Handbook of Electrical Engineering

INDUCTION MOTORS 127

Figure 5.7 Circuit diagram for an induction motor using a star-delta starter.

between 65% and 80%. Table 5.14 illustrates the effect of reduced voltage on the starting current,
line current and torque for various ratios. A disadvantage of the method is that two extra three-phase
circuit breakers or contactors are necessary, thus making three in total for the motor circuit, which
require space to be allocated. Retro-fitting a Korndorfer starter may therefore be difficult if space
is scarce.

5.10.3 Soft-start power electronics method

A standard design of motor is used. An external rectifier-inverter is connected between the main
circuit breaker, or contactor, and the motor during the starting and run-up period. The starter varies
the frequency and voltage magnitude of the applied three-phase supply to the motor. Upon starting
the frequency and voltage are set to their lowest values, and thereafter they are slowly raised as
the shaft speed increases. The intent is to operate the motor in its near-synchronous speed state
for each frequency that the motor receives. The process is explained in more detail in Chapter 14.
The rectifier-inverter equipment is expensive when compared with other switching and transformer
methods, but it has several advantages:-

• The starting current can be limited to a value that is equal to or a little higher than the full-load
  current of the motor.


• The torque created in the motor during the whole run-up period can be in the order of the full-load
  value, and so the shape of the inherent torque-speed curve of the motor is not a critical issue for
  most standard designs of motors.