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 (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:
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
