0195136047.pdf

13.2 INDUCTION MACHINES 579

winding currents causes the motor to start. The auxiliary winding is disconnected by a centrifugal
switch or relay when the motor comes up to about 75% of the synchronous speed. The torque–speed
characteristic of the split-phase motor is of the form shown in Figure 13.2.12(c). A split-phase
motor can develop a higher starting torque if a series resistance is inserted in the starting auxiliary
winding. A similar effect can be obtained by inserting a series inductive reactance in the main
winding; this additional reactance is short-circuited when the motor builds up speed.

2. Capacitor motors:Capacitor motors have a capacitor in series with the auxiliary winding
   and come in three varieties: capacitor start, two-value capacitor, and permanent-split capacitor.
   As their names imply, the first two use a centrifugal switch or relay to open the circuit or reduce
   the size of the starting capacitor when the motor comes up to speed. A two-value-capacitor motor,
   with one value for starting and one for running, can be designed for optimum starting and running
   performance; the starting capacitor is disconnected after the motor starts. The relevant schematic
   diagrams and torque–speed characteristics are shown in Figures 13.2.13, 13.2.14, and 13.2.15.
   Motors in which the auxiliary winding and the capacitor are not cut out during the normal running
   conditions operate, in effect, as unbalanced two-phase induction motors.


3. Shaded-pole motors:The least expensive of the fractional-horsepower motors, generally
   rated up to^1 / 20 hp, they have salient stator poles, with one-coil-per-pole main windings. The
   auxiliary winding consists of one (or rarely two) short-circuited copper straps wound on a portion
   of the pole and displaced from the center of each pole, as shown in Figure 13.2.16(a). The shaded-

(a)

+

−

Centrifugal switch

Cage rotor

Auxiliary winding

Torque, per unit

Torque during starting due to main

and auxiliary windings

Torque due to main winding only

Main

winding

(c)

(b)

I

I

Im

Im

V V

Ia

Ia

4

3

2

1

S = 1

(zero speed)

S = 0

(synchronous speed)

Speed or slip

Switching

speed

Figure 13.2.12Split-phase motor.(a)Schematic diagram.(b)Phasor diagram at starting.(c)Typical torque–
speed (or slip) characteristic.