VFD Braking Resistor Formulas

Braking resistor formulas for a drive: the resistance from the braking power and the DC bus voltage, the braking energy and the average power in the resistor.

The formulas

Vb is the DC bus voltage at which the braking chopper switches on, Pb the peak braking power, J the inertia of the motor and load in kg·m², n the speed in rpm, ts the stopping time and tcycle the time between the stops. The resistance must not be lower than the minimum value of the drive, and the resistor must carry the peak power for the braking time and the average power over the cycle. The energy that the machine has is shared between the resistor and the losses of the motor and the load, so the figure is a safe upper estimate. A load that pulls the motor all the time, such as a lowering hoist, needs a continuous rating. The voltage, the inertia and the times here are examples, and the drive maker gives the resistance range and the duty.

Worked example: a 15 kW peak braking power on a bus that brakes at 750 V

1. R = 750² / 15,000 = 37.5 Ω

Worked example: stopping a load of 2 kg·m² from 1,480 rpm in 4 s, once a minute

1. ω = 2π × 1480 / 60 = 154.99 rad/s
2. E = ½ × 2 × 154.99² = 24.02 kJ
3. Pav = 24.02 / 4 = 6.005 kW during the stop
4. Pcyc = 24.02 / 60 = 0.4003 kW over the cycle

Resistance for a 750 V bus by braking power

Frequently asked questions

Why a resistor at all?

A motor that slows a load turns into a generator and feeds energy back into the DC bus of the drive. A drive with a diode front end cannot return it to the supply, so the chopper sends it to the resistor as heat.

Why not use the lowest resistance?

A low resistance gives a high current in the chopper, and the drive has a minimum value. A high resistance gives less braking power for the same bus voltage.

What is the difference between the peak and average power?

The resistor takes the peak power only during the stop. Its continuous rating follows the average over the cycle, and its pulse rating covers the peak.

Is there a way to avoid the resistor?

A drive with a regenerative front end returns the energy to the supply, which saves the heat but costs more.

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