VTdrive What is Variable Frequency AC Drive Dynamic Braking Resistor0 pages

Understanding How Dynamic Braking Works
When an induction motor’s rotor is turning slower than the synchronous speed
set by the drive’s output power, the motor is transforming electrical energ y
obtained from the drive into mechanical energ y available at the drive shaft of
the motor. This process is referred to as motoring. When the rotor is turning
faster than the synchronous speed set by the drive’s output power, the motor is
transforming mechanical energy available at the drive shaft of the motor into
electrical energy that can be transferred back to the drive. This process is
referred to as regeneration.
Most AC PWM drives convert AC power from the fixed frequency utility grid into
DC power by means of a diode rectifier bridge or controlled SCR bridge before it
is inverted into variable frequency AC power. Diode and SCR bridges are cost
effective, but can only handle power in the motoring direction. Therefore, if the
motor is regenerating, the bridge cannot conduct the necessary negative DC
current, the DC bus voltage will increase and cause an overvoltage fault at the
drive. More complex bridge configurations use SCRs or transistors that can
transform DC regenerative electrical power into fixed frequency utility electrical
energ y. This process is known as line regeneration.
A more cost effective solution can be provided by allowing the drive to feed the
regenerated electrical power to a resistor which transforms it into thermal energ
y. This process is referred to as dynamic braking.

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