Dynamic Braking Resistors

Braking Energy Has to Go Somewhere

When a variable speed drive rapidly slows a heavy load, the motor acts as a generator, feeding energy back onto the drive’s DC bus. Motor and drive losses dissipate some of that energy, but a demanding stop can generate more than they can absorb. The resulting rise in bus voltage can force a longer stopping time or trigger an overvoltage trip, interrupting the controlled stop.

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When the Load Drives the Motor

Slowing a high-inertia load faster than friction turns the motor into a generator. An overhauling load does the same by supplying mechanical power to the motor, as when a hoist lowers a suspended weight. In either case, energy flows back onto the DC bus.

Without regenerative capability, the drive cannot return that energy to the supply. The bus capacitors provide limited storage, while electrical losses dissipate some energy as heat. If these are insufficient, bus voltage rises. The control system may extend the deceleration time or trip on overvoltage to protect the equipment.

For the process, that can mean a load travelling beyond its intended stopping point, an interrupted machine cycle, or production waiting while a fault is investigated and reset. With an overhauling load, the braking demand can continue throughout the movement, rather than occurring only during deceleration.

The required stopping time and operating cycle determine how much braking energy must be handled, and how quickly.

What Creates the Braking Demand?

Operating condition
Effect on braking demand

Rapid deceleration

Slowing a high-inertia load quickly returns stored mechanical energy through the motor to the DC bus.

Overhauling loads

A load such as a descending hoist supplies energy to the motor, requiring braking throughout the movement, not just during stopping.

Frequent stop cycles

Repeated braking leaves less time for the resistor to cool, increasing the average heat it must dissipate.

How a Braking Resistor Works

AC IN RECTIFIER DC BUS CAPACITORS BRAKE CHOPPER INVERTER MOTOR M DB RESISTOR ACTS AS A GENERATOR HEAT The brake chopper connects the resistor across the DC bus, allowing regenerated energy to dissipate as heat.
1

The motor becomes a generator

As the drive slows the load, or the load overhauls the motor, power flows backward through the inverter and charges the DC bus.

2

The bus voltage rises

The drive’s braking circuit watches the bus. At its turn-on level, the brake chopper switches on, connecting the resistor across the DC bus. Without sufficient energy dissipation, the drive may extend the stopping time or trip on overvoltage.

3

The resistor turns it into heat

The braking resistor absorbs the energy and dissipates it as heat. Its ohmic value determines how much power it can dissipate at a given bus voltage and must stay within the braking chopper’s current limits. Properly sized for the load and operating cycle, it supports faster stopping while keeping the stop under the drive’s control.

Sizing for the application

Every Stop Adds to the Thermal Load

A braking resistor converts the regenerated energy it receives into heat. Its resistance must allow sufficient braking power within the drive’s limits, while its power rating and thermal capacity must suit the operating cycle. Get the sizing wrong and the result can be a resistor that overheats, a stop that takes too long, or a braking circuit pushed past its limits.

Sizing parameter
What it determines

Ohms

Determines how much current flows through the resistor and how much braking power it can dissipate at a given DC bus voltage. Lower resistance allows more current and greater power dissipation.

Watts

The continuous power the resistor can dissipate. Average braking power depends on the energy in each stop and how often stops occur; peak power and energy per stop must also stay within the resistor’s ratings.

Duty cycle

Braking time divided by total cycle time. Short braking periods can exceed the continuous power rating within specified overload limits. Long or frequent braking requires sufficient thermal capacity and cooling.

Estimate Your Braking Resistor

Enter your drive rating and braking cycle to estimate the resistor requirements. Use the results as a starting point for sizing, not a final product selection.

HP
V AC
%
sec
sec

Deceleration: braking torque falls as the load slows, so the average power into the resistor is about half of the peak.

111.9 kW
Peak braking power
5.4 Ω
Maximum resistance
9.3 kW
Continuous power rating
560 kJ
Energy per stop

Illustrative estimate based on your inputs and the calculator’s assumptions. Motor and drive losses are not deducted, which increases the estimated energy to be dissipated. Final selection must account for the resistor’s thermal and overload ratings, the actual braking threshold, and the minimum resistance and maximum braking current specified by the drive manufacturer.

PRODUCT IMAGE — I-Gard dynamic braking resistor

I-Gard Dynamic Braking Resistors

Enclosed resistor assemblies built to your resistance and power requirements, helping prevent overvoltage trips during braking.

  • Connect through a braking chopper across the DC bus, with electrical components rated and tested to 1000 V DC.
  • Resistance selected to support the required braking power within the drive’s limits, with power and thermal ratings matched to the operating cycle.
  • Resistor elements mounted and interconnected at the factory, with a terminal block and over-temperature switch.
  • NEMA 1 enclosures as standard, with NEMA 3R and 4X options for outdoor and washdown applications.
  • Element options for heavy-load braking and demanding environments, including cranes, elevators, and mining equipment.

Brochure (C-800EB) CSA Certificate (C22.2 No.14)
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Five Resistor Technologies,
Matched to the Duty

Braking resistors don’t all see the same duty. Voltage, shock and vibration, overload requirements, and braking frequency vary by application. I-Gard builds each assembly using the resistor type that suits the duty, selecting the most economical style for your resistance and power requirements.

Type
Element
Where It Fits

CSW Smoothwound

Stainless resistive alloy wire wound on a porcelain tube, with a refractory cement coating

High resistance values or applications requiring an adjustable slider band

FDL Wirewound

Open helical stainless alloy wire wound around a grooved porcelain core

Most dynamic braking applications. The open element withstands considerably higher overloads than enamel-type resistors and cools faster because every turn is exposed to ambient air.

EW Edgewound

Stainless steel strip wound on its edge and supported by grooved porcelain supports

Applications requiring robust construction, with stainless steel terminals welded to the element

SSG Stainless Steel Grid

Stainless steel grid

Higher wattage per bank allows fewer resistors, less space, and lower cost. Suited to the shock and vibration of overhead and tower cranes, mining equipment, and pumps.

SSR Stainless Steel Ribbon

A 4-inch-wide Type 406 stainless steel element, formed into a serpentine shape from a continuous strip nearly 30 feet long, mounted on mica-insulated through-bolts

Large motor control and continuous duty in extreme conditions. Resistance increases by only 6% at the rated temperature rise of 375°C. Double-isolated to ground and rated at 1500 V for high-horsepower, high-voltage motors.

Why resistance stability matters: As resistance increases with temperature, the power the resistor can dissipate at a given DC bus voltage decreases. A stable resistance helps maintain braking capability throughout a long stop.

Built for DC Duty

I-Gard builds each assembly to your resistance and power requirements, with electrical components selected for the elevated DC bus voltages encountered during braking.

  • Rated and tested to 1000 V DC. All electrical components are rated and tested for operation at up to 1000 V DC.
  • CSA approved. Assemblies include a terminal block, electrical spacing, and over-temperature protection designed to meet the applicable certification requirements.
  • Thermal protection as standard. Every enclosure includes a normally closed over-temperature switch for connection to the installation’s protective circuit, helping prevent resistor burnout. A normally open switch is available as an option.

FAQ

Frequently Asked Questions

Answers to our most commonly asked questions. Can’t find yours? Ask here

It absorbs the energy a motor regenerates while a variable speed drive slows a load, and releases it as heat. Connected across the drive’s DC bus, it keeps the bus voltage from rising to the point where the drive trips on overvoltage, so the stop stays under the drive’s control.

Slowing a high-inertia load turns the motor into a generator, and that energy flows back onto the drive’s DC bus. If the bus voltage rises past the drive’s limit, the drive shuts down as a protective measure and the motor coasts to a stop. A braking resistor gives the energy somewhere to go.

No. For light loads and gentle stops, the drive’s DC bus capacitors and the mechanical losses in the system can absorb enough energy. A braking resistor is needed when the application demands faster deceleration, when the load is highly inertial, or when the load overhauls the motor.

In deceleration braking, the required torque falls as the load slows, so the resistor handles roughly half the power of an overhauling load. In an overhauling load, such as a hoist lowering, the load keeps driving the motor and the required braking torque stays constant for the whole cycle, so the resistor must be sized for about twice the power.

By three numbers: ohms, watts, and duty cycle. The ohmic value is the DC bus turn-on voltage squared, divided by the peak braking power, and it determines the braking torque. The wattage is the average power the resistor must dissipate, which depends on the energy in each stop and how often stops occur. Duty cycle is braking time divided by total cycle time. The calculator above gives a starting estimate.

Lower resistance does brake harder, but it also draws more current and generates more heat, and it can never go below the minimum ohm rating set by the drive or its braking module. A value that is too low can cause permanent damage to the drive or the braking chopper.

Every electrical component in the assembly is rated and tested to 1000 V DC, to suit the elevated DC voltages a braking resistor sees under braking. The stainless steel ribbon type is double-isolated to ground and rated 1500 V.

Every enclosure includes a built-in terminal block and a normally closed over-temperature switch as standard, to prevent resistor burnout. Wired into the drive’s control circuit, it lets the drive or control system respond to an over-temperature condition. A normally open thermal switch is available as an option.

The drive’s input voltage and horsepower, the braking torque required (usually 100% or 150%), the duty cycle (braking on-time and off-time), whether the load is decelerating or overhauling, and the minimum ohm rating or maximum braking current specified by the drive manufacturer.

Yes. NEMA 1 ventilated enclosures are standard for indoor locations, and NEMA 3R and 4X enclosures are available for outdoor and washdown locations.

Yes. I-Gard dynamic braking resistors are CSA approved, File C22.2 No. 14.

Sizing a Braking Resistor for Your Drive?

Send us your drive voltage and horsepower, braking torque, duty cycle, load type, and the minimum ohm rating from your drive manufacturer. I-Gard will build the resistor to your ohm and wattage requirements, with standard delivery in two weeks.

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