Page 512 - Handbook of Electrical Engineering

502 HANDBOOK OF ELECTRICAL ENGINEERING

The authors of References 6 and 27 give analyses of the short-circuit current of an induction
motor that has only one winding in each axis of the rotor. These analyses result in a simple equation
of the form,
E
−t
1
I = e T
1
X
where E is the air-gap phase-to neutral voltage before the fault was applied,

X

T =
R 1
2
X m

and X = X 1 + X m −
X 2 + X m
Which approaches X 1 when X m is large compared with X 1 and X 2 .
The DC off-set has been ignored in the above equation, which is a reasonable assumption for
small motors.
A more comprehensive treatment of the subject is given in Reference 23 in which comparisons
were made with actual test results taken from large motors. The treatment also takes account of the
DC off-set and the ‘deep-bar’ effect of the rotor conductors and slots. These are important factors to
consider, especially with large high-voltage motors. The problem of delayed zero crossing is discussed
in sub-section 7.2.11 in relation to the breaking current duty of circuit breakers. The problem arose
with generators from the possibility that a poor combination of the armature time constant T a and the
sub-transient reactance X could occur. A very similar effect can occur with large motors. Kalsi et al

d
in Reference 23 showed that the peak value of the current in the first half-cycle could be as high as
12 times the rated peak current, largely due to the full DC off-set that can occur, see Figure 20.4.

Figure 20.4 Short-circuit current decrement for a 2500 kW and a 37 kW induction motor. These motors have
a relatively high armature time constant Ta that causes the initial offset of the waveform. The ‘deep bar’ effect
in the rotor has been taken into account.