Induction Motor
Part of Generators and Motors
How a rotating magnetic field induces currents in a squirrel-cage rotor to produce torque without any electrical connection to the rotor.
Why This Matters
The induction motor is the workhorse of industrial civilization. It has no brushes, no commutator, no electrical connection to its rotor — it works entirely through electromagnetic induction. This makes it rugged, nearly maintenance-free, and capable of running for decades with only bearing replacements. For a rebuilding civilization that needs reliable mechanical power from electrical energy, the induction motor is the most important motor type to understand and build.
Unlike DC motors, which require a commutator that wears and sparks, or synchronous motors, which require rotor excitation, the induction motor is self-starting from AC power and has essentially no wearing parts. The rotor conductors carry induced currents, not externally supplied ones, so there are no slip rings or brushes to maintain.
Understanding induction motors also illuminates the relationship between frequency, pole count, and speed — essential knowledge for matching motor speed to mechanical loads, and for understanding why grid frequency matters to every motor connected to it.
The Rotating Magnetic Field
The induction motor works because the stator produces a magnetic field that rotates in space. In a three-phase motor, three sets of stator windings, displaced 120° around the stator and fed with three-phase currents displaced 120° in time, produce a field that rotates continuously. At any instant, the combined field from the three phases produces a single resultant that sweeps around the stator bore at the supply frequency.
The speed of this rotating field is the synchronous speed: Ns = 120 × f / P, where f is frequency in Hz and P is the number of poles. For a 50 Hz supply and 4-pole motor: Ns = 120 × 50 / 4 = 1500 RPM. For a 2-pole motor at 60 Hz: Ns = 3600 RPM.
Even single-phase AC can produce a rotating field with the right winding arrangement. A capacitor-start motor uses a capacitor to shift the phase of current in a start winding by about 90°, creating a two-phase approximation that starts the rotor turning. Once rotating, a single-phase motor sustains torque through a combination of effects. For rebuilt civilization applications, single-phase motors are simpler to connect but require capacitors, which may be scarce.
The Squirrel-Cage Rotor and Slip
The rotor of a standard induction motor consists of aluminum or copper bars embedded in slots around an iron core, short-circuited at each end by end rings. This assembly, resembling a squirrel cage in shape, has no external connections. It is entirely self-contained.
When the stator field rotates faster than the rotor (which it always does under load), the rotor conductors experience a changing magnetic field — they are being “lapped” by the rotating field. By Faraday’s law, this induces currents in the rotor bars. These rotor currents, in the presence of the stator field, experience a force (F = BIL) that drives the rotor to follow the field.
Slip is the difference between synchronous speed and actual rotor speed, expressed as a fraction: s = (Ns − Nr) / Ns. A motor running at 1450 RPM on a 1500 RPM synchronous field has slip of (1500 − 1450)/1500 = 0.033, or 3.3%. Slip is essential to operation — zero slip means no relative motion, no induced current, no torque. At full load, typical slip is 2–8%.
Building a Simple Induction Motor
Constructing an induction motor from scratch requires making a stator with three-phase windings and a squirrel-cage rotor. Both require precision but are achievable with hand tools and patience.
Stator construction: Cut a stack of iron laminations (silicon steel sheet, 0.35–0.5 mm thick) with slots around the inner bore. The slot count should be divisible by the number of phases times poles — for a 3-phase 4-pole motor, 36 slots is common. Stack the laminations to the desired core length and bind them tightly. Wind three sets of coils (one per phase) in the slots using enameled copper wire. Connect the ends of each phase winding to form either a star (Y) or delta connection.
Rotor construction: The squirrel cage is most easily made by die-casting aluminum into rotor slots, but this requires a foundry. An alternative is to press copper or aluminum bars into machined slots and weld or braze end rings at each end. The bars must fit snugly in the slots to ensure good electrical contact. Laminate the rotor core just as the stator to reduce eddy currents.
Air gap: The clearance between rotor outer diameter and stator bore must be uniform and small — 0.3 to 0.8 mm for machines under 10 kW. A non-uniform air gap causes vibration and unequal current distribution in phases. Use precision bearings and a accurately bored stator to achieve this.
Torque-Speed Characteristics
The torque produced by an induction motor varies with slip in a predictable way. Starting from rest (slip = 1.0), starting torque depends on rotor resistance — high rotor resistance gives high starting torque but poor running efficiency. At normal running slip (2–8%), torque is proportional to slip: double the load, double the slip, approximately double the torque.
At very high slip (rotor nearly stalled), the motor enters a breakdown torque region where additional load causes the motor to stall suddenly rather than slow gradually. The breakdown torque is typically 2–3 times the rated torque. Operating continuously above rated torque overheats the motor because rotor copper losses increase as the square of current.
For a rebuilding civilization, the practical implication is to match motor size generously to load. An undersized motor running near breakdown torque will fail quickly. An oversized motor running at 50–60% of rated load runs cooler and lasts longer, at the cost of somewhat lower power factor.
Starting, Protecting, and Running
Induction motors draw 5–7 times rated current at starting (locked rotor current). This large starting current stresses the windings and can trip circuit breakers. For small motors under 5 kW, direct-on-line starting is acceptable. For larger motors, reduce starting current by temporarily reducing the applied voltage (star-delta starter, autotransformer starter) or by using a wound-rotor motor where external resistance limits starting current.
Protection: Every induction motor needs overload protection (thermal relays or fuses set to 115–125% of rated current) to prevent burnout from sustained overload, and short-circuit protection (fuses or breakers at 200–400% of rated current) for fault clearing. Without overload protection, a stalled motor will burn out its windings in minutes.
Cooling: Standard motors are totally enclosed fan-cooled (TEFC) — a shaft-mounted fan blows air over external fins. For hand-built motors without a fan, ensure air can circulate around the stator. The stator winding temperature must not exceed the insulation class rating: class B insulation (standard) allows 130°C total temperature, class F allows 155°C.
Diagnosing Induction Motor Problems
An induction motor that fails to start may have an open winding (check with ohmmeter across each phase — all should be equal and non-zero), a failed starting capacitor (single-phase motors), or a mechanical jam. If it hums but does not rotate, suspect a single-phasing condition — one phase is missing from the supply.
A motor that runs but produces less than rated torque may have a partial winding short (turns shorted together — hard to detect without a megohmmeter) or high slip from a worn rotor bar. Cracked or broken rotor bars cause speed fluctuations at twice slip frequency — a characteristic hunting or pulsing at light load that disappears under heavy load.
Overheating without obvious mechanical cause usually means inadequate ventilation, sustained overload, or incorrect voltage. Motors run hot if the voltage is significantly below rated (say, 10% low) because they must draw more current to deliver the same torque. A motor rated 380 V being fed 340 V will run noticeably hotter. Check supply voltage under load and correct if needed.