Brake Chopper Circuits in VFDs

Braking circuits are essential to the proper operation of systems that employ a variable frequency drive (VFD) and electric motor controlling a high inertia load. Cranes, elevators, and centrifuges are just a few applications in which the VFD will be subject to potentially large amounts of regenerated energy from the motor.

What Is a Brake Chopper Circuit?

In a VFD, a brake chopper circuit is an electric circuit that activates when excess energy from a decelerating motor raises the DC bus voltage beyond a safe level. The chopper rapidly switches a braking resistor on and off to dissipate that excess energy as heat. This prevents overvoltage faults and protects the drive.

When a motor slows down or is driven by a load, it can act like a generator. This creates a counter electromotive force (CEMF), which sends energy back to the drive. The rate of deceleration/overhauling and size of the load will determine how much CEMF voltage is generated. This voltage will add to the voltage already present on the drive’s DC bus capacitors.

In general, VFDs are not designed to return energy from a motor back to the utility. The drive’s input rectifiers allow power to flow in, but not back to the utility.

Related Article: How Pulse Width Modulation Works in a VFD
Consequently, the DC bus voltage will increase with the voltage received back from the motor. If the voltage level rises above the maximum rating of the capacitors, then damage will result.

When Is a Brake Chopper Required in a VFD?

A brake chopper is required when a motor connected to a VFD generates significant regenerative energy during deceleration or when driving an overhauling load. In these situations, the returned energy raises the DC bus voltage, and the brake chopper dissipates the excess energy through a braking resistor to prevent overvoltage faults.

Let’s take a look at how this is accomplished with the KEB F6 inverter.

Brake Chopper

A braking circuit, often called a chopper circuit, is controlled by the drive. It uses a power transistor and braking resistors connected across the DC bus.

F6 drives come in both 200V and 400V models. Working with a 400V model F6 on a 3-phase supply voltage of 480VAC, the drive will measure an idle bus voltage of 672VDC (480VAC x √3). The default voltage level at which the F6 will trigger the braking transistor is 780VDC (380VDC for 200V models) – this level is programmable.

The F6 will monitor the bus voltage and if it begins to rise above the trigger level, it will turn on the braking transistor, sending current flow to the braking resistor(s) where the energy will be used up in the form of heat.

Once the bus voltage falls below the trigger level, the F6 will turn off the braking transistor. The F6’s maximum DC bus voltage level is 840VDC (420VDC for 200V models). If for some reason there is a failure in the braking circuit (i.e. open braking resistor) and the excess voltage can’t be dispersed, the F6 will trigger the error E.OP (over-potential) when the DC bus voltage reaches higher than the rated maximum value. When the error is triggered, the drive will stop operation.

Brake Chopper Circuits: Common Industries

• Material Handling & Logistics. These systems often lift or move heavy loads and then decelerate quickly, causing the motor to regenerate energy that must be managed by a brake chopper. Examples: overhead cranes, hoists, and conveyors.

• Elevators & Vertical Transportation. When elevators descend with heavy loads, the motor becomes a generator and sends energy back into the drive, requiring braking circuits to protect the DC bus. Examples: passenger elevators, freight elevators, and escalators.

• Industrial Processing & Manufacturing. These machines often involve large rotating masses that must slow down quickly during production cycles. Examples: centrifuges, rolling mills, and large mixers.

• Transportation & Motion Systems. High-speed motors in these systems frequently transition between motoring and braking modes, creating significant regenerative energy. Examples: electric trains and rail systems.

• Theater & Stage. Motorized fly systems, winches, and hoists are used to raise and lower scenery, lighting trusses, curtains, and stage props.

Integrated Brake Transistor

The KEB F6 Combivert excels in applications that demand muscular and reliable braking. It has a strong, integral braking transistor rated for a 100% duty cycle. Some VFD manufacturers do not incorporate the braking transistor in their drives and instead, offer a separate braking module. This means more space is required to accommodate extra hardware at added cost.

Moreover, the braking transistor duty rating of other manufacturers’ drives is often less than 100%, which means that the drive or braking module may need to be oversized in order to handle the braking current.

Given these considerations, it is easy to see why the F6 is the VFD of choice for many OEMs.

When implementing a braking circuit, the size and specifications of the braking resistor(s) must also be taken into account. They must be sized correctly or damage can occur. If the resistance is too low, the braking transistor’s rated maximum braking current could be exceeded, causing damage to the braking circuit.

If the resistance is too high, then the excess voltage will not be sufficiently dissipated, resulting in over-voltage. Over time, repeated exposure to over-voltage can cause damage to the DC bus capacitors, resulting in drive failure. For assistance with proper resistor sizing, KEB provides the minimum and typical resistance ratings along with other braking circuit data in the front of the drive manual.

Regenerative Drives

As an alternative to the traditional braking circuit, KEB offers the R6 regenerative unit. This unit acts as the power supply for the VFD and transfers regenerative energy from the motor back to the line. The returned energy can be used by other electrical loads in the building and since there is no heat being generated by a braking resistor, less energy is used for cooling the machine room. This can mean significant energy cost savings.

The R6 is coupled to the DC bus of the VFD and like the VFD, it constantly monitors the DC bus voltage. The R6 detects the supply voltage level and will automatically configure itself for 200VAC or 400VAC operation. When the bus voltage starts to go above 103% of the nominal DC bus voltage, the R6 will change from standby to regen mode; returning the regenerative energy to the mainline (480VAC supply = 672VDC bus; the R6 would regenerate around 692VDC).

Auxiliary braking resistors may still be needed when the R6 unit cannot operate in regen mode (i.e. building running off of UPS supply or battery backup). The F6 triggers its braking transistor at a higher voltage level than the regenerative voltage level of the R6, giving the braking system some redundancy when needed.

Brake Choppers vs. Regenerative Drives

Brake choppers and regenerative drives both take on the problem of excess energy generated when a motor slows down. The brake chopper protects the VFD by diverting the energy to a braking resistor. There, the energy is safely dissipated as heat.

A regenerative drive, on the other hand, takes a different approach. Regenerative drives feed recovered energy back into the facility’s electrical supply. This then allows for the energy to be reused by other equipment rather than being lost, improving overall system efficiency.

Whatever the application, the KEB F6 and R6 provide the perfect braking solution.

FAQs

Why is a brake chopper used in VFD?
A brake chopper is used to handle extra energy created when a motor slows down. It sends that energy to a braking resistor so the VFD’s DC bus voltage does not rise too high.

How does a chopper circuit control the speed of a DC motor?
A chopper circuit controls speed by switching the voltage to the motor on and off very quickly. By changing how long the voltage stays on, it adjusts the average voltage and the motor speed.

What happens if a VFD does not have a brake chopper?
Without a brake chopper, energy from a slowing motor can cause the DC bus voltage to rise too high. This may trigger an overvoltage fault or damage the drive over time.