4-13phase difference between the two windings is obtained by connecting the auxiliary winding in series witha capacitor and starting switch. When the motor is first energized, the starting switch is closed. Thisplaces the capacitor in series with the auxiliary winding. The capacitor is of such value that the auxiliarycircuit is effectively a resistive-capacitive circuit (referred to as capacitive reactance and expressed asX_{C}). In this circuit the current leads the line voltage by about 45º (because XCabout equals R). The mainwinding has enough resistance-inductance (referred to as inductive reactance and expressed as X_{L}) tocause the current to lag the line voltage by about 45º (because XLabout equals R). The currents in eachwinding are therefore 90º out of phase - so are the magnetic fields that are generated. The effect is thatthe two windings act like a two-phase stator and produce the rotating field required to start the motor.Figure 4-11.—Capacitor-start, ac induction motor.When nearly full speed is obtained, a centrifugal device (the starting switch) cuts out the startingwinding. The motor then runs as a plain single-phase induction motor. Since the auxiliary winding is onlya light winding, the motor does not develop sufficient torque to start heavy loads. Split-phase motors,therefore, come only in small sizes.RESISTANCE-START.—Another type of split-phase induction motor is the resistance-start motor.This motor also has a starting winding (shown in fig. 4-12) in addition to the main winding. It is switchedin and out of the circuit just as it was in the capacitor-start motor. The starting winding is positioned atright angles to the main winding. The electrical phase shift between the currents in the two windings isobtained by making the impedance of the windings unequal. The main winding has a high inductance anda low resistance. The current, therefore, lags the voltage by a large angle. The starting winding isdesigned to have a fairly low inductance and a high resistance. Here the current lags the voltage by asmaller angle. For example, suppose the current in the main winding lags the voltage by 70º. The currentin the auxiliary winding lags the voltage by 40º. The currents are, therefore, out of phase by 30º. Themagnetic fields are out of phase by the same amount. Although the ideal angular phase difference is 90ºfor maximum starting torque, the 30-degree phase difference still generates a rotating field. This suppliesenough torque to start the motor. When the motor comes up to speed, a speed-controlled switchdisconnects the starting winding from the line, and the motor continues to run as an induction motor. Thestarting torque is not as great as it is in the capacitor-start.

Integrated Publishing, Inc.

6230 Stone Rd, Unit QPort Richey, FL34668

Phone For Parts Inquiries: (727) 493-0744
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