Page 201 - Power Quality in Electrical Systems

Electric Motor Drive Equipment 183

800
Conditions: 2.5% unbalance, 5 hp drive, half load, DC bus C = 56 uF/hp
700

600
Increasing La
DC bus voltage
500
Voltage (V), current (A) 300
400

200

100 input current × 5 Rectified input voltage
0
Increasing La
–100 Increasing La

–200
0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018
Time (Sec.)
Figure 12.11 Operation under conditions of Figure 12.10. Waveforms of line current,
DC bus voltage, and “raw-unfiltered” rectified AC line voltage for line reactance rang-
ing, L S 1 percent to 2 percent [12.10].
[© 2004, IEEE, reprinted with permission]

line current is shown in Figure 12.12. It includes the triplens as well
as the 5th, 7th, 11th, and 13th harmonics, characteristic of three-phase
operation [12.10].

Protective measures
Protective measures to maintain the operation of ASDs in the face of line
voltage sags and interruptions include the following [12.7]:

■ Restart
■ Kinetic buffering
■ Boost converter
■ Active front end
The second, third, and fourth measures reduce the sensitivity of the
ASD by increasing its tolerance to the depth and duration of line volt-
age sags and interruptions. External equipment, such as a UPS or E/G
set, is required to maintain ASD operation for longer-duration voltage
interruptions.