Page 202 - Power Quality in Electrical Systems

184 Chapter Twelve

110
Conditions: 2.5% unbalance, 5 hp drive, half load
100 DC bus C = 56 uF/hp. La = 1%

90
Input current harrmonic amplitude (%) 70
80

60
50

40
30

20
10

0
0 5 10 15 20 25 30 35 40 45 50
Harmonic orders
Figure 12.12 5 hp ASD. 2.5-percent line voltage unbalance, L S 1%. Harmonic spec-
trum under unbalanced conditions [12.10].
[© 2004, IEEE, reprinted with permission]
Restart options are provided on commercial ASDs. Figure 12.13 shows
a simplified version of the operation between power failure and recovery.
The dc-bus voltage declines as the bus capacitor discharges while supply-
ing the inverter. The inverter is shut down by the dc-bus under-voltage trip.
The motor coasts down. The dc-bus voltage recovers when power is restored.
The motor and inverter will only restart on a reset command [12.11].

Power failure Power recovery
Set value: 0
Main circuit Under
DC voltage voltage

Time
Figure 12.13 Commercial ASD
Output operations following power fail-
frequency ure, inverter shutdown by DC-
bus undervoltage trip. No reset
command. Motor coasts to a stop
[12.11].
LU trip On