Calculator Inputs
Formula Used
The calculator first converts speed into angular velocity.
ω = 2π × rpm ÷ 60
Reflected load inertia is moved to the motor shaft.
Jref = Jload ÷ gear ratio²
Total energy removed during braking is:
E = 0.5 × J × (ω1² − ω2²)
Average stop power is:
P = E ÷ t
For a DC bus chopper, resistance from power is:
R = V² ÷ P
The current limit gives:
Rmin = V ÷ Imax
For armature braking, generated voltage is estimated from motor constant.
Eg = Ke × ω
The external resistor is estimated as:
Rext = Eg ÷ I − Ra
How to Use This Calculator
Choose the braking method first. Use the DC bus method for a drive with a brake chopper. Use armature braking for a resistor connected across the motor armature.
Enter speed, inertia, gear ratio, braking time, voltage, and current limit. Add duty data using stops per hour. Enter a proposed resistor only when you want to test a real part.
Press Calculate. The result appears above the form. Review resistance, current, peak power, continuous wattage, pulse energy, and warnings. Use CSV or PDF buttons to save the same input result.
Example Data Table
| Case | Voltage | Total inertia | Speed | Stop time | Current limit | Typical use |
|---|---|---|---|---|---|---|
| Small conveyor | 180 V | 0.045 kg·m² | 1200 rpm | 8 s | 12 A | Light duty stopping |
| Machine spindle | 320 V | 0.180 kg·m² | 1750 rpm | 5 s | 25 A | Frequent controlled stops |
| Hoist lowering | 560 V | 0.420 kg·m² | 1450 rpm | 4 s | 40 A | High energy braking |
DC Motor Braking Resistor Guide
What the Calculator Solves
A DC motor braking resistor turns stored motion into heat. The motor acts like a generator during braking. The resistor gives that energy a safe path. This calculator estimates the resistor value, peak power, average thermal load, and pulse energy rating.
Why Inertia Matters
The most important input is inertia. A heavy load stores more energy. A fast motor also stores more energy. Doubling speed raises energy by four times. That is why speed and inertia must be entered with care. Gear ratio also changes the load inertia seen by the motor shaft.
Braking Time and Current
A short stop needs high braking power. High power means more current or a lower resistance. The current limit protects the drive, chopper, motor, and resistor. The calculator compares the power-based resistance with the current-based minimum resistance. It then recommends the safer value.
Thermal Rating
Resistors have two important ratings. Peak rating covers the short braking pulse. Continuous rating covers repeated stops. A resistor may survive one stop but fail during frequent cycling. The stops per hour value estimates duty average power. The derating input adds practical headroom for heat, airflow, enclosure limits, and manufacturer tolerance.
Chopper and Armature Methods
The DC bus chopper method suits modern drives. It assumes the resistor is switched across the DC bus near the brake voltage. The armature method suits a motor disconnected from supply and connected to a resistor. That method uses motor voltage constant and armature resistance.
Using the Result
Pick a resistor near the recommended resistance. Never go below the drive minimum resistance. Check peak watts, continuous watts, and joules per pulse. Use parts made for dynamic braking. Keep wiring short. Allow airflow. Confirm final selection with the drive manual and resistor datasheet.
FAQs
1. What is a DC motor braking resistor?
It is a resistor used to absorb regenerated energy during braking. The motor produces electrical energy as it slows. The resistor converts that energy into heat.
2. Why is the recommended resistance not always the lowest value?
A lower resistance causes higher current. The calculator checks the maximum braking current. It raises the resistor value when current would exceed the safe limit.
3. What does pulse energy rating mean?
Pulse energy is the joules the resistor must absorb during one braking event. It is different from continuous watts. Repeated braking needs both ratings checked.
4. Should I use the chopper method or armature method?
Use the chopper method for a drive with a DC bus brake circuit. Use armature braking when a resistor is connected across the DC motor armature.
5. Why does gear ratio affect the result?
Gear ratio changes load inertia reflected to the motor shaft. A higher motor-to-load speed ratio usually reduces reflected load inertia at the motor.
6. Can I enter an existing resistor?
Yes. Enter its resistance in the proposed resistor field. The calculator will estimate current, peak power, and stop time for that resistor.
7. Why is a safety factor needed?
Real systems have tolerance, poor airflow, voltage variation, and changing loads. A safety factor adds headroom to power and energy ratings.
8. Is this calculator enough for final design?
It gives engineering estimates. Final parts should be checked against the motor data, drive manual, braking unit limits, resistor datasheet, and local safety rules.