Page 118 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

INDUCTION MOTORS

INDUCTION MOTORS 6.7

FIGURE 6.6 The development of induced torque in an induction motor. (a) The rotating stator field B
S
induces a voltage in the rotor bars. (b) The rotor voltage produces a rotor current flow, which lags behind
the voltage because of the inductance of the rotor. (c) The rotor current produces a rotor magnetic field B
R
lagging 90° behind itself, and B interacts with B to produce a counterclockwise torque in the machine.
R net
The reactance of a rotor depends on the rotor inductance and the frequency of the volt-
age and current in the rotor. If the rotor inductance is L , the rotor reactance is given by
R
X L 2 f L
R r R r R
Since f sf, the rotor reactance becomes
r e
X 2 sf L
R e R
s(2 f L )
e R
sX
R 0
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