Page 193 - Power Quality in Electrical Systems

Electric Motor Drive Equipment 175

speed or restart, when power was restored. The hazard depends on
the inertia and function of the load that the motor was driving.
2. The motor damages itself from high stator current or negative shaft
torque when it attempts to recover its speed (particularly under high
inertia load) after a short-time sag or interruption of line voltage [12.3].

Phenomena
In the normal operation of the induction motor, the rotating air-gap
magnetic field links both the stator windings and the rotor windings
(squirrel-cage bars). When the stator voltage sags or is interrupted, two
phenomena occur: the rotor slows down (depending on the load inertia)
and the air-gap magnetic field declines in amplitude but is supported
by the rotor currents.
When the stator voltage is restored, depending upon the outage time,
the stator-produced magnetic field and the declining rotor air-gap field
can be out of phase. The result is nearly double the line voltage acting
on the leakage reactance, resulting in a large transient stator current
and negative shaft torque. The oscillograms for a 10-hp motor with an
inertia load of 4.28 per unit (pu) of the rotor inertia and a 7.3 cycle inter-
ruption are shown in Figure 12.2 [12.3]. The peak current is 1.67 pu of

800
Voltage (V) 400 0

–400
–800
300
Current (A) –150 0
150

–300
5000
Torque (in-lb) –5000 0

–10000
2000
Speed (rpm) 1600
1800
1400
1200
1000
0.00 0.10 0.20 0.30 0.40
Time (s)
Figure 12.2 Motor voltage, current, shaft torque, shaft speed versus time for 7.3 cycle volt-
age interruption for a 10-hp motor. [12.3].
[© 2006, IEEE, reprinted with permission]