Chapter 9: single- and two-phase motors  (Page 3/9)

9.2.2 Capacitor-Type Motors

• Capacitors can be used to improve motor starting performance, running performance, or both, depending on the size and connection of the capacitor. The capacitor-start

Figure 9.4 Capacitor-start motor: (a) connections, (b) phasor diagram at starting,

and (c) typical torque-speed characteristic.

motor is also a split-phase motor, but the time-phase displacement between the two

currents is obtained by means of a capacitor in series with the auxiliary winding,

as shown in Fig. 9.4a. Again the auxiliary winding is disconnected after the motor has started, and consequently the auxiliary winding and capacitor can be designed at minimum cost for intermittent service.

• By using a starting capacitor of appropriate value, the auxiliary-winding current ${\hat{I}}_{\text{aux}}$ at standstill can be made to lead the main-winding current ${\hat{I}}_{\text{main}}$ by 90 electrical degrees, as it would in a balanced two-phase motor (see Fig. 9.4b). In practice, the best compromise between starting torque, starting current, and cost typically results with a phase angle somewhat less than ${\text{90}}^o$ . A typical torque-speed characteristic is shown in Fig. 9.4c, high starting torque being an outstanding feature. These motors are used for compressors, pumps, refrigeration and air-conditioning equipment, and other hard-to-start loads. A cutaway view of a capacitor-start motor is shown in Fig. 9.5.
• In the permanent-split-capacitor motor, the capacitor and auxiliary winding are not cut out after starting; the construction can be simplified by omission of the switch, and the power factor, efficiency, and torque pulsations improved. For example, the capacitor and auxiliary winding could be designed for perfect two-phase operation (i.e., no backwards flux wave) at any one desired load. The losses due to the backward field at this operating point would then be eliminated, with resulting improvement in efficiency. The double-stator-frequency torque pulsations would also be eliminated, with the capacitor serving as an energy storage reservoir for smoothing out the pulsations in power input from the single-phase line, resulting in quieter operation.
• Starting torque must be sacrificed because the choice of capacitance is necessarily a compromise between the best starting and running values. The resulting torque-speed characteristic and a schematic diagram are given in Fig. 9.6.
• If two capacitors are used, one for starting and one for running, theoretically optimum starting and running performance can both be obtained. One way of accomplishing this result is shown in Fig. 9.7a. The small value of capacitance required for

Figure 9.5 Cutaway view of a capacitor-start induction motor.

The starting switch is at the right of the rotor. The motor is of

drip-proof construction. (General Electric Company.)

Figure 9.6 Permanent-split-capacitor motor

and typical torque-speed characteristic.

Figure 9.7 Capacitor-start, capacitor-run motor

and typical torque-speed characteristic.

optimum running conditions is permanently connected in series with the auxiliary winding, and the much larger value required for starting is obtained by a capacitor connected in parallel with the running capacitor via a switch with opens as the motor comes up to speed. Such a motor is known as a capacitor-start, capacitor-run motor.