Handbook of Electrical Engineering

128 HANDBOOK OF ELECTRICAL ENGINEERING

Figure 5.8 Circuit diagram for an induction motor using a Korndorfer auto-transformer starter.

Table 5.14. Auto-transformer starting of an induction motor

Voltage applied

to motor in

per cent of

rated voltage

Line current

in per cent of

locked rotor

current

Motor current

in per cent of

locked rotor

current

Locked rotor

torque in per cent

of full voltage

value

100 100 100 100

90 81 90 81

80 64 80 64

70 49 70 49

60 36 60 36

50 25 50 25

5.10.4 Series reactor method

A standard design of motor is used. This is a simple method that requires the insertion of a series
reactor during starting. The reactor is bypassed once the motor reaches its normal working speed.
Only one extra circuit breaker or contactor is required. The amount of reactance is calculated on the
basis of the desired reduction of line current during starting, but the limiting factor is the reduced
starting torque. The torque is reduced for two reasons, firstly because the total circuit impedance is
increased and secondly the reactance-to-resistance ratio is increased.