Electric Motors
1. How an Electric Motor Works
Every motor relies on the same core principle: a current-carrying conductor placed in a magnetic field experiences a force (the Lorentz force, F = B I L). Arrange conductors on a rotating body (the rotor) inside a fixed magnetic field (the stator), and that force becomes torque.
| Part | Role |
|---|---|
| Stator | Stationary part that produces (or hosts) the magnetic field. |
| Rotor | Rotating part that carries current or magnets and produces torque. |
| Windings | Coils of wire that carry current and generate magnetic fields. |
| Commutator / ESC | Switches current direction to keep torque flowing one way. |
| Bearings | Support the shaft and allow smooth rotation. |
| Air gap | Small clearance between rotor and stator; smaller = stronger coupling. |
Key relationships:
- Torque is proportional to current:
T = Kt * I - Back-EMF (voltage generated by rotation) is proportional to speed:
V_bemf = Ke * w - Mechanical power out:
P = T * w(torque times angular velocity)
2. Motor Family Overview
| Family | Power Source | Commutation | Typical Use |
|---|---|---|---|
| Brushed DC | DC | Mechanical (brushes) | Toys, small robots, automotive actuators |
| Brushless DC (BLDC) | DC (via ESC) | Electronic | Drones, EVs, fans, power tools |
| Stepper | DC (pulsed) | Open-loop steps | 3D printers, CNC, precise positioning |
| AC Induction | AC | None (slip) | Pumps, fans, industrial machinery |
| AC Synchronous / PMSM | AC / DC drive | Electronic | EV traction, servos, high-efficiency drives |
| Universal | AC or DC | Mechanical | Power tools, vacuum cleaners |
3. DC Motors
3.1. Brushed DC
A brushed DC motor uses carbon brushes and a commutator to mechanically switch current in the rotor windings, keeping the torque in one direction as the rotor spins.
- Pros: Cheap, simple to drive (just apply DC voltage), easy speed control via voltage.
- Cons: Brushes wear out, generate sparks and EMI, lower efficiency, limited lifespan.
- Control: Vary voltage (PWM) for speed; reverse polarity for direction (H-bridge).
3.2. Brushless DC (BLDC)
A BLDC motor puts the permanent magnets on the rotor and the windings on the stator. An electronic speed controller (ESC) energizes the stator coils in sequence, replacing the mechanical commutator.
- Pros: High efficiency, high power density, long life (no brushes), quiet.
- Cons: Requires an ESC and rotor-position sensing (Hall sensors or sensorless back-EMF).
- Control: Trapezoidal (six-step) or sinusoidal (FOC) commutation via the ESC.
| Aspect | Brushed DC | Brushless DC |
|---|---|---|
| Efficiency | 75-80% | 85-95% |
| Maintenance | Brush wear | Effectively none |
| Driver | H-bridge | 3-phase ESC |
| Cost | Low | Higher |
| Lifespan | 1,000-3,000 hr | 10,000+ hr |
4. Stepper Motors
A stepper motor divides a full rotation into a fixed number of discrete steps (commonly 200 steps = 1.8 degrees each). By energizing coils in sequence, the rotor moves one precise step at a time, enabling accurate open-loop positioning without feedback.
| Type | Description | Application |
|---|---|---|
| Permanent Magnet | Magnetized rotor, coarse steps, low cost. | Simple positioning. |
| Variable Reluctance | Toothed iron rotor, no magnet. | Legacy / specialty. |
| Hybrid | Combines PM + reluctance; fine steps, high torque. | 3D printers, CNC, cameras. |
- Microstepping: Driving coils with intermediate current levels subdivides each step (e.g. 1/16, 1/256) for smoother, finer motion.
- Trade-off: Excellent low-speed holding torque and precision, but torque drops at high speed and steps can be lost if overloaded (no feedback).
- Drivers: A4988, DRV8825, TMC2209 (silent, sensorless load detection).
5. AC Motors
5.1. Induction (Asynchronous)
The stator’s rotating magnetic field induces current in the rotor (no direct electrical connection). The rotor always spins slightly slower than the field, a difference called slip.
- Squirrel-cage: Rugged, cheap, self-starting; the workhorse of industry.
- Wound-rotor: External rotor resistance for high starting torque and speed control.
- Control: A Variable Frequency Drive (VFD) changes the supply frequency to vary speed.
5.2. Synchronous
The rotor locks to and rotates exactly at the supply frequency (no slip). Permanent Magnet Synchronous Motors (PMSM) are the high-efficiency choice for EV traction and servo drives.
| Aspect | Induction | Synchronous / PMSM |
|---|---|---|
| Speed vs. supply | Slightly slower (slip) | Exactly locked |
| Efficiency | Good | Excellent |
| Cost | Lower | Higher (magnets) |
| Starting | Self-starting | Needs drive / starter |
5.3. Universal
A series-wound motor that runs on either AC or DC. High speed and good starting torque make it ideal for handheld power tools and vacuum cleaners, at the cost of brush wear and noise.
6. Motor Control and Drivers
| Motor | Typical Driver | Method |
|---|---|---|
| Brushed DC | H-bridge (L298N, DRV8871) | PWM voltage + polarity for direction. |
| BLDC | 3-phase ESC | Six-step or Field-Oriented Control (FOC). |
| Stepper | Step/dir driver (A4988, TMC2209) | Sequenced coil energizing, microstepping. |
| AC Induction | Variable Frequency Drive (VFD) | Vary frequency and voltage (V/f or vector). |
| PMSM / Servo | Servo drive | Closed-loop FOC with encoder feedback. |
Key control concepts:
- PWM (Pulse Width Modulation): Rapidly switch power on/off; the average voltage sets speed.
- H-Bridge: Four switches that let current flow through a motor in either direction, enabling forward/reverse and braking.
- FOC (Field-Oriented Control): Controls the motor’s current vector for smooth, efficient torque; the standard for high-performance BLDC/PMSM drives.
- Closed vs. open loop: Servos use encoder feedback (closed loop) for accuracy; steppers usually run open loop.
7. Key Specifications
When selecting a motor, these are the numbers that matter:
| Spec | Meaning |
|---|---|
| Voltage (V) | Nominal operating voltage. |
| Current / Stall Current | Running current and worst-case locked-rotor current. |
| Torque (N.m or kg.cm) | Rotational force; rated vs. stall (max) torque. |
| Speed (RPM) | No-load speed; drops under load. |
| Kv (for BLDC) | RPM per volt with no load (higher Kv = faster, lower torque). |
| Power (W) | Mechanical output = torque x angular velocity. |
| Efficiency (%) | Mechanical power out / electrical power in. |
| Duty cycle | Continuous vs. intermittent operation rating. |
A motor’s torque-speed curve captures most of this: torque is highest at stall (zero speed) and falls linearly to zero at no-load speed; peak power sits near the middle.
8. Choosing the Right Motor
| Need | Best Choice |
|---|---|
| Cheap and simple | Brushed DC |
| High efficiency and long life | BLDC / PMSM |
| Precise positioning, open loop | Stepper |
| Precise positioning, high performance | Servo (BLDC + encoder) |
| Constant-speed industrial load | AC Induction (with VFD) |
| High-speed handheld power tool | Universal |
| Drone / RC propulsion | BLDC (outrunner, high Kv) |
Rules of thumb:
- Match stall torque with margin above your worst-case load; do not size to rated torque alone.
- Check the stall current your driver and power supply must survive.
- For battery devices, prioritize efficiency (BLDC/PMSM) to extend runtime.
- For anything that must hold or step to exact positions cheaply, use a stepper; if it must not lose position under load, use a servo.
9. Summary
| Motor Type | Strength | Watch Out For |
|---|---|---|
| Brushed DC | Simple, cheap, easy to drive | Brush wear, EMI, efficiency |
| BLDC | Efficient, powerful, long-lived | Needs an ESC and position sense |
| Stepper | Precise open-loop positioning | Torque falls at speed, lost steps |
| AC Induction | Rugged, cheap, self-starting | Slip, needs VFD for speed control |
| PMSM / Synchronous | Top efficiency, exact speed | Magnet cost, requires a drive |
| Universal | AC or DC, high speed | Brush wear, noise |
The right motor is a balance of torque, speed, efficiency, control complexity, and cost for the specific job.