0195136047.pdf

580 ROTATING MACHINES

(a)

+

−

Centrifugal switch

Auxiliary winding

Torque, per unit

Torque during starting due to

main and auxiliary windings

Torque due to main winding only

(c)

(b)

I

I

Im

Im

V V

Ia

C

Ia

4

3

2

1

S = 1

(zero speed)

S = 0

(synchronous speed)

Switching Speed or slip

speed

Main

winding

Figure 13.2.13Capacitor-start motor.(a)Schematic diagram.(b)Phasor diagram at starting.(c)Typical

torque–speed characteristic.

pole motor got its name from these shading bands. Induced currents in the shading coil cause the

flux in the shaded portion of the pole to lag the flux in the other portion in time. The result is

then like a rotating field moving in the direction from the unshaded to the shaded portion of the

pole. A low starting torque is produced. A typical torque-speed characteristic is shown in Figure

13.2.16(b). Shaded-pole motors have a rather low efficiency.

Applications for Induction Motors

Before specifying a particular motor for a given application, the designer must know the load

characteristics, such as horsepower requirement, starting torque, acceleration capability, speed

variation, duty cycle, and the environment in which the motor is to operate. Typical speed–torque

curves for squirrel-cage induction motors with NEMA design classification A, B, C, and D are

shown in Figure 13.2.17. Having selected the appropriate motor for a given application, the next

step is to specify a controller for the motor to furnish proper starting, stopping, and reversing

without damaging the motor, other connected loads, or the power system. The ranges of standard

power ratings given by NEMA for single-phase motors are listed in Table 13.2.1. Two-phase

induction motors with high rotor resistance are employed as servomotors for control-system

applications that usually require positive damping over the full speed range.