Rockwell Automation AK DBU - Dynamic Braking Unit 600/690 VAC - Manual (Page 10)

P-6 Overview
Theory of Operation
When the rotor of an induction motor is turning slower than the
synchronous speed set by the drive’s output power, the motor is
transforming electrical energy obtained from the drive into mechanical
energy 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 into the utility grid. 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 can only handle power in the motoring direction.
Therefore, if the motor is regenerating, the bridge cannot conduct the
necessary negative DC current. Depending on parameter setting, the
drive regulator will either increase the DC bus voltage and cause a Bus
Overvoltage trip at the drive, or extend the set deceleration rate or
increase the output frequency.
When a drive is dissipating regenerative electrical energy on an
occasional or periodic basis, a DBU connected to the DC bus of a drive
and feeding a power resistor can be specified. When a drive is
consistently operating in the regenerative mode, a regenerative unit or
RGU should be used to transform the DC regenerated energy to the fixed
frequency utility energy.
Parameter Setting on the Inverter
ATTENTION: When the braking unit is connected to a drive, the drive
Bus Regulation should be turned off. In addition the Regen Power Limit
should be set to the motor power limit or the braking unit power limit,
whichever is greater.
For example on a PowerFlex 700
• Parameters 161/162 [Bus Reg. Mode A/B] must be set to 0 (Disabled)
• Parameter 163 [DB Resistor Type] must be set to 2 (None)
• Parameter 153 [Regen Power Lim] must be set to the motor power
limit or braking unit power limit, whichever is greater.