Machinery's Handbook, 31st Edition
Electric Motor Applications 2651 Types of Single-Phase Alternating-Current Motors.— Most of the single-phase alternating-current motors are basically induction motors distinguished by different arrangements for starting. (A single-phase induction motor with only a squirrel-cage rotor has no starting torque.) In the capacitor-start single-phase motor, an auxiliary winding in the stator is connected in series with a capacitor and a centrifugal switch. During the starting and accelerating period the motor operates as a two-phase induction motor. At about two-thirds full-load speed, the auxiliary circuit is disconnected by the switch and the motor then runs as a single-phase induction motor. In the capacitor-start, capacitor-run motor, the auxiliary circuit is arranged to provide high effective capacity for high starting torque and to remain connected to the line but with reduced capacity during the running period. In the single-value capacitor or capacitor split-phase motor, a relatively small continuously-rated capacitor is permanently connected in one of the two stator windings and the motor both starts and runs like a two-phase motor. In the repulsion-start single-phase motor, a drum-wound rotor circuit is connected to a commutator with a pair of short-circuited brushes set so that the magnetic axis of the rotor winding is inclined to the magnetic axis of the stator winding. The current flowing in this rotor circuit reacts with the field to produce starting and accelerating torques. At about two-thirds full load speed the brushes are lifted, the commutator is short circuited and the motor runs as a single-phase squirrel-cage motor. The repulsion motor employs a repul sion winding on the rotor for both starting and running. The repulsion-induction motor has an outer winding on the rotor acting as a repulsion winding and an inner squirrel-cage winding. As the motor comes up to speed, the induced rotor current partially shifts from the repulsion winding to the squirrel-cage winding and the motor runs partly as an induc- tion motor. In the split-phase motor, an auxiliary winding in the stator is used for starting with either a resistance connected in series with the auxiliary winding ( resistance-start ) or a reactor in series with the main winding ( reactor-start ). The series-wound single-phase motor has a rotor winding in series with the stator wind ing as in the series-wound direct-current motor. Since this motor may also be operated on direct current, it is called a universal motor. Characteristics of Single-Phase Alternating-Current Motors.— Single-phase motors are used in sizes up to about 7 1 ⁄ 2 horsepower (5.6 kW) for heavy starting duty chiefly in home and commercial appliances for which polyphase power is not available. The capacitor- start motor is available in normal starting torque designs for such applications as cen trifugal pumps, fans, and blowers and in high-starting torque designs for reciprocating compressors, pumps, loaded conveyors, or belts. The capacitor-start, capacitor-run motor is exceptionally quiet in operation when loaded to at least 50 percent of capacity. It is available in low-torque designs for fans and centrifugal pumps and in high-torque designs for applications similar to those of the capacitor-start motor. The capacitor split-phase motor requires the least maintenance of all single-phase mo- tors, but has very low starting torque. Its high maximum torque makes it potentially useful in floor sanders or in grinders where momentary overloads due to excessive cutting pres - sure are experienced. It is also used for slow-speed direct connected fans. The repulsion-start, induction-run motor has higher starting torque than the capacitor motors, although for the same current, the capacitor motors have equivalent pull-up and maximum torque. Electrical and mechanical noise and the extra maintenance sometimes required are disadvantages. These motors are used for compressors, conveyors and stok- ers starting under full load. The repulsion-induction motor has relatively high starting torque and low starting current. It also has a smooth speed-torque curve with no break and a greater ability to withstand long accelerating periods than capacitor type motors. It is particularly suitable for severe starting and accelerating duty and for high inertia loads such as laundry extractors. Brush noise is, however, continuous.
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