and cuts across the rotor conductor. This induces voltage and causes the rotor current to flow. So, motor torque is developed by the interaction of the rotor current and the magnetic revolving field. The motor stator and rotor are shown here to illustrate the details a little more clearly for the large industrial type of three-phase motor. The diagram below illustrates how the three phases are produced:

The purpose of the iron rotor is to reduce the air-gap and concentrate the magnetic flux through the rotor conductors. Induced current flows in one direction in half of the rotor conductors and in the opposite direction in the remainder. The shorting rings on the ends of the rotor complete the path for rotor current. A two-pole field is assumed to be rotating in a counterclockwise direction at synchronous speed.

The stator of a polyphase (3ø) induction motor consists of a laminated steel ring with slots on the inside circumference. The motor winding is similar to the AC generator stator winding and is generally of the two-layer distributed, preformed type. Stator phase windings are symmetrically placed on the stator and may be either wye or delta connected.

Torque

The revolving field produced by the stator windings cuts through the rotor conductors and induces a voltage in the conductors. Rotor currents flow because the rotor end rings provide continuous metallic circuits. The resulting torque tends to turn the rotor in the direction of the rotating field. This torque is proportional to the product of the rotor current, the field strength, and the rotor power factor.

By using the transformer comparison, it is possible to see that the primary is the stator and the secondary is the rotor. At start, the frequency of the rotor current is that of the primary stator winding. The reactance of the rotor is relatively large compared with its resistance, and the power factor is low and lagging by almost 90°. The rotor current therefore lags the rotor voltage by approximately 90°. Because almost half of the conductors under the south pole carry current inward, the net torque on the rotor as a result of the interaction between rotor and rotating field is small.

As the rotor comes up to speed in the same direction as the revolving field, the rate at which the revolving field cuts the rotor conductors is reduced and the rotor voltage and frequency of rotor currents are correspondingly reduced. Hence, at almost synchronous speed the voltage induced in the rotor is very small. The rotor reactance also approaches zero.

Applications of Three-Phase Motors

Three-phase motors are used in commercial and industrial applications for machine tools, industrial pumps and fans, air compressors, and air-conditioning equipment. They are recommended wherever polyphase power supply is available. They provide high starting and breakdown torque with smooth pull-up torque. They are efficient to operate and are designed for 208-230/460-V operation, with horsepower ratings from ¼ to the hundreds. They can be obtained for 50-Hz as well as 60-Hz operation.

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