How an E-Bike Controller Works: MOSFETs, PWM and Phase Wires
The controller is the most misunderstood part of an e-bike. It is the device that decides how much battery power reaches the motor and when. This article explains its inner workings in plain terms.
The Controller’s Job
A battery stores DC (direct current) at a fixed voltage. A BLDC motor needs three-phase AC whose timing changes thousands of times per minute. The controller is the translator: it reads rider inputs and motor position, then switches battery DC into the correct phase pattern to produce smooth, controllable torque.
DC to Three-Phase AC
Inside, the controller has three half-bridges, one per motor phase. By turning the high and low sides on and off in sequence, it effectively creates alternating current in the motor windings. The order and timing of switching, guided by Hall sensors, determine direction, speed and torque.
MOSFETs and PWM
The switches are MOSFETs (metal-oxide-semiconductor field-effect transistors). Rather than fully on/off, they are pulsed using pulse-width modulation (PWM): the fraction of time a MOSFET is ‘on’ sets the effective voltage and thus the motor power. More MOSFETs in parallel and better cooling let a controller handle higher current without overheating.
Wiring: Phase, Hall, Throttle, Brake
The three thick phase wires carry motor power. The thin Hall wires report rotor position. The throttle and pedal-assist inputs tell the controller how much power to deliver, and the brake signal cuts power the instant you squeeze the lever for safety.
Reading a Controller Spec Sheet
| Parameter | What it means |
|---|---|
| Rated voltage | Pack voltage it supports (36/48/52/72 V) |
| Continuous current | Sustained amps before overheating |
| Peak current | Short-burst amps for acceleration |
| Low-voltage cut-off | Protects battery from over-discharge |
Frequently Asked Questions
Can a controller damage a motor?
Yes, if mis-matched. Too much current overheats windings; wrong phase timing causes rough running. Always match voltage and keep continuous current within the motor’s rating.
What do the three phase wires do?
They deliver the switched three-phase power from the controller to the motor. Swapping any two of them reverses rotation.
How many amps do I need?
A rough rule: amps x volts ≈ watts. A 48 V 1000 W motor needs about 22 A continuous; size the controller with some headroom, e.g. 25-30 A.