Braking Resistor Calculator

Braking Resistor Calculator

When a motor decelerates, it acts as a generator and feeds kinetic energy back into the VFD’s DC bus. If that energy is not absorbed, the bus voltage rises until the drive trips on overvoltage or the capacitors are damaged. A braking resistor, switched across the DC bus by the drive’s internal braking transistor (IGBT), converts that regenerative energy into heat. The calculator above sizes the resistor: enter the DC bus voltage and motor power, and it returns the minimum resistance, recommended power rating, and energy absorbed per braking cycle.

Core Formula

A resistance below R_min draws too much current and risks damaging the braking IGBT. The continuous power rating accounts for duty cycle — a resistor that brakes 10% of the time only dissipates 10% of peak power on average, but the 1.5× margin covers thermal lag and worst-case cycling.

Calculator Inputs

V_DC — DC bus voltage inside the VFD. For a 230 V single-phase drive: ~325 V (230 × √2). For a 460 V three-phase drive: ~650 V. Check the drive manual for the exact braking threshold voltage.

P (Motor Power) — rated motor power in kW, watts, or horsepower. Sets the maximum regenerative power the braking circuit handles.

T (Braking Time) — duration of each braking event in seconds. Optional, but needed for energy-per-cycle calculation.

D (Duty Cycle) — percentage of total operating time spent braking. A conveyor that brakes 3 seconds every 60 seconds = 5% duty cycle. Determines the resistor’s continuous power rating.

T% (Braking Torque) — percentage of rated motor torque during braking. 100% = full-torque stop. Servo applications may use 150% for rapid deceleration.

Worked Example — 460 V / 15 kW Industrial Drive

V_DC = 650 V, motor power = 15 kW, braking torque = 100%, braking time = 4 seconds, duty cycle = 10%.

Select a braking resistor of ≥28.2 Ω (standard value: 30 Ω or 33 Ω) with a continuous power rating of at least 2,250 W. The peak current at the instant braking engages: I = 650 / 28.2 ≈ 23 A.

Servo Example — 1 kW with 150% Braking Torque

V_DC = 325 V, motor power = 1 kW, T% = 150%, braking time = 0.5 s, duty cycle = 30%.

A 75 Ω resistor rated for at least 675 W continuous handles this servo application. The high duty cycle (30%) is the dominant sizing factor — even though the motor is small, frequent braking demands serious heat dissipation. For detailed power dissipation analysis, check the resistor’s thermal derating curve against its mounting conditions.

Why Minimum Resistance Matters

The braking IGBT inside the VFD has a maximum current rating. R_min ensures the peak braking current (V_DC / R) stays below that limit. Going lower than R_min does not stop the motor faster — it overloads the transistor. Most VFD manufacturers specify a minimum allowable braking resistance in the drive manual. If your calculated R_min is lower than the manufacturer’s spec, use the manufacturer’s value.

Duty Cycle and Continuous Power Rating

The duty cycle determines the average thermal load on the resistor over time. A 15 kW peak braking load at 5% duty cycle only requires 750 W average dissipation (plus safety margin). This is why braking resistors are often rated for far less than the peak braking power — they cool down between cycles.

However, the resistor must survive each individual braking pulse without overheating. Wire-wound and stainless-steel grid resistors are designed for this: high thermal mass absorbs the pulse energy, then radiates the heat before the next cycle. If duty cycle exceeds ~20%, consider forced-air cooling or a larger resistor bank.

DC Bus Voltage by Drive Type

The DC bus voltage depends on the AC supply and the rectifier topology:

The braking chopper typically activates at a threshold above the nominal bus voltage — often 115–120% of the rectified value. Use the threshold voltage from the drive manual for the most accurate R_min calculation.

Resistor Construction Types

Wire-Wound Resistors

Ceramic-housed wire-wound resistors handle moderate power (up to a few kW) and are common in servo and small industrial drives. They are compact but have limited pulse energy capacity. Adequate for low duty cycles and short braking events.

Stainless-Steel Grid Resistors

Grid resistors stack stamped metal elements in a frame with natural or forced-air cooling. They handle tens of kW continuously and absorb large pulse energies. Standard choice for industrial VFD braking on conveyors, cranes, hoists, and centrifuges.

Resistor Banks

For very high power or massive energy pulses, multiple resistors are connected in series or parallel to share the load. Series increases total resistance and voltage handling. Parallel decreases resistance and increases current capacity. The calculator’s R_min and power rating guide the bank configuration.

Common Applications

Conveyors and Material Handling

Conveyors brake frequently but briefly — low duty cycle, moderate braking power. A single wire-wound or small grid resistor is usually sufficient.

Cranes and Hoists

Lowering a load converts potential energy into electrical energy continuously. Duty cycles can reach 40–60%, demanding a high continuous power rating and forced-air cooling.

Centrifuges and Flywheels

High-inertia loads store enormous kinetic energy. Braking events are infrequent but long — the resistor must absorb tens or hundreds of kJ per cycle. Energy per cycle (E = P × T) is the dominant sizing factor.

Servo and CNC Axes

Rapid, frequent deceleration at 100–150% torque. Short braking times but high duty cycles. Compact wire-wound resistors mounted near the drive are typical.

Frequently Asked Questions

Last updated: March 2026