An electronic speed controller, or ESC, is the power and control link between a robot controller and a motor. In many drones, rovers, and robotic arms, the ESC converts a small throttle command into carefully timed high-current pulses. This matters because brushless DC motors cannot spin from a simple DC connection alone.
They need electronic commutation so the rotating magnetic field stays synchronized with the rotor.
A typical ESC for a 3-phase brushless DC motor contains a microcontroller, gate drivers, and three half-bridges made from MOSFETs. The microcontroller reads the throttle input, estimates or measures rotor position, and switches the MOSFETs in a timed sequence. In sensorless control, the ESC watches the back-EMF voltage on the unpowered phase to detect when the rotor has reached the next commutation point.
By changing duty cycle and timing, the ESC controls motor speed, torque, direction, braking, and efficiency.
Key Facts
- A 3-phase BLDC ESC uses three half-bridges, one for each motor phase: A, B, and C.
- Motor speed is mainly controlled by PWM duty cycle, where duty cycle = on time / period.
- Electrical input power is approximately P = V I for the battery side of the ESC.
- For an ideal motor drive, mechanical output power is P = torque x angular speed, or P = τω.
- In six-step commutation, two phases are driven at a time while the third phase is often used for back-EMF sensing.
- Back-EMF increases with speed and is often modeled as E = k_e ω.
Vocabulary
- Electronic Speed Controller
- An electronic circuit that controls motor speed and torque by switching power to the motor phases.
- Half-bridge
- A pair of high-side and low-side switches that can connect one motor phase to the positive supply or to ground.
- PWM
- Pulse width modulation is a method of controlling average voltage by rapidly switching a signal on and off with a chosen duty cycle.
- Back-EMF
- Back electromotive force is the voltage generated by a spinning motor that opposes the applied voltage and reveals rotor motion.
- Commutation
- Commutation is the timed switching of motor phase currents so the stator magnetic field pulls the rotor around.
Common Mistakes to Avoid
- Connecting a BLDC motor directly to a battery, which is wrong because the motor needs phased switching to create a rotating magnetic field.
- Ignoring ESC current rating, which is wrong because MOSFETs and traces can overheat if motor current exceeds the controller limit.
- Confusing PWM frequency with motor speed, which is wrong because PWM frequency is the switching rate while motor speed depends on duty cycle, load, voltage, and commutation.
- Assuming sensorless ESCs work perfectly at zero speed, which is wrong because back-EMF is very small when the rotor is not moving.
Practice Questions
- 1 An ESC is connected to an 11.1 V LiPo battery and draws 18 A during steady operation. Estimate the electrical input power in watts.
- 2 A PWM signal has a period of 50 microseconds and is on for 15 microseconds each cycle. What is the duty cycle as a percent?
- 3 In a sensorless BLDC ESC using six-step commutation, why is one phase often left unpowered during part of the cycle, and what information can the ESC gain from it?