21/10/13 Dynamic Braking Resistors Calculations - Engineers Edge

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Dynamic Braking Resistors Calculations

Electronic Electrical Devices | Dynamic Braking Resistor Supplier Distributors

Dynamic Braking Resistors About

Current of Dynamic Braking Motor ( Resistors ) based on Power

Where: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)

Where:

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

Where:

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 (manufacturerspecified). 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

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21/10/13 Dynamic Braking Resistors Calculations - Engineers Edge

www.engineersedge.com/instrumentation/components/dynamic_braking_resistors_calculations.htm 2/2

Where:

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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