Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 202)

202 Rockwell Automation Publication 750-RM002B-EN-P - September 2013
Chapter 4 Motor Control
the drive can trip off due to transient DC bus overvoltage problems. Once the
choice of the approximate Ohmic value of the Dynamic Brake Resistor is made,
the wattage rating of the Dynamic Brake Resistor can be made.
The wattage rating of the Dynamic Brake Resistor is estimated by applying the
knowledge of the drive motoring and regenerating modes of operation. The
average power dissipation of the Regenerative mode must be estimated and the
wattage of the Dynamic Brake Resistor chosen to be slightly greater than the
average power dissipation of the drive. If the Dynamic Brake Resistor has a large
thermodynamic heat capacity, the resistor element is able to absorb a large
amount of energy without the temperature of the resistor element exceeding the
operational temperature rating. Thermal time constants in the order of 50
seconds and higher satisfy the criteria of large heat capacities for these
applications. If a resistor has a small heat capacity, the temperature of the resistor
element could exceed the maximum temperature limits during the application of
pulse power to the element and could exceed the safe temperature limits of the
resistor.
The peak regenerative power can be calculated in English units (Horsepower), in
The International System of Units (SI) (Watts), or in the per unit system (pu),
which is dimensionless for the most part. In any event, the final number must in
Watts of power to estimate Dynamic Brake Ohmic value. Calculations in this
page are demonstrated in SI units.
Speed, Torque, Power Profile
The following figure is a typical dynamic braking application. The top trace
represents speed and is designated by the omega symbol. In the profile the motor
is accelerated to some speed, it holds that speed for a period of time and is then
decelerated. This deceleration is not necessarily to zero speed. The cycle is then
repeated.
The middle trace represents motor torque. Torque starts out high as the motor is
accelerated then drops down to maintain the commanded speed. Then the
torque turns negative as the motor is decelerated. The cycle is then repeated.
The bottom trace represents motor power. Power increases as the motor speed
increases. Power decreases some to maintain the commanded speed then goes