Page 128 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

INDUCTION MOTORS

INDUCTION MOTORS 6.17

FIGURE 6.21 Flux linkage in a deep bar rotor. (a) For a current flowing in the top of the bar,
the flux is tightly linked to the stator, and leakage inductance is small. (b) For a current flowing
in the bottom of the bar, the flux is loosely linked to the stator, and leakage inductance is large. (c)
The resulting equivalent circuit of the rotor bar as a function of depth in the rotor.

cross section. Since the rotor resistance is high during starting conditions, the starting
torque is relatively higher and the starting current is relatively lower than for a class A
motor. This design is known as class B (Fig. 6.20).
Figure 6.18c illustrates a cross-sectional view of a double-cage rotor. The inner bars are
large (low resistance). They are buried deeply in the rotor. The upper bars are small (low resis-
tance). They are located near the surface of the rotor. This design is similar to the deep bar
rotor, but the difference between the low- and high-slip operation is even more exaggerated.
During starting conditions, the rotor resistance is high because the small bars are effec-
tive only. Hence, the starting torque is high. However, during normal operation, both bars
are effective, resulting in low resistance. Double-cage rotors of this type are used to produce
NEMA class B and C characteristics (Fig. 6.20). The main disadvantage of double-cage
rotors is that they are more expensive than squirrel-cage rotors.

STARTING INDUCTION MOTORS

Since the inrush current of induction motors can exceed 8 times the normal operating
current, across-the-line starting may not be acceptable because it may cause a dip in the
power system voltage. In wound rotor induction motors, extra resistance can be inserted in
the rotor circuit during starting. This results in an increase in the starting torque and a reduc-
tion in the starting current. The starting current of a squirrel-cage induction motor can vary
widely depending on the motor’s rated power and the effective resistance of the rotor at
starting conditions.

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