What is a braking resistor? (For VFD

What is a braking resistor?

What Is a Braking Resistor?

A braking resistor is a power resistor used to turn regenerative energy into heat when a motor controlled by a variable frequency drive is slowing down. On high-inertia machines, hoists, winding systems and applications with frequent deceleration, the motor can temporarily stop behaving like a load and begin behaving like a generator.

The regenerated energy flows back into the drive's DC bus. When the DC-link voltage reaches the braking threshold, the internal brake chopper or an external braking unit connects the resistor to the DC link. The resistor converts the excess electrical energy into heat and helps prevent the DC-bus voltage from continuing to rise.

The simplified energy path is: Rotating load → Motor → VFD → DC Bus → Brake Chopper → Braking Resistor → Heat

How does it work? · How is it selected? · Braking-energy calculation · FAQ

Important: The interactive model below is an educational illustration, not a resistor-sizing calculator. Always use the drive manufacturer's minimum resistance, brake-chopper limits and permitted braking duty when selecting real hardware.

Frequency 50 Hz Inertia 0.35

How does a braking resistor work?

During normal motoring, electrical power flows from the drive to the motor. During rapid deceleration or when the mechanical load drives the motor, the direction of power flow can reverse. The motor then enters a regenerative operating condition and converts part of the machine's mechanical energy back into electrical energy.

This energy returns to the DC bus. Many conventional VFDs use a diode rectifier at the input, so this energy cannot simply flow back into the AC supply. The DC-link voltage therefore rises.

When the voltage reaches the braking threshold, the brake chopper connects the resistor across the DC link. The excess energy is dissipated as heat. For the wider concept, see dynamic braking and regenerative energy.

When is a braking resistor needed?

Not every VFD application requires one. A fan or pump with a long deceleration time may lose enough energy naturally through aerodynamic, hydraulic and mechanical losses. A crane, hoist, centrifuge, press, winding machine or high-inertia conveyor may have to remove far more energy in a much shorter time.

Vertical-motion systems add another important factor. A descending load can release gravitational potential energy continuously. This is one reason why braking systems for cranes and hoists cannot be selected from motor kW alone.

How do you select a braking resistor?

1. Start with the drive manufacturer's permitted resistance. Never choose a resistance below the minimum value specified for the particular drive or brake chopper. Too little resistance can cause excessive chopper current and damage the braking transistor or power stage.

2. A higher resistance is not automatically safer. Increasing resistance reduces the braking power available at a given DC-link voltage. Under heavy regenerative loading, the resistor may then be unable to remove energy quickly enough and the drive may trip on DC-bus overvoltage.

3. Thermal power is a separate requirement. Resistance determines an important part of instantaneous chopper current and braking power. The resistor's wattage and pulse-energy capability determine how much heat it can safely absorb.

4. Braking time and repetition rate matter. A machine that brakes every ten seconds has a very different thermal duty from a machine that stops once an hour. Braking time, cycle time and duty ratio must therefore be considered together.

5. Check pulse-energy capability. A resistor can experience short bursts of power well above its continuous thermal rating, but only within the limits specified by its manufacturer.

Understanding braking energy

The rotational kinetic energy stored in a mechanical system can be approximated by:

E = ½ × J × ω²

where J is the total moment of inertia referred to the motor shaft and ω is angular speed. The relevant inertia may include the motor rotor, gearbox, drum, pulley, rolls and the machine load.

As a simple example, a system with a total inertia of 5 kg·m² rotating at 1500 rpm stores roughly 62 kJ of rotational energy. Removing that energy in five seconds corresponds to about 12 kW of average mechanical braking power before motor, drive and mechanical losses are considered.

This illustrates why rules such as “use a resistor equal to the motor kW” or “half the motor power is enough” are not reliable engineering methods.

Ohms and watts describe different things

Resistance in ohms strongly influences chopper current and instantaneous braking power. Power and energy ratings describe the resistor's ability to survive and dissipate heat.

If the chopper is assumed to be continuously on, instantaneous resistor power can be illustrated by P = V² / R. A real VFD brake chopper normally switches the resistor in pulses, however, so this equation alone is not a complete sizing method.

Resistance too low: excessive chopper current.
Resistance too high: insufficient braking power and possible DC-bus overvoltage.
Thermal rating too low: resistor overheating.
Duty too high: insufficient cooling time between braking events.

Braking resistor, DC injection braking and regenerative drive: are they the same?

No. With DC injection braking, the drive applies DC current to the motor stator to create braking torque. A significant part of the resulting heat is produced inside the motor.

With dynamic resistor braking, regenerated mechanical energy flows into the DC link and is then dissipated mainly in the braking resistor.

A regenerative drive or regenerative front end uses a different topology and can return suitable braking energy to the AC supply instead of converting it into heat. This can be particularly useful on machines that regenerate frequently or for long periods.

Installation and thermal safety

Braking resistors can reach very high surface temperatures during operation. They must be installed away from combustible materials with the clearances and ventilation specified by the manufacturer.

High-duty applications should use appropriate thermal protection, such as a resistor temperature switch or sensor where provided. Cable size, protective earth, terminal torque and maximum permitted resistor-cable length must follow the drive manufacturer's instructions.

If a resistor repeatedly overheats, simply installing a larger wattage unit should not be the first response. Check resistance, duty cycle, braking time, ventilation, chopper operation and whether the application would be better served by a regenerative system.

Braking resistor FAQ

Does every VFD need a braking resistor?
No. A resistor is required only when the application produces regenerative energy that the drive cannot otherwise manage within the required deceleration time.

Does a higher-wattage resistor produce stronger braking?
Not by itself. Braking behaviour depends on resistance, DC-link voltage and chopper capability. The wattage mainly describes thermal and energy-handling capacity.

Can I use a resistance below the value in the manual?
Do not go below the manufacturer's minimum permitted resistance. Doing so can overload the braking transistor or external chopper.

What happens if resistance is too high?
Available braking power decreases. During demanding deceleration the DC-link voltage may continue rising until the drive trips on overvoltage.

Is it normal for a braking resistor to become hot?
Yes. Converting electrical energy into heat is its purpose. Exceeding the resistor's specified temperature, pulse-energy or duty limits is not normal.

Does an OV / Overvoltage alarm always mean the resistor is undersized?
No. Short deceleration time, excessive inertia, incorrect resistance, an open resistor circuit, wiring problems, brake-chopper faults and high supply voltage are among the other possibilities.

Does a braking resistor recover energy?
No. It dissipates regenerative energy as heat. Returning that energy to the supply requires a regenerative drive or another suitable energy-recovery system.

Can two VFDs with the same kW rating use the same resistor?
Not necessarily. Minimum resistance, DC-link voltage, braking-transistor capability and permitted duty vary between manufacturers and drive models.

Braking resistor product groups

All Braking Resistors · 500 W · 1 kW · 2 kW and above · 10 kW