How do I calculate the Duty Cycle % for a Braking Resistor on a VFD?

The provided information will allow you to calculate the duty cycle of a braking resistor for the purpose of selecting the correct part for the application.

The duty cycle of a VFD braking resistor refers to the percentage of time that the resistor is active and dissipating energy during the braking process. The duty cycle can be calculated using the following formula:

Duty Cycle = (Braking Time / (Braking Time + Rest Time)) x 100%

where:

• Braking Time: The time duration for which the VFD is actively braking the motor and the braking resistor is dissipating energy.
• Rest Time: The time duration for which the VFD is not actively braking the motor and the braking resistor is not dissipating energy.

For example, let's say that the VFD is programmed to apply the braking resistor for 2 seconds and then rest for 8 seconds. In this case, the total cycle time would be 10 seconds (2 seconds of braking time + 8 seconds of rest time). Therefore, the duty cycle would be:

Duty Cycle = (2 / (2 + 8)) x 100% Duty Cycle = 20%

So in this example, the braking resistor would be active and dissipating energy for 20% of the total cycle time.