VFD Braking Resistors: Braking Choppers, Ohmic Sizing & Thermal Safety

When a Variable Frequency Drive (VFD) commands an AC induction motor to decelerate rapidly or hold a overhauling load (such as a crane hoist or unwinder), the motor acts as a generator. Kinetic energy flows backwards from the mechanical load into the drive’s DC bus.

Without a way to handle this surplus energy, the DC bus voltage rises rapidly until the drive trips on an Overvoltage Fault (OU/OV) to protect its internal capacitors.

Equipping a VFD with a Braking Chopper and an external Braking Resistor safely dissipates this regenerative energy as heat, allowing precise deceleration and stopping control.

This guide explains dynamic braking mechanics, how to select the correct resistance and power ratings, and critical installation safety standards.

1. How Dynamic Braking Works in VFDs

Dynamic braking involves three key hardware elements operating on the drive’s DC bus:

[ Decelerating Motor ] ── (Regenerative Energy) ──> [ VFD Inverter Stage ]
                                                           │
                                                           ▼
                                                   [ DC Bus Voltage Rises ]
                                                           │
                                                           ▼
                                              [ Braking Chopper (IGBT Switch) ]
                                                           │  (Fires when voltage > threshold)
                                                           ▼
                                               [ External Braking Resistor ]
                                                  (Dissipates Heat to Air)

1. Generator Action: As motor synchronous speed drops below actual rotor speed, energy flows back through the drive’s inverter stage into the DC bus capacitors.

2. Chopper Activation: A braking chopper (an internal or external IGBT switch) monitors the DC bus voltage. When the voltage exceeds a pre-set threshold (typically around 380V DC for 230V drives, or 750V DC for 400V/480V drives), the chopper transistor switches on.

3. Resistor Dissipation: The braking chopper routes the excess DC energy into the external braking resistor bank, where it is safely converted into thermal energy.

2. Key Steps for Selecting a Braking Resistor

Selecting the wrong resistor can burn out the drive’s IGBT switch or fail to stop the motor in time. Selection depends on two primary metrics: Ohmic Value ($\Omega$) and Power Rating (Watts).

Step A: Determine Minimum Resistance ($R_{min}$)

The VFD’s braking chopper transistor has a strict maximum current limit. The total resistance of the connected braking resistor bank must never be lower than the drive’s specified $R_{min}$ value.

• Too Low (R < $R_{min}$): Draws excess current, destroying the chopper transistor.

• Too High (R > $R_{ideal}$): Limits current flow, meaning the drive cannot dissipate energy fast enough to prevent an overvoltage trip.

Step B: Calculate Peak Braking Power ($P_{peak}$)

Peak power occurs at the instant deceleration begins from maximum speed:

P_peak = (V_dc)^2 / R

Where $V_{dc}$ is the chopper activation voltage threshold, and R is the selected resistance value.

Step C: Calculate Continuous Power Rating ($P_{br}$) Based on Duty Cycle

Braking resistors are rated for continuous heat dissipation. To avoid over-specifying large, expensive resistors, multiply peak power by the Braking Duty Cycle:

Duty Cycle (%) = (Braking Time / Total Cycle Time) x 100
P_br = P_peak x Duty Cycle (%)

3. Installation & Thermal Safety Rules

Because braking resistors convert electrical energy into high temperatures (often exceeding 200°C under heavy duty cycles), proper installation is essential:

1. Mount Outside the Main Electrical Enclosure: Never install high-wattage braking resistors inside the main control panel with the PLC or VFD. Heat rise will damage sensitive electronics. Mount them on top of the cabinet or in a dedicated ventilated enclosure.

2. Wire a Thermal Overload Switch: Always select resistors with an integrated thermal switch (NC contact). Wire this contact into the drive’s enable circuit or upstream main contactor to drop power if the resistor overheats (e.g., if a chopper IGBT fails short-circuit).

3. Maintain Clearance & Vertical Orientation: Mount resistor banks vertically to encourage natural convection airflow, leaving at least 150 mm to 200 mm clearance from surrounding walls or heat-sensitive components.