### Dynamic Braking Resistors Equations

**Electronic Electrical Devices****
Dynamic
Braking Resistors About**

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

**Current of Dynamic Braking Motor ( Resistors ) based on
Power**

Where:

I_{res} = Dynamic resistor current (amps)

P_{motor} = 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:

R_{max} = Resistance

I_{res} =
Dynamic resistor current (amps) see above

With the R_{max}, which is based on the I_{res}, 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:

P_{res }= 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**

Where:

V_{brake }= Voltage during braking

R = Dynamic Breaking Resistor rating (ohms)

**Braking Torque:**

Combine equations above and:

Torque_{braking} is the effective braking torque
created by the electric motor. Torque is in Newton-meters