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2
Section
How to Determine Dynamic Brake Requirements
When a drive is consistently operating in the regenerative mode of
operation, serious consideration should be given to equipment that will
transform the electrical energy back to the fixed frequency utility grid.
As a general rule, Dynamic Braking can be used when the need to
dissipate regenerative energy is on an occasional or periodic basis. In
general, the motor power rating, speed, torque, and details regarding the
regenerative mode of operation will be needed in order to estimate what
Dynamic Brake Resistor value is needed.
The Peak Regenerative Power of the drive must be calculated in order
to determine the maximum resistance value of the Dynamic Brake
Resistor. Once the maximum resistance value of the Dynamic Brake
Resistor current rating is known, the required rating and number of
Dynamic Brake Resistors can be determined. If a Dynamic Brake
Resistance value greater than the minimum imposed by the choice of the
peak regenerative power is made and applied, the drive can trip off due
to transient DC bus overvoltage problems. Once the approximate
resistance value of the Dynamic Brake Resistor is determined, the
necessary power rating of the Dynamic Brake Resistor can be calculated.
The power rating of the Dynamic Brake Resistor is estimated by
applying what is known about the drive's motoring and regenerating
modes of operation. The Average Power Dissipation must be estimated
and the power rating of the Dynamic Brake Resistor chosen to be greater
than that average. If the Dynamic Brake Resistor has a large
thermodynamic heat capacity, then the resistor element will be 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 (defined as thermal time constants less than 5 seconds) the
temperature of the resistor element could exceed its maximum.
Peak Regenerative Power can be calculated as:
• Horsepower (English units)
• Watts (The International System of Units, SI)
• Per Unit System (pu) which is relative to a value
The final number must be in watts of power to estimate the resistance
value of the Dynamic Brake Resistor. The following calculations are
demonstrated in SI units.
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