This electric street-car brake example uses the traction motor as a generator and sends the generated energy to a resistor grid. The calculator compares two entered grid-power operating points at a specified resistor voltage and vehicle speed. It estimates electrical current, equivalent resistance and the ideal retarding force associated with the higher power.
Electric Street-car Brake Interactive Calculator
Compare two resistor-grid power points for a streetcar dynamic brake. Enter actual grid voltage and vehicle speed; explore current, equivalent resistance and ideal retarding force.
Equation Used
- Known actual grid voltage and two prescribed electrical power points.
- Ideal lossless conversion for force equivalence only.
- No motor, adhesion, cooling or low-speed capability model.
- Vehicle depiction and power-flow arrows are schematic.
A specified operating-point comparison, not available brake capacity or a stopping simulation.
From moving wheels to heat
During electrical braking, the driven wheels turn the traction machine and mechanical energy is converted into electrical energy. Power electronics route the generated power to a resistor grid in the rheostatic mode illustrated here. The resistor transfers that energy to heat.
The vehicle drawing identifies a roof resistor enclosure, traction equipment beneath the floor and wheels coupled through gearing. Its proportions and component locations are illustrative. The lower drawing shows energy flow, not a wiring or switching schematic.
Regenerative braking sends energy back toward the supply instead. Electrically operated friction brakes are another different system: an electromagnet may apply brake shoes without using a traction motor as a generator. This example and its power calculator specifically address resistor braking.
Compare operating points
Enter the actual voltage across the equivalent resistor grid, two electrical power points and vehicle speed. The calculator sorts the two powers so the lower and higher result labels remain correct even when point A exceeds point B.
The grid voltage depends on the vehicle’s conversion and braking circuit and need not equal the nominal overhead supply. Use a known grid operating voltage for this comparison.
The wheel animation runs faster as entered vehicle speed increases. It shows one steady operating point, not the changing speed of a braking vehicle. The grid and current results remain visible while the animation plays.
Power, current and equivalent resistance
For grid electrical power P in watts and grid voltage V, current is I=P/V and equivalent resistance is R=V²/P. The calculator converts entered kilowatts to watts before evaluating these electrical quantities.
Under the ideal assumption that all mechanical braking power reaches the grid, P=Fv. With P in kilowatts and speed v in metres per second, F=P/v is in kilonewtons. Vehicle speed in km/h is divided by 3.6 to obtain metres per second.
The resistance result belongs to the higher power point at the entered voltage. Holding voltage fixed while changing power implies a different effective resistance or controlled operating state. It is not a model of an unchanged passive resistor at arbitrary power.
Default 600 V comparison
At an actual grid voltage of 600 V, 200 kW corresponds to approximately 333.3 A and 400 kW to approximately 666.7 A. The equivalent resistance at 400 kW is 0.90 Ω.
At 30 km/h, or 8.333 m/s, 400 kW corresponds to an ideal retarding force of 48.0 kN. This is a power-balance result, not a claim that a particular streetcar can supply that force.
Doubling voltage at fixed power halves current and quadruples equivalent resistance. Doubling speed at fixed power halves the ideal force. Real control limits can prevent the entered constant-power condition from being achievable.
What this calculator does not predict
Motor torque and speed limits, wheel adhesion, converter efficiency, resistor temperature, cooling, excitation and brake blending are not inputs. The resulting force is therefore an ideal equivalent of the entered grid power, not an available-force curve or brake design rating.
Fixed positive power divided by speed rises as speed falls; that equation alone does not describe a real dynamic brake near a stop. A complete vehicle model must account for low-speed electrical limits and other braking systems. The display compares the two entered power points at a constant speed.
Braking capability depends on the vehicle, electrical equipment and operating conditions. The model does not calculate stopping distance, emergency performance or parking hold.
Electric street-car brake questions
Is the voltage the overhead-line voltage?
No. It is the actual voltage across the modeled equivalent resistor grid. Supply and grid voltages can differ.
Why does the vehicle not slow down in the animation?
The inputs define one operating point. No vehicle mass or deceleration history is supplied, so the animation displays motion at that steady speed.
Does the calculated force include friction brakes?
No. It converts entered resistor power into an ideal mechanical-force equivalent.
Is this the same as an old solenoid-operated streetcar brake?
No. A solenoid can operate mechanical brake rigging. This example instead shows traction generation and resistor dissipation.
References
- General Electric, US5396214A: Dynamic braking grid resistor configuration for reducing EMI in an electric traction motor vehicle — transit traction generation, power conversion and resistor energy dissipation. The illustration is a simplified functional view, not a reproduction of the patent circuit.
- Federal Railroad Administration, Railroad Electromagnetic Compatibility: Locomotive (1981) — dynamic braking with traction motors generating into resistor grids.
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