How a Braking Resistor Works

The operation of a braking resistor in a drive system with a variable frequency drive (VFD) is relatively simple.
When a DC voltage is applied to a braking resistor, current flows according to Ohm’s law (I = U / R).
The electrical energy is then converted into heat according to the power formula (P = I² × R).

The braking resistor must safely dissipate this heat to the surrounding environment.
In the short term, the resistor can absorb heat due to its thermal capacity.
This depends on the material properties, mass of the resistor and the temperature rise (ΔT).

If the temperature rises too high, damage may occur.
For this reason, braking resistors are typically designed with a large cooling surface to dissipate heat through natural air cooling.
In some applications, forced air cooling or water cooling is used.

Braking Resistors in Regenerative Operation

Braking resistors are used when an electric motor feeds energy back into the drive system during operation.
This occurs, for example, when a motor decelerates a load or controls a lowering load.

When a motor is controlled by a
variable frequency drive (VFD),
electrical energy can be generated during so-called regenerative operation.
This energy increases the voltage in the DC bus of the VFD.

If the DC bus voltage rises too high, the VFD may trip or shut down to protect the electronics.
By using a brake chopper (often integrated in the VFD) in combination with a braking resistor,
the excess energy is safely dissipated.

More technical background on VFDs and braking systems can be found in the
Fluxcon VFD wiki.

When is a Braking Resistor Required?

In many industrial applications, a braking resistor is required to safely dissipate excess energy.
This is common in situations such as:

• Hoisting systems where loads are lowered
• Fast deceleration or precise positioning of loads
• Applications with high inertia, such as large fans
• Situations where motors temporarily oppose each other

When a heavy load must be decelerated or an external force must be controlled,
regenerative energy is generated that must be dissipated by the drive system.

Selection of a Braking Resistor

When selecting a braking resistor, the correct resistance value (Ohms) is critical.
Each VFD has an allowable resistance range.

A resistance that is too low can lead to excessive current and damage to the VFD.
A resistance that is too high may result in insufficient energy dissipation.

In addition, the thermal power rating of the braking resistor is important.
Depending on the application, different amounts of energy must be converted into heat over short or extended periods.

For questions about selecting braking resistors or their application with VFDs,
please visit our VFD FAQ.

Aluminum Braking Resistors IP67

• easy installation
• waterproof
• brand-independent

Braking Resistor Selection

When selecting a braking resistor, the correct resistance value is essential.
Each VFD specifies an acceptable resistance range.

If the resistance is too low, current may become too high and damage the drive.
If the resistance is too high, the drive may not be able to dissipate sufficient electrical power.

The thermal rating is also critical.
Depending on the application, more or less energy must be dissipated as heat,
and for different durations.

For advice, please contact Fluxcon specialists.

Brake Chopper

Up to 15 kW, brake choppers are integrated in the
compact VFD series.
These integrated brake choppers can handle up to 50% of the nominal current to the braking resistor.

An external brake chopper can handle up to 150% of the nominal current.

Above 15 kW, VFDs are typically not equipped with a built-in brake chopper.
In these cases, an external brake chopper is required when regenerative energy is present.

The brake chopper monitors the DC bus voltage and activates when the voltage exceeds a threshold,
diverting energy to the braking resistor.

Brake Choppers

• optional on VFD
• up to 150% nominal current
• brand-independent

Fluxcon Drive Product Range

Fluxcon offers a range of electronic components for controlling electric motors,
including variable frequency drives, soft starters, braking resistors, brake choppers, line reactors and EMC filters.

Customers choose Fluxcon for its practical approach, excellent price-performance ratio and fast delivery.
Fluxcon engineering provides solutions based on real-world experience.

Simple solutions are often the best.
Fluxcon values transparency and works closely with customers to find the optimal solution for each application.

All products are brand-independent.
For example, braking resistors can also be used with third-party VFD brands.