Electric Motor Regenerative Braking: Brake Resistor, Chopper and DC Bus Overvoltage Management

An electric motor is not always an energy-consuming element; when the load starts driving the motor, for example while a lift descends, a crane lowers a load or a high-inertia machine decelerates rapidly, the motor turns into a generator and pumps energy back into the drive's DC bus. If this returning energy is not managed, the drive's DC bus voltage rises dangerously and the drive trips with an overvoltage fault to protect itself; in severe cases hardware damage occurs. In this article, as HEM Motor, we cover regenerative and dynamic braking in electric motors, how DC bus overvoltage arises, how energy is burned with a braking resistor and chopper, how energy is returned to the grid with regenerative feedback units (AFE), resistor sizing, and which solution is correct for which application.

Where Does the Energy Go During Braking?

While the motor decelerates or is driven by the load, the kinetic energy of the rotating mass or the potential energy of the load is converted into electrical energy. In a drive-based system this energy cannot flow back to the grid through the diode bridge at the drive's input stage, because a standard diode bridge is unidirectional. As a result, the energy accumulates in the DC bus capacitors inside the drive and raises the voltage. When the DC bus voltage exceeds a certain threshold, the drive issues a "DC bus overvoltage" fault. There are three fundamental ways to solve this: convert the energy into heat in a resistor (dynamic braking), return the energy to the grid (regenerative braking/AFE), or extend the deceleration to reduce the returning power.

For the basis of brake motor and duty type selection, our article Duty Type S7, S8, S9: Braked and Variable-Load Operation helps; for the distinction between continuous and intermittent operation, Duty Type (S1-S6) Selection provides a good foundation.

Dynamic Braking: Braking Resistor and Chopper

The most common and economical solution is to burn the returning energy as heat in a braking resistor. In this system the braking chopper is a semiconductor switch that continuously monitors the DC bus voltage. When the voltage exceeds the set threshold, the chopper engages, connects the braking resistor to the DC bus and the excess energy is dissipated as heat in the resistor; when the voltage drops, the chopper disengages. In many small-to-medium power drives the chopper is built in and it is enough to connect only an external braking resistor; at higher powers an external chopper module is required.

• Advantage: Simple, economical, reliable; sufficient for most applications.
• Disadvantage: Energy is not recovered, it is wasted as heat; heating and energy waste with frequent braking.
• Caution: Since the resistor heats up, ventilation and fire safety are important.

Braking Resistor Sizing

A braking resistor is sized by two fundamental parameters: resistance value (ohm) and power capacity (watt). The resistance value must not fall below the minimum resistance allowed by the chopper and the drive; otherwise the switching current stresses the drive. The power capacity is selected according to the average power produced in the resistor during braking and the frequency of the braking cycle (duty ratio). An application that brakes frequently and for long periods needs a much higher wattage resistor than one that brakes briefly.

For the motor's heating limit during braking and frequent start-stop behaviour, our article S3/S4 Intermittent Duty: Heating Limit helps; for heating in jogging and inching applications, Jogging and Frequent Start-Stop (Inching) is directly relevant.

Regenerative Braking: Feedback (AFE) Units

For applications that want to return energy to the grid instead of dumping it as heat, regenerative (feedback) units are used. These systems have an active input stage (Active Front End, AFE) that allows bidirectional power flow instead of a standard diode bridge. During braking, the AFE pumps the excess energy arriving at the DC bus back to the grid in a controlled manner; thus energy is recovered rather than turned into heat. In applications that brake frequently and heavily, with long running hours and high energy cost, this solution provides significant energy savings compared with a braking resistor.

AFE units can also contribute to power quality by providing low-harmonic input current; for the topic of drive-induced grid harmonics and THD, our article Drive-Induced Grid Harmonics and Power Quality (THD) is a related resource.

Which Solution for Which Application?

The correct braking solution depends on the application's braking frequency, the amount of returning energy and the energy cost:

• Fan, pump: Usually little braking need; free coasting is often sufficient.
• Conveyor, transport: Moderate braking; a braking resistor is usually sufficient and economical.
• Crane, lift, hoisting: Energy returns continuously while lowering a load; a regenerative (AFE) solution makes sense.
• Centrifuge, high inertia: AFE saves energy on frequent and large decelerations.
• Test bench, dynamometer: Continuous generator mode; AFE is almost mandatory.

For the supply of brake motors in conveyor and crane applications, our article IE4 Brake Motor: Conveyor and Crane helps; for correct ordering with accessory options such as brake, encoder and forced fan, Accessory Options is complementary.

Not Confusing Mechanical Brake With Electrical Braking

An important distinction: the regenerative and dynamic braking described in this article is the electrical deceleration of the motor. The mechanical brake (electromagnetic disc brake) mounted at the rear of the motor is a separate component, usually used to stop the motor or hold it at standstill (holding brake). In crane, lift and hoisting applications the two are used together: while electrical braking decelerates the load in a controlled manner, the mechanical brake handles safe stopping and holding. In a correct system these two braking types complement each other; electrical braking protects the mechanical brake from overload and extends brake pad life.

DC Bus Voltage and the Braking Threshold

Understanding numerically how and why DC bus overvoltage arises is the basis of choosing the right braking solution. A three-phase drive rectifies the grid voltage and produces a DC bus voltage roughly equal to the peak value of the line voltage. When the motor turns into a generator during braking, this voltage begins to rise rapidly. The drive triggers the chopper when a certain braking threshold voltage is exceeded; if there is no chopper or braking resistor, the voltage reaches the protection threshold and the drive trips. The greater the braking torque and the faster the deceleration, the more the returning power and therefore the rate of voltage rise. So an application wanting to stop a high-inertia load in a very short time is the most demanding braking scenario and requires a resistor at the highest peak power.

Extending the braking time (slow ramp) lowers the returning peak power and can sometimes solve the problem without extra hardware; but if the process requires fast stopping, a braking resistor or AFE is unavoidable. To see the effect of rated voltage and frequency on motor behaviour, our article Rated Voltage and the 50/60 Hz Difference helps.

Braking Resistor Mounting and Safety

A braking resistor is a component that reaches high temperature in operation; therefore its mounting and safety must not be neglected. The resistor must be mounted away from flammable materials, in a well-ventilated location, with adequate clearance above and around it. Many braking resistors include a thermal switch for overheating protection; this contact must be wired into the control circuit to stop the drive when the resistor reaches a dangerous temperature. The resistor wiring must be kept as short as possible due to high-frequency switching and chosen in a suitable cross-section. For resistors mounted inside a panel, ventilation or external mounting should be considered so that the generated heat does not raise the panel temperature. These measures prevent both fire risk and early failure.

• Mount the resistor away from flammable surfaces, in a ventilated place.
• Wire the thermal protection contact into the drive's stop circuit.
• Choose short, suitable-section, heat-resistant cable.
• Provide ventilation against heat build-up for in-panel mounting.

For the economics of energy recovery and monitoring winding temperature under frequent braking, see Winding Temperature Monitoring: PT100 and PTC; for grounding and EMC in a VFD system, Grounding and EMC is complementary.

Energy Sharing With a Common DC Bus

In plants where several drives operate together, a smart way to manage braking energy is the common DC bus architecture. In this approach the DC buses of several drives are connected together; thus the energy produced by one motor while braking can be used directly by another motor on the same bus that is accelerating at that moment. This requires neither converting the energy into heat nor returning it to the grid; the energy simply shifts within the system. Especially in multi-axis machines where one axis brakes while another starts, in textile and paper lines and in winding-unwinding applications, this architecture provides significant energy savings. The common DC bus is often combined with a single central regenerative (AFE) unit to provide both internal sharing and return of excess energy to the grid. This is one of the most efficient braking solutions in large, continuous processes. For motor supply for uninterrupted production in continuous processes, see Paper and Textile Lines in Continuous Processes.

Frequently Asked Questions

Should I choose a braking resistor or a regenerative unit?

On applications that brake infrequently and briefly, a braking resistor is economical and sufficient. On applications that recover frequent and large energy, such as cranes, lifts and dynamometers, a regenerative (AFE) unit is more sensible in the long run thanks to energy savings.

Why does the DC bus overvoltage fault occur?

When the motor turns into a generator during braking and pumps energy into the DC bus, if this energy is not burned or returned, the bus voltage rises and the drive trips with an overvoltage fault to protect itself. The solution is to add a braking resistor or a regenerative unit.

Why is the wattage of the braking resistor important?

The wattage (continuous power) of the resistor must be chosen according to the braking frequency and duration. A low-wattage resistor overheats and may burn out under frequent braking. Peak power, average power and the duty ratio must be calculated together.

Run Your Motor Safely With the Right Braking Solution

As HEM Motor, we clarify with you whether your application's braking profile needs a braking resistor and chopper or a regenerative feedback (AFE) solution, and evaluate resistor sizing and motor-drive compatibility together. Share your load type, inertia and braking frequency; let us identify the right solution and provide a tailored quote for fast delivery from manufacturer stock so you can build a safe system without experiencing the DC bus overvoltage problem. Contact us to request a quote.