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9. SELECTION OF DYNAMIC BRAKING UNITS AND DISCHARGING
RESISTORS
The synchronous speed of the motor decreases relative to a decrease in set frequency.
When this happens, the synchronous speed of the motor is less than the rotational speed of the
rotor and the slip becomes negative. Consequently, the torque generated by the motor operates
as a braking touque. See Figure 9.1.
This braking torque is a function of voltage verses frequency. For example, the difference
between the torque at frequency f1 and the torque at frequency f3 in Figure 9.2 is caused by the
difference between their voltage-vs-frequency ratios.
Speed
Torque-vs-speed curve
at frequency f3
Torque-vs-speed curve at frequency f2
Torque-vs-speed curve at frequency f1
Output voltage (V)
Frequency
(f)
Figure 9.1 Frequency change and motor torque
Figure 9.2 Voltage-vs-frequency
characteristics
In this status, the motor operates as a generator. The rotational energy of the motor is
converted into electrical energy and feed back into the inverter circuit. See Figure 9.3.
Dynamic braking unit
Discharging resistor
Motor
Regeneration energy
Power supply
DC smoothing
circuit
Converter
section
Inverter
circuit
Figure 9.3 Dynamic braking by the inverter
The regenerated energy is stored in the DC bus circuit of the inverter.
(Capacitor feedback type)
