Why VFDs Need a Brake Resistor
When an induction motor decelerates, it enters regenerative mode and pumps energy back into the DC bus of the variable frequency drive. If that energy has nowhere to go, bus voltage climbs above the drive's overvoltage trip threshold — and the VFD either faults on an OV error or sustains permanent damage to its DC-link capacitors.
A brake resistor connects across the DC bus through a braking chopper transistor and dissipates the excess energy as heat. This lets you:
- achieve very short stopping times — essential for hoists, centrifuges, winding machines and test benches;
- keep the DC bus voltage stable during deceleration of high-inertia loads;
- eliminate nuisance OV faults without reducing ramp rates.
Soft starters have no internal braking chopper, so for applications requiring controlled, fast deceleration a VFD with an external brake resistor is the right tool.
How the Braking Circuit Works
Most drives from 1.5 kW upward have an internal braking chopper transistor. It switches on when the DC bus voltage exceeds a set threshold (typically 680–760 V on a 380 V supply). Current flows through the external resistor, converting kinetic energy to heat.
If your drive lacks a built-in chopper (common in budget models below 0.75 kW), you will need an external brake unit in addition to the resistor.
Brake Resistor Calculation Formula
Two parameters must be determined: resistance (Ω) and power rating (W).
Minimum Resistance
Too low a resistance will destroy the braking transistor. The floor value is:
R_min = U_DC² / P_VFD
where U_DC is the DC bus voltage at chopper turn-on (≈ 680 V for a 380 V supply) and P_VFD is the rated drive power in watts.
Example for a 7.5 kW drive:
R_min = 680² / 7500 ≈ 61.6 Ω → select a standard 68 Ω or 72 Ω resistor
Required Power Rating
The continuous power rating depends on the braking duty cycle (ED%):
P_R = P_brake × ED%
where P_brake is the peak braking power ≈ 1.0–1.5 × P_VFD and ED% is the fraction of total cycle time that the resistor is active.
Typical ED% values: 5–10% for occasional stopping, 20–40% for repetitive duty (hoists, presses, conveyors).
Example for a 7.5 kW drive, ED = 10%:
P_R = 7500 × 1.0 × 0.10 = 750 W → select a 68 Ω / 1000 W resistor
Brake Resistor Selection Table by VFD Power
Values are for a 3×380 V supply with ED = 10%. At ED 20–40%, multiply resistor wattage by 2–4 accordingly.
| VFD Power | R min, Ω | Recommended R, Ω | Resistor Power (ED 10%) | Resistor Power (ED 30%) |
|---|---|---|---|---|
| 1.5 kW | 308 | 300 | 150 W | 450 W |
| 4 kW | 116 | 120 | 400 W | 1200 W |
| 7.5 kW | 62 | 68 | 750 W | 2250 W |
| 15 kW | 31 | 33 | 1500 W | 4500 W |
| 22 kW | 21 | 22 | 2200 W | 6600 W |
| 37 kW | 12.5 | 13 | 3700 W | 11 kW |
| 55 kW | 8.4 | 10 | 5500 W | 16.5 kW |
Wiring a Brake Resistor
The resistor connects to the PB and + terminals on the drive's DC bus (some manufacturers label them B1/B2 or P/PB). Follow these steps:
- Switch off mains power and wait at least 5 minutes for the DC bus to discharge before touching any terminals.
- Run a cable from the + (P, P+) terminal to one end of the resistor.
- Run a cable from the PB (B, BR) terminal to the other end of the resistor.
- Use heat-resistant cable rated at least 105°C, sized for the peak braking current.
- Mount the resistor outside the enclosure or in a dedicated ventilated compartment — it gets very hot during braking.
- In the VFD parameters, enable the braking chopper function and set the overvoltage trip threshold (typically 680–760 V).
Keep cable runs as short as possible. Long leads add inductance that can spike voltage across the chopper transistor. Stay under 5 metres; use shielded cable for longer runs.
Practical Tips
- Resistor type: choose wirewound or ribbon resistors made from stainless-steel alloy. They handle repeated thermal shock far better than metal-oxide types.
- Overheat protection: wire an NC thermostat or thermal fuse in series with the resistor and connect it to the VFD's external fault input — if the resistor overheats, the drive trips safely.
- Multiple resistors: for drives above 37 kW it is common to parallel or series-connect several smaller resistors for easier installation, replacement and cooling.
- Never go below R_min: under-resistance is the single most common cause of braking transistor failure — do not do it.
Where to Buy
Chastotnik.ua stocks brake resistors compatible with INVT, Veichi, Danfoss, Delta and Mitsubishi drives. You can also browse the full range of VFD accessories — DC link chokes, EMC filters and remote keypads.
Need help selecting the right resistor for your application? Contact our engineers via the feedback form or by phone — we will match the correct ohm and watt rating to your specific drive and duty cycle.
For applications where fast stopping is not required, explore our range of electric motors with high holding torque, or consider a soft starter with a built-in controlled deceleration ramp.
