Dynamic Braking with VSD
- When a stop signal is given to a VSD, the VSD output decelerates to stop and is switched off, but the motor may carry on running due to load inertia. Mechanical energy now is running the motor and motor acts as a generator.
- Energy flows back from the motor into the DC link capacitors within the drive. This causes the DC link voltage to rise.
- If the DC link voltage exceeds 750V then the drive will trip to protect the capacitors and the inverter power devices.
- The amount of energy that can be absorbed in the capacitors is relatively small; typically more than 20% braking torque will cause the drive to trip on over- volts.
- Dynamic braking is a means of increasing the braking capability of the drive by dissipating the excess energy in a high power resistor connected across the DC link.
The amount of energy produced by the motor during regeneration depends on the ramp down time parameter and the Inertia of the load.
Low inertia and slow ramp times do not produce sufficient energy to require a dynamic braking unit.
In some units the braking transistor is built in with connection terminals for external resistor.
Braking torque of 20% and up to 125% (with resistor) can be provided.