Electric motors drive almost everything that moves: ceiling fans, water pumps, mixers, washing machines, lifts, conveyors, CNC machines and electric vehicles. But "motor" covers many very different machines. A ceiling fan motor, a 3D-printer stepper and a 75 kW factory pump motor work on different principles, are wired differently and fail in different ways.

This guide is the starting point of the OhmForge Motor Library. It classifies the main motor types, explains how each one works in simple terms, and shows where each is commonly used.

What is an electric motor?

An electric motor converts electrical energy into mechanical energy (rotation or linear motion). It does this using magnetic fields: current in the motor's windings creates a magnetic field that interacts with another field (from magnets or other windings) and produces torque on the rotor.

Every rotating motor has two main parts:

• Stator: the stationary outer part, usually laminated steel with windings or magnets.
• Rotor: the rotating inner part, connected to the shaft. (In "outrunner" motors the rotor is on the outside.)

How electric motors are classified

There is no single official family tree, but engineers usually group motors by the supply they use (AC or DC) and how they create rotation (induction, synchronous, brushed commutation, electronic commutation or stepping).

1. AC induction motors

The induction motor is the most widely used industrial motor. The stator winding creates a rotating magnetic field. This field induces current in the rotor, and the rotor follows the field. The rotor always turns slightly slower than the field; this difference is called slip. Most induction motors use a rugged squirrel-cage rotor with no brushes, which makes them reliable and low-maintenance.

Three-phase induction motors

• Squirrel-cage: the workhorse of industry. Self-starting, simple and robust. Used for pumps, fans, compressors, conveyors and machine tools.
• Slip-ring (wound-rotor): the rotor has windings brought out through slip rings and brushes, so external resistance can be added for high starting torque with lower starting current. Found in some cranes, hoists, crushers and older heavy-duty drives.

Single-phase induction motors

A single-phase winding alone does not produce a rotating field, so these motors need a starting arrangement:

2. AC synchronous motors

A synchronous motor runs at exactly the speed of the rotating magnetic field, with no slip. That speed depends on supply frequency and the number of poles:

An induction motor with the same pole count runs a little below these speeds because of slip; the exact rated speed is printed on its nameplate.

• Wound-field synchronous: rotor field supplied with DC excitation. Used in large industrial drives and can help with power factor correction.
• Permanent-magnet synchronous motor (PMSM): magnets on the rotor, driven by an inverter. High efficiency and power density. Widely used in electric vehicles, inverter air conditioners and modern industrial servo systems.
• Synchronous reluctance (SynRM): rotor has no magnets or windings; torque comes from magnetic saliency. Used with VFDs for efficient pump and fan drives.
• Line-start permanent-magnet: has a cage for direct-on-line starting plus magnets for synchronous running.

3. DC brushed motors

A brushed DC motor uses a commutator and carbon brushes to reverse current in the armature coils as the rotor turns. Speed is controlled easily by changing the voltage, and direction is reversed by changing polarity (for permanent-magnet types). Brushes and commutators wear, so they need periodic inspection.

4. Brushless DC (BLDC) motors

A BLDC motor replaces brushes and commutator with electronic commutation. Permanent magnets are on the rotor, three-phase windings are on the stator, and an electronic controller switches current to the windings in sequence. The controller knows the rotor position from Hall sensors or, in sensorless designs, from the back-EMF of the windings.

• Inrunner: rotor inside the stator; higher speed. Used in power tools, pumps and many appliances.
• Outrunner: rotor on the outside; higher torque at lower speed. Used in drones, e-bike hub motors and BLDC ceiling fans.

Advantages include high efficiency, low maintenance and quiet running. The trade-off is that a BLDC motor always needs a matching controller; it cannot be connected directly to a battery or the mains. BLDC and PMSM motors are closely related. The difference is mainly in winding design and drive waveform (trapezoidal for BLDC, sinusoidal for PMSM), and manufacturers do not always use the terms consistently.

5. Universal motors

A universal motor is a series-wound commutator motor that can run on AC or DC. It reaches very high speeds and gives high power for its size, which is why it is used in mixer-grinders, vacuum cleaners, many portable power tools and some older washing machines. The downsides are brush wear, sparking, noise and a shorter service life than induction motors.

6. Stepper motors

A stepper motor moves in fixed angular steps when its windings are energised in sequence by a stepper driver. A common hybrid stepper has a 1.8° step angle, which gives 200 full steps per revolution. Position can be controlled without feedback (open loop), as long as the motor is not overloaded and does not miss steps.

• Types: permanent-magnet, variable-reluctance and hybrid.
• Wiring: bipolar (usually 4 wires) or unipolar (5, 6 or 8 wires).
• Uses: 3D printers, hobby CNC machines, camera sliders, valves and positioning stages.

7. Servo motors

"Servo" describes a closed-loop system: a motor, a feedback device (encoder or resolver) and a servo drive that continuously corrects position, speed or torque. The motor itself is often a PMSM (industrial AC servo) or a DC motor (small hobby servos).

• Industrial AC servo: CNC axes, robots, packaging and printing machines.
• RC hobby servo: small DC motor, gearbox, potentiometer and control board in one housing, controlled by a pulse signal.

8. Special-purpose motors

• Linear motors: produce straight-line motion directly; used in high-speed positioning and some transport systems.
• Torque / direct-drive motors: high torque at low speed with no gearbox; used in rotary tables and robotics.
• Vibration (ERM) motors: small DC motors with an off-centre weight; used in phones and controllers.
• Geared motors: any motor combined with a gearbox to trade speed for torque.
• Hermetic compressor motors and submersible pump motors: sealed assemblies designed for one specific application.

Comparison of the main motor types

Which motor is used where? (quick map)

How to identify a motor

1. Read the nameplate. Look for phase (1~ or 3~), voltage, frequency (Hz), rated current, power (kW or HP), speed (rpm), connection (Y/Δ), insulation class, duty and IP rating.
2. Check for a capacitor. A capacitor mounted on or near the motor usually indicates a single-phase capacitor motor.
3. Look for brushes. Brush holders, a commutator or visible sparking indicate a brushed DC or universal motor.
4. Count the terminals or leads. Six terminals with links usually indicate a three-phase motor that can be connected in star or delta. Three thick wires plus several thin signal wires usually indicate a BLDC motor with Hall sensors.
5. Check the controller. If the motor is fed from an electronic controller, drive or ESC, it is likely BLDC, PMSM, stepper or servo.

Choosing the right motor: key questions

• What supply is available: single-phase, three-phase, or DC battery?
• What torque and speed does the load need, and does it need high starting torque?
• Is speed control or precise positioning required?
• What is the duty cycle (continuous, intermittent, frequent starts)?
• What is the environment: dust, water, heat, explosive atmosphere? (Check the IP rating and any hazardous-area certification.)
• What efficiency is required? Many regions set minimum efficiency levels using the IE classes defined in IEC 60034-30-1.

Frequently asked questions

What is the most common type of electric motor?

In industry, the three-phase squirrel-cage induction motor. In homes, single-phase induction motors (fans, pumps), universal motors (mixers, tools) and, increasingly, BLDC motors.

What is the difference between BLDC and PMSM?

Both have permanent magnets on the rotor and need electronic drives. BLDC motors are usually driven with trapezoidal (block) commutation, PMSMs with sinusoidal current. In practice, the names overlap.

Is a servo motor a type of motor or a system?

A servo is best understood as a system: motor, feedback device and drive working together in a closed loop.

Why do induction motors run below synchronous speed?

Torque is produced only when rotor current is induced, and that requires the rotor to turn slower than the rotating field. This speed difference is called slip.

What to read next

• How a three-phase induction motor works (coming soon)
• How to read a motor nameplate (coming soon)
• AC vs DC: what's the difference?

References

• IEC 60034 series, Rotating electrical machines (ratings, efficiency classes IEC 60034-30-1, terminal markings IEC 60034-8).
• NEMA MG 1, Motors and Generators (North American practice).
• Motor manufacturers' catalogues, nameplates and installation manuals for specific products.

Technical note: this is an educational overview. Motor designs vary between manufacturers and models; always use the nameplate and manufacturer documentation for installation, wiring and repair.