VFD Dynamic Braking Resistor Calculator

A VFD dynamic braking resistor absorbs regenerative energy when a motor-driven load decelerates faster than the drive and load losses can dissipate that energy. The calculator converts reflected rotating inertia and a speed change into resistor energy per stop, then screens peak resistor power, minimum resistance, and repeated-duty loading.

The primary result is Resistor energy per stop. That value represents the portion of rotational kinetic energy assigned to the braking resistor during one commanded deceleration. It is used with the deceleration time to establish resistor pulse power, with DC bus voltage to screen resistance, and with repeated braking frequency and duty to review thermal capability.

Dynamic braking resistor selection is not a branch-circuit conductor, raceway-fill, or voltage-drop calculation. It is a drive-system application calculation. The final resistor assembly must still be compatible with the VFD braking chopper, the drive manufacturer’s permitted minimum resistance, resistor pulse-energy rating, enclosure temperature, clearances, wiring method, grounding, and the installation requirements enforced by the AHJ.

Braking Energy at the Motor Shaft

The calculator begins with load inertia referred to the motor shaft and the change in motor speed.

Use Reflected load inertia (kg·m²) for the combined inertia seen by the motor shaft. This can include the motor rotor, gearbox-reflected load inertia, couplings, rolls, fans, flywheels, and other rotating equipment. Entering inertia at the wrong shaft location can substantially understate or overstate braking energy.

Initial speed (rpm) is the speed at the start of deceleration. Final speed (rpm) is the speed at the end of braking. The calculation uses the difference between the kinetic energy at those two speeds, not simply the rpm difference.

The calculator applies the entered Braking energy fraction (%) to account for the portion of mechanical braking energy expected to reach the resistor. This field should be based on the drive and application conditions being evaluated. Mechanical losses, process loading, regeneration limits, and drive behavior can affect the actual energy delivered to the DC bus and resistor.

The energy calculation is:

\(\displaystyle E_R=\frac{1}{2}J\left(\omega_i^2-\omega_f^2\right)\times f_B\)

Where:

• \(E_R\) = resistor energy per stop, in joules
• (J) = reflected load inertia, in kg·m²
• \(\omega_i\) = initial angular speed, in radians per second
• \(\omega_f\) = final angular speed, in radians per second
• \(f_B\) = braking energy fraction as a decimal

Angular speed is converted from rpm:

\(\omega=\frac{2\pi \times rpm}{60}\)

A higher reflected inertia, a larger speed reduction, or a larger braking energy fraction increases the energy the resistor must absorb.

Resistor Power and Resistance Screen

Deceleration time (s) establishes the duration over which the calculated energy is released. A shorter deceleration time produces a higher braking power requirement even when the stopping energy remains unchanged.

The calculator’s Peak resistor power is:

\(\displaystyle P_{peak}=\frac{E_R}{t}\)

Where:

• \(P_{peak}\) = peak resistor power, in watts
• \(E_R\) = resistor energy per stop, in joules
• (t) = deceleration time, in seconds

The DC bus voltage (V) is then used to calculate the Minimum resistance screen:

\(\displaystyle R_{min}=\frac{V_{DC}^2}{P_{peak}}\)

This produces the resistance that corresponds to the entered DC bus voltage and calculated peak power. It is not automatically the resistor value that may be installed. The VFD manufacturer’s braking-transistor or braking-chopper documentation controls the allowable minimum resistance. Selecting a resistance below the drive’s approved limit can overload or damage the braking circuit even if the arithmetic screen produces that value.

A higher resistor value reduces current and braking-chopper loading but can also reduce available braking torque or prevent the resistor from absorbing energy at the required rate. A lower resistor value increases braking current and chopper stress. The selected resistor must therefore satisfy both the drive’s resistance limit and the application’s braking requirement.

Repeated Braking Duty

Braking cycles per hour (cycles/h) identifies how often the braking event is expected to repeat. This input addresses thermal loading over repeated operations rather than the single-stop pulse alone.

Resistor duty fraction (%) represents the allowed average duty fraction for the resistor. It is used by the calculator to produce the Duty-adjusted power screen.

The calculator displays Average resistor power from the braking energy per stop multiplied by braking cycles per hour:

\(\displaystyle P_{avg\ screen}=E_R \times N\)

Where (N) is braking cycles per hour.

It then applies the duty fraction:

\(\displaystyle P_{duty\ screen}=\frac{P_{avg\ screen}}{D}\)

Where (D) is the resistor duty fraction as a decimal.

Because the displayed Average resistor power is calculated from joules per stop multiplied by cycles per hour, confirm the unit basis and thermal interpretation against the resistor manufacturer’s pulse-duty and continuous-power data before selecting a resistor. A true average thermal power calculation normally requires a consistent time basis. The resistor’s continuous wattage rating, short-time overload curve, pulse-energy capacity, and cooling arrangement must be evaluated separately.

Calculation Example

Use the following application values:

Field	Entered value
Reflected load inertia (kg·m²)	2
Initial speed (rpm)	1800
Final speed (rpm)	600
Deceleration time (s)	5
Braking energy fraction (%)	90
DC bus voltage (V)	700
Resistor duty fraction (%)	20
Braking cycles per hour (cycles/h)	4

First, convert motor speeds to angular velocity:

\(\omega_i=\frac{2\pi \times 1800}{60}=188.4956\ rad/s\)

\(\omega_f=\frac{2\pi \times 600}{60}=62.8319\ rad/s\)

Then calculate braking resistor energy:

\(\displaystyle E_R=\frac{1}{2}(2)\left(188.4956^2-62.8319^2\right)(0.90\)

\(\displaystyle E_R=28{,}424.4607\ J\)

The calculated peak resistor power for a five-second stop is:

\(\displaystyle P_{peak}=\frac{28{,}424.4607}{5}=5{,}684.8921\ W\)

Using a 700 V DC bus basis, the minimum resistance screen is:

\(\displaystyle R_{min}=\frac{700^2}{5{,}684.8921}=86.1934\ \Omega\)

The calculator produces the following results:

Result	Value
Resistor energy per stop	28,424.4607 J
Peak resistor power	5,684.8921 W
Average resistor power	113,697.8427 W
Minimum resistance screen	86.1934 ohm
Duty-adjusted power screen	568,489.2135 W

For this application, the resistor must be reviewed for at least the calculated braking pulse energy of 28.42 kJ, the approximate 5.68 kW pulse over five seconds, the VFD’s approved resistance range, and the actual repeated-duty thermal loading.

Drive and Field Verification

Use Drive or load note to document the drive model, braking-chopper threshold, and resistor manual reference outside the calculation values. Those details establish whether the drive includes an internal braking transistor, requires an external braking unit, permits the calculated resistance range, or has a published resistor selection method that overrides a generic screen.

Verify these conditions before finalizing the braking resistor:

• The drive’s published minimum and maximum braking-resistor resistance requirements.
• The braking resistor’s pulse-energy, overload, continuous-power, and duty-cycle ratings.
• The actual deceleration profile, including whether the VFD reaches the commanded ramp or extends deceleration because of DC bus overvoltage.
• Motor and load inertia values referred to the motor shaft.
• Braking frequency during normal operation, fault recovery, emergency stops, and high-cycle process conditions.
• Resistor enclosure temperature, ventilation, combustible-material clearance, and guarding against contact with hot surfaces.
• Conductor insulation temperature rating, terminal rating, equipment grounding, disconnecting means, and wiring requirements applicable to the VFD-resistor circuit.

The calculated Minimum resistance screen and Duty-adjusted power screen support early VFD commissioning and application review. Final equipment selection must follow the VFD and resistor manufacturer documentation for the specific drive, braking chopper, resistor assembly, and installation environment.