Motors

Why it matters

Motors turn most of the world's industrial electricity into motion: pumps, fans, conveyors, compressors and machine tools. Industrial machinery mechanics replace and align them, electricians wire and protect them, and engineers select them. Knowing how the two big families, DC motors and AC induction motors, work explains their nameplates, why their speeds behave the way they do and what usually goes wrong.

The principle

A current-carrying conductor in a magnetic field feels a force. Bend the conductor into a loop on a shaft, and the forces on its two sides make a torque that turns the shaft. Every electric motor is a way of keeping that torque pointing the same way as the shaft rotates.

Two main parts:

• Stator: the stationary part (frame, magnets or field windings).
• Rotor (in DC machines often called the armature): the rotating part on the shaft.

DC motors

In a brushed DC motor, the rotor carries the windings. A commutator (a segmented copper ring) and carbon brushes reverse the current in each rotor coil every half-turn, so the torque keeps the same direction.

• Speed rises with applied voltage. Lower the voltage and the motor slows; that makes speed control simple.
• Direction reverses when you reverse the polarity of the supply (for a permanent-magnet motor).
• Torque rises with current. At stall (rotor held still) the current is limited only by winding resistance, so stall current is high and windings overheat quickly.
• Wear: brushes and commutators wear and spark, so brushed motors need maintenance.

Brushless DC (BLDC) motors put permanent magnets on the rotor and use electronics to switch the stator coils. They are efficient and low-maintenance, and are found in drones, computer fans, power tools and electric vehicles.

DC motors also appear in older industrial drives (steel mills, cranes, elevators), many of which have been replaced by AC drives.

AC induction motors

The three-phase induction motor is the workhorse of industry: rugged, cheap and almost maintenance-free.

• The three stator windings, fed by three-phase AC 120° apart, create a rotating magnetic field.
• The rotor is usually a squirrel cage: aluminium or copper bars shorted together by end rings. There is no electrical connection to it.
• The rotating field sweeps past the bars and induces current in them (like a transformer secondary). Those currents, in the field, produce torque.

Synchronous speed is the speed of the rotating field:

Ns (rpm) = 120 × f / P

where f is the supply frequency and P the number of poles. At 60 Hz, a 4-pole motor's field turns at 120 × 60 / 4 = 1,800 rpm.

The rotor always turns a little slower than the field; otherwise the bars would see no change in field and no current would be induced. The difference is slip:

slip = (Ns − N) / Ns

A 4-pole, 60 Hz motor with a nameplate speed of 1,750 rpm has a slip of 50 / 1,800 ≈ 2.8%.

• Direction: swapping any two of the three supply phases reverses the rotation.
• Starting current (inrush) is often 5–8 times the running current, which is why motor circuits use time-delay protection, and why larger motors use soft starters or variable-frequency drives (VFDs).
• Speed control: because speed depends on frequency, a VFD changes the frequency to change the speed.
• Single-phase induction motors (in homes) need a start winding, often with a capacitor, because a single-phase field pulses instead of rotating.

Comparing them

Common failures (awareness)

• Bearings wear from misalignment, belt over-tension or poor lubrication: noise, heat, vibration.
• Winding insulation breaks down from overheating, moisture or voltage spikes; tested with an insulation resistance tester by trained staff.
• Single phasing: a three-phase motor losing one phase draws high current and overheats if protection does not trip.
• Brush wear in DC motors.

Safety and real-world requirements

Motors start automatically, store rotating energy and are supplied at high voltage. Inspection or repair requires the machine to be isolated and locked out under the employer's lockout/tagout program (https://www.osha.gov/control-hazardous-energy), and electrical work on motor circuits is done by trained, qualified people following NFPA 70E (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e). Checked on 2026-10-23. A small battery-powered DC toy motor is the only motor to experiment with at home.

Recap

• Motors make torque from current in a magnetic field.
• DC: commutator and brushes; speed follows voltage; reverse by polarity.
• Induction: rotating field from three-phase AC; Ns = 120f / P; slip makes torque; reverse by swapping two phases; VFDs control speed.

Try it

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