13.2 INDUCTION MACHINES 573I2 start′ =122. 9
√
( 0. 281 + 0. 15 )^2 +( 0. 489 + 0. 2 )^2=122. 9
0. 813= 151 .2ATstart=1
125. 73 × 151. 22 × 0. 15 = 81 .8N·mFor such applications as fans and blowers, a motor needs to develop only a moderate starting
torque. Some loads, like conveyors, however, require a high starting torque to overcome high
static torque and load inertia. The motor designer sometimes makes the starting torque equal to
the maximum torque by choosing the rotor-circuit resistance at startup to be
R2 start′ =√
(R 1 ′′)^2 +(X′′ 1 +Xl′ 2 )^2 (13.2.17)which can easily be obtained from Equation (13.2.13) withSmaxT=1.
Speed and Torque Control of Polyphase Induction Motors
The induction motor is valuable in so many applications because it combines simplicity and
ruggedness. Although a good number of industrial drives run at substantially constant speed,
quite a few applications need variable speed. Speed-control capability is essential in such
applications as conveyors, hoists, and elevators. Because the induction motor is essentially a
constant-speed machine, designers have sought creative ways to easily and efficiently vary its
speed continuously over a wide range of operating conditions. We only indicate the methods of
speed control here.
The appropriate equation to be examined, based on Equation (13.1.8), is
n=( 1 −S)n 1 =( 1 −S) 120 fs/P (13.2.18)
wherenis the actual speed of the machine in revolutions per minute,Sis the per-unit slip,fs
is the supply frequency in hertz,Pis the number of poles, andn 1 is the synchronous speed in
revolutions per minute. Equation (13.2.18) suggests that the speed of the induction motor can
be varied by varying either the slip or the synchronous speed, which in turn can be varied by
changing either the number of poles or the supply frequency. Any method of speed control that
depends on the variation of slip is inherently inefficient because the efficiency of the induction
motor is approximately equal to 1−S. On the other hand, if the supply frequency is constant,
varying the number of poles results only in discrete and stepped variation in motor speed.
Indeed, all methods of speed control require some degree of sacrifice in performance, cost,
and simplicity. These disadvantages must be weighed carefully against the advantages of speed
variability.
The following are methods available for speed and torque control of induction motors.
- Pole-changing method
- Variable-frequency method
- Variable-line-voltage method
- Variable-rotor-resistance method
- Rotor-slip frequency control
- Rotor-slip energy recovery method
- Control by auxiliary devices (Kramer control, Scherbius control, Schrage motor)