© ElectroCraft 2022
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High Power Universal Drive User Manual
13.3.2 Calculating power value of the braking resistor
The power rating of a braking resistor decides its maximum heat capacity. A brake resistor with large heat
capacity can dissipate large amount of energy without the temperature of the resistor element exceeding
the operational (safe) temperature rating.
Maximum Power value = Resistor On voltage x Maximum deceleration current of the motor
The On voltage and Off voltage of the brake resistor is programmed into the drive by the user via the PC
configuration software. Brake resistor On and Off voltages are calculated as given below:
Brake resistor On voltage = Power supply output v 4 volts
Brake resistor Off voltage = Power supply output v 2 volts
An example to calculate the maximum power of a braking resistor with a power supply voltage of 48 V
and motor deceleration current of 24 A is as follows:
Maximum Power value = (48 + 4) x 24 = 1248 Watts
The power rating of the braking resistor
need not
be the maximum value as the value of power dissipated
across the resistor depends on the duty cycle of braking.
Duty Cycle = Time in Braking / Total cycle Time
Calculation for average power value of the braking resistor using duty cycle is as follows:
Average Power value = Resistor On voltage x Maximum deceleration current of the motor x Duty Cycle
For example, consider an application with power supply voltage of 48 V and motor deceleration current of
24 A, so a 2 ohm resistor is picked (according to section 13.3.1). The load accelerates for 6 seconds and
brakes for 4 seconds, which gives a total cycle time of 10 seconds. Example parameters are referenced
in the table below:
Parameter
Value
Unit
Total cycle Time
10
Seconds
Time in Braking
4
Seconds
Volts during braking
52
Volts
Deceleration
current
24
Amps
Duty Cycle = 4/10 = 0.4
Average Power value = 52 x 24 x 0.4 = 500 Watts
Therefore, in this example a 500 Watts resistor will be able to handle the braking power of 1248 watts for
4 seconds and cools down for 6 seconds as long as the maximum current of the resistor is not exceeded,
and the thermal sensitive switch does not trip.
Considering the same application as referenced in the
above example but the “Total cycle Time” is 10
minutes and “Time in Braking” is 4 minutes.
Duty Cycle = 4/10 = 0.4
Average Power value = 500 Watts
A 500 watts resistor will not handle 1248 watts of braking power for 4 minutes. The brake resistor will get
too hot, and it will trip the user supplied temperature sensitive switch (refer to section 13.5), in turn
disconnecting the braking resistor from the drive. The drive will then fault due to shunt overpower or bus
over voltage after the braking resistor is disconnected from the drive while braking. Hence, the Average
power value of the resistor also gets affected by the duration of the braking time. The selected power
rating for any external brake resistor is application dependent. Usually, a heavy-duty wire wound resistor
will work best.
