Dynamic Braking Resistors Equations

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The following are general equations that define dynamic braking systems performance parameters.

Current of Dynamic Braking Motor ( Resistors ) based on Power

Ires = Dynamic resistor current (amps)
Pmotor = Motor power
n = Motor efficiency (.95 to .90) for most large electric motor applications, (.90 - .83) for smaller motors.
800 = Constant

Next calculate the dynamic resistor resistance max (ohms)

Rmax = Resistance
Ires = Dynamic resistor current (amps) see above

With the Rmax, which is based on the Ires, consult your dynamic braking resistor manufacturer specification for your operating voltage to define your required braking resistor.

Resistor Power Rating

D = Duty cycle, D=1 when resistor is dissipating energy continually.

Power Dissipation:

240 v installation

480 v Installations

600 v installation


Pres = Power dissipation
D = Duty cycle
R = Resistance in ohms

The power dissipation number is between 0 and 1. Typically, D will never be specified less than 0.2. This will ensure that the minimum power rating of the resistor will always be sufficient. The duration of the braking period determines the temperature rise of the resistor as determined by the thermal time constant of the resistor (manufacturer specified). Typical braking applications are intermittent (D <1), the resistor should not be designed for continuous operation. Braking resistors are normally sized for the average power dissipation over the braking cycle time.

Maximum braking power


Vbrake = Voltage during braking
R = Dynamic Breaking Resistor rating (ohms)

Braking Torque:

Combine equations above and:

Torquebraking is the effective braking torque created by the electric motor. Torque is in Newton-meters

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