AC Motors
Part of Generators and Motors
AC induction motors convert alternating current into rotation without brushes or commutators — self-starting, robust, and the most common motor type in industrial civilization.
Why This Matters
The AC induction motor is one of the most important machines ever built. Nikola Tesla’s invention powers roughly 70% of all electricity consumed in industry worldwide. Its advantages are compelling: no brushes to wear out, no commutator to arc and fail, simple construction with few moving parts, and the ability to run directly from AC mains without any conversion electronics.
For a rebuilding civilization that has established AC generation (alternators rather than DC dynamos), the induction motor becomes the standard workhorse for all mechanical power needs — pumps, fans, lathes, mills, grinders, and blowers. Understanding how to build, connect, and troubleshoot AC motors is as fundamental as understanding how to use a hammer.
Even where only DC power is available initially, understanding AC motor principles prepares you for the transition to AC systems, which become increasingly advantageous as power transmission distances grow.
Induction Motor Principles
The AC induction motor works on a beautifully simple principle: a rotating magnetic field induces currents in a rotor, and the electromagnetic force on those currents creates torque.
Rotating magnetic field: Three-phase AC (three voltages 120° apart in phase) fed to three stator windings spaced 120° around the stator creates a magnetic field that rotates at the AC frequency. At 50 Hz, the field rotates at 3,000 RPM (for a 2-pole machine) or 1,500 RPM (4-pole).
Synchronous speed: N_s = 120 × f / P, where f is frequency in Hz and P is number of poles. For 50 Hz, 4-pole: N_s = 1,500 RPM.
Induction: The rotating field passes through the rotor conductors, inducing voltages (by Faraday’s law) and therefore currents in the rotor. These rotor currents create their own magnetic field, which interacts with the stator field to produce torque.
Slip: The rotor never quite reaches synchronous speed — if it did, there would be no relative motion between field and rotor, no induced current, and no torque. The rotor runs at 2–8% below synchronous speed under load. This difference is called slip. Slip increases with load, which is why loaded motors run slower than unloaded.
Single-phase induction: Single-phase AC does not inherently create a rotating field (it creates a pulsating field). Single-phase motors need a starting mechanism — a starting winding with capacitor phase shift, or a shaded pole (a shorted copper band on part of each stator pole that creates a lagging flux). Once running, single-phase motors continue rotating.
Stator Construction
The stator is the stationary outer part containing windings that produce the rotating magnetic field.
Laminated core: The stator iron must be laminated (thin insulated layers) to prevent eddy current losses. Slot the laminations for winding insertion. Lamination thickness: 0.3–0.5 mm is standard; 0.5 mm is achievable with basic rolling mills.
Core material: Silicon steel (3–5% silicon) has low hysteresis losses and high permeability. If unavailable, soft iron laminations work with somewhat higher losses. Avoid thick solid iron — eddy current losses make it nearly useless.
Slot winding: Three-phase windings occupy slots in the stator core. Each phase winding is distributed across multiple slots to create a smooth rotating field.
Simplified construction: For a 4-pole, 3-phase motor:
- Prepare 24 slots in the stator core (24/4 poles × 3 phases × 2 coil sides per slot = 24)
- Wind coils of magnet wire (enameled copper or cotton-covered copper)
- Place coils in slots, insulating between phases with varnished cloth or plastic film
- Connect coils in each phase in series or parallel; configure for delta or star connection
- Impregnate completed winding with shellac or varnish and bake to seal
Rotor Construction (Squirrel Cage)
The squirrel cage rotor is the simplest and most robust rotor design.
Construction: Aluminum or copper bars fit into slots around the rotor core and are short-circuited at each end by aluminum or copper end rings.
Fabrication options:
- Cast aluminum: Melt aluminum and cast it directly into the rotor slots with integral end rings (commercial manufacturing method — requires precise mold)
- Fabricated copper: Machine individual copper bars, press into slots, braze or weld to copper end rings (buildable with basic metalworking)
- Welded flat bars: Cut copper or aluminum strips, bend to fit slots, weld end rings
Rotor core: Same laminated silicon steel as stator. Press onto the shaft with a tight interference fit or keyed joint.
Shaft and bearings: Steel shaft, turned to fit ball bearings at each end. Ball bearings must be preloaded and sealed against moisture and dust.
Single-Phase Motor Starting
For generators producing single-phase AC (simpler to build than 3-phase), single-phase motors require starting assistance.
Capacitor-start motor: A starting winding in series with a capacitor creates a phase-shifted current, producing enough asymmetry for self-starting. A centrifugal switch disconnects the starting winding once the motor reaches ~75% speed.
Shaded-pole motor: Simplest construction — a copper ring shorted around one side of each stator pole creates a lagging flux. Self-starting in one direction; low starting torque and lower efficiency than capacitor motors. Good for fans and small pumps.
Split-phase: A thinner-wire auxiliary winding with higher resistance provides phase shift. Self-starting. Centrifugal switch disconnects auxiliary winding at speed. Simple but moderate starting torque.
Practical Considerations
Motor ratings: Size motor for the load, with 20–50% margin for starting torque. Motors can draw 3–7× rated current during starting — ensure your generator and wiring can supply this briefly.
Cooling: AC motors rely on their fan for cooling. Do not block ventilation, and do not run motors at low speeds for extended periods without external cooling.
Overload protection: Fit thermal overload relays (bimetallic strips that trip when motor draws excess current). Without protection, motor winding temperature rises until insulation fails.
Direction reversal: Swap any two of the three phase connections to reverse rotation direction. For single-phase motors, reversal usually requires opening the motor and reconnecting the starting winding.
AC motors represent a major manufacturing step above DC motors but offer dramatic operational advantages. Once your community can produce laminated cores and wind coils, AC motors should become the standard for every stationary mechanical application.