Page 199 - Power Quality in Electrical Systems

Electric Motor Drive Equipment 181

■ The overcurrent protection system can be triggered by increased cur-
rent during the sag, or by high inrush current at voltage recovery to
recharge the DC-link capacitor.
The effect of a constant-torque loading level on the sensitivity of the
ASD in Figure 12.7 is shown in Figure 12.8 [12.9]. At 25-percent torque,
the ASD can tolerate a voltage interruption of 50 ms and a voltage sag
to 65 percent of the nominal line voltage up to 500 ms. Decreased load-
ing results in better ride-through capabilities.

Voltage unbalance
Unbalance in the three-phase voltages supplying an ASD can produce
the following effects [12.10]:

■ The three-phase input diode rectifier reverts to single-phase mode,
resulting in increased ripple in the dc-link voltage.
■ The line current contains triplen harmonics.
■ The diodes and the dc-bus capacitor are stressed by increased ripple,
with possible life reduction.
■ The motor develops uncharacteristic torque ripple, with increased
noise and vibration.

100

90
80

70
Voltage, (%) 60

50
40

30 Constant torque load type
20 T = 100% of T n
T = 50% of T n
10 T = 25% of T n
T = 0% of T (no load)
n
0
0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 500
Time, (ms)
Figure 12.8 Influence of torque, 100 percent to 0 percent, on sensitivity of ASD to a bal-
anced three-phase line-voltage sag [12.9].
[© 2005, IEEE, reprinted with permission]