Page 206 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

VARIABLE-SPEED DRIVES

VARIABLE-SPEED DRIVES 10.5

L S

T 1 T 2 T 3
V S D 1 D 2 D 3
2
V S
Three- a
phase Rectifier C b
inverter c

T 4 T 5 T 6
V S D 4 D 5 D 6
2

FIGURE 10.6 A three-phase voltage source inverter using power transistors.

FIGURE 10.7 The three-phase voltage commands given to the VSD at the required output fre-
quency.

INPUT SOURCES FOR REGENERATION OR
DYNAMIC SLOWDOWN

If the inverter is made to regenerate (i.e., to try to pump back the mechanical energy in the
load to the ac supply), the source diode bridge cannot reverse its current flow. Thus, the
absorbed energy will cause the dc bus voltage to increase. If this higher voltage is not
absorbed, the source bridge, load inverter, and capacitor bank could get damaged.

Dynamic Braking
The simplest way to absorb the load energy that is regenerated is by using a resistor across
the dc link (dynamic braking resistor). This is simply a large power-dissipating resistor that

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