How to size a braking resistor for an inverter

During braking, the motor regenerates energy back into the DC bus: if that energy isn’t managed, the voltage rises until the protections trip.

The resistor absorbs it and dissipates it as heat. The key is choosing the right one for the application, and the variable that really makes the difference is the duty cycle, not just the motor power.

Let’s look at what you need to arrive at the right value.

Motor power alone is not enough: you need to cross-check the inverter’s characteristics, the energy to be dissipated and, above all, the actual duty cycle of the application.

When is a braking resistor needed?

A braking resistor is needed when the energy regenerated during deceleration exceeds what the drive can handle on its own.

This typically happens with:

• rapid decelerations;
• high load inertia;
• frequent acceleration and braking cycles;
• handling of vertical loads;
• the need to quickly reduce speed or stop the motor.

The more frequent the cycles, the less margin remains before the chopper kicks in, and that is where sizing by guesswork costs the most.

The resistor connected to the braking circuit converts the energy into heat and dissipates it. This is typical, for example, of lifting, industrial handling, machine tools and railway applications.

What data do you need to size a braking resistor?

Before choosing the resistor, you need to gather some information about the inverter and the application.

The main data include:

These elements make it possible to determine not only the resistance value, but also the energy and power the component will need to handle.

Ohmic value: why it must be compatible with the inverter

One of the fundamental aspects is the resistance value, expressed in ohms (Ω).

The first reference should always be the inverter manufacturer’s documentation, which normally states the minimum allowed value or a range of usable values.

Going below the specified minimum resistance can cause excessive current in the braking circuit and subject the chopper to stresses beyond its ratings.

At the same time, choosing an excessively high resistance can limit the available braking power.

This is why the ohmic value should not be chosen on the basis of the motor alone: it must be checked against the specific characteristics of the drive.

Peak power and average power are not the same thing

Another possible mistake is to consider only the resistor’s rated power.

A braking resistor may have to dissipate very high power for a few seconds and then have a long enough interval to cool down.

That’s why, when sizing, it is important to distinguish between:

Two machines that require the same power during braking may therefore need different resistors if one brakes occasionally and the other repeats the same operation many times per minute.

Duty cycle: how often does braking occur?

The duty cycle indicates how long the resistor is engaged in braking relative to the total cycle time.

For example, braking that lasts 5 seconds and repeats every 50 seconds corresponds to a 10% duty cycle.

This figure is fundamental because it determines the time available to dissipate the accumulated heat.

As the frequency and duration of braking increase, so does the thermal load on the resistor.

That’s why knowing the braking power alone is not enough: you need to know for how long and how often that power will have to be dissipated.

How is the braking resistance calculated?

As a first approximation, when the DC bus voltage at which the braking circuit trips and the peak power to be dissipated are known, the relationship between these values is:

Even knowing the voltage and peak power, the resulting value remains a first theoretical reference: it cannot be used in isolation.

It must be checked against the limits specified by the inverter manufacturer and the characteristics of the braking chopper.

In particular, the chosen value must never fall below the minimum resistance allowed by the drive.

How do you determine the required power?

To size the resistor correctly, you need to assess how much energy must be dissipated during each braking event and how often the cycle occurs.

With a predominantly inertial load, the stored kinetic energy depends on the total moment of inertia and the rotational speed.

Once the energy to be dissipated by the resistor has been determined, it can be related to the braking time and to the total cycle time to assess, respectively, the power required during braking and the average power.

In more complex applications, such as lifting, vertical loads or systems with variable cycles, the other energy components present in the system must also be taken into account.

A simplified example

This is precisely why two resistors with the same ohmic value may not be equivalent for the same application.

Don’t forget the installation conditions

Electrical sizing is only part of the choice.

The resistor converts braking energy into heat and must be able to dissipate it under the real conditions in which it will be used.

You therefore also need to consider:

• ambient temperature;
• natural or forced ventilation;
• installation inside or outside the control panel;
• available space;
• required degree of protection;
• presence of dust, humidity or corrosive agents;
• mounting method.

A solution that is correctly sized from an electrical standpoint may require different construction features depending on the installation environment.

When a standard resistor isn’t enough

Not every application can be solved by choosing a catalogue component.

There may be constraints on size, power, duty cycle, degree of protection, connections or environmental conditions that call for a specific configuration.

In these cases, it is possible to design a custom resistor based on the electrical, thermal and mechanical characteristics of the application.

Grid resistors

RAG model

Resistor banks made of GRF-type grids assembled in packs, for starting and braking. →