Page 119 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

INDUCTION MOTORS

6.8 CHAPTER SIX

FIGURE 6.7 The transformer model of an induction motor, with rotor and stator connected by an ideal
transformer of turns ratio a eff.

FIGURE 6.8 The rotor circuit model of an induc- FIGURE 6.9 The rotor circuit model with all the
tion motor. frequency (slip) effects concentrated in resistor R .
R

The rotor equivalent circuit is shown in Fig. 6.8. The rotor current can be found as
E
R
I
R R jX R
R
sE
I
R 0
R R jsX R 0
R
The voltage supplied E R can be treated as constant. The final rotor equivalent circuit
0
is shown in Fig. 6.9. The equivalent impedance containing all the effects of varying rotor
slip is
R
Z R jX
R, eq R 0
s
Figure 6.10 illustrates the variation of rotor current with speed.
At high slips (rotor speed is much lower than normal operating speed or synchronous
is much larger than R /s. The rotor current approaches a steady-state value. At
speed), X R 0 R
low slips (rotor speed is near normal operating speed or synchronous speed), the resistive
term R /s is much larger X R . The rotor resistance is the dominant term, and the rotor current
R 0
vary linearly with slip.

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