3.2 AC Induction Motors: Construction, Slip & Synchronous Speed

AC Induction Motors: Construction, Slip, and Synchronous Speed

The alternating current (AC) induction motor is the dominant motor type in industrial and commercial installations, powering the majority of pumps, fans, compressors, and conveyors that an RME-licensed electrician will encounter. Its dominance comes from simple, rugged construction: because the most common rotor design has no brushes or commutator, induction motors require very little maintenance, are inexpensive to manufacture, and are highly reliable in continuous industrial duty.

Construction

An induction motor has two main parts:

• Stator — the stationary outer structure, built from laminated iron with slots holding the motor's windings (typically three-phase). Energizing these windings produces the rotating magnetic field.
• Rotor — the rotating inner member, available in two designs:
  • Squirrel-cage rotor — conductive bars (usually aluminum or copper) embedded in the rotor core and short-circuited at both ends by end rings, forming a structure resembling a rotating cage. There are no external electrical connections, no brushes, and no slip rings — the rotor is essentially maintenance-free. This is by far the most common rotor type.
  • Wound-rotor — the rotor itself carries windings (typically three-phase) that are brought out to slip rings on the shaft, allowing external resistance to be connected into the rotor circuit. This adds cost and complexity but allows controlled starting torque and some external speed adjustment, useful for large fans, hoists, and cranes needing smooth acceleration.

The Rotating Magnetic Field

When three-phase currents, each 120 electrical degrees apart in time, flow through three sets of stator windings spaced 120 mechanical degrees apart around the stator, their combined magnetic effect is a magnetic field of constant magnitude that rotates around the stator bore at a constant speed. This rotating magnetic field sweeps past the rotor conductors, inducing voltages and currents in them by transformer action, and the resulting rotor currents interacting with the stator's field produce torque — hence the name "induction" motor, since the rotor is never directly connected to the supply.

Synchronous Speed

The speed at which the stator's magnetic field rotates is called synchronous speed (Ns), and it depends only on the supply frequency and the number of magnetic poles built into the stator winding:

Ns = 120f / P

where f is the supply frequency in hertz (Hz) and P is the number of poles (always an even number, since poles occur in north/south pairs).

Worked Example — Synchronous Speed at 60 Hz

These are the standard synchronous speeds you will see on induction motor nameplates and in exam problems for a 60 Hz supply, which is the Philippine standard utility frequency.

Slip — Why the Rotor Always Lags

If the rotor could somehow spin at exactly synchronous speed, the rotor conductors would experience no relative motion with respect to the rotating field, so no voltage would be induced in them, no rotor current would flow, and no torque would be produced. For an induction motor to develop torque, the rotor must always rotate slightly slower than the rotating field, maintaining the relative motion needed to keep inducing rotor current. This speed difference is called slip, defined as:

s = (Ns - Nr) / Ns

where Nr is the actual rotor speed. Slip is usually expressed as a percentage. At standstill (motor not yet turning), slip = 1 (100%) — the maximum relative motion and maximum induced rotor current. At full load, slip on standard induction motors is typically small, roughly 2% to 5%, meaning the rotor runs only slightly below synchronous speed. Because the rotor speed is never quite equal to (synchronous with) the stator field speed, induction motors are sometimes called asynchronous motors.

Worked Example — Calculating Slip

A four-pole, 60 Hz induction motor has a nameplate full-load speed of 1,750 rpm. First find synchronous speed: Ns = 120 x 60 / 4 = 1,800 rpm. Then slip = (1,800 - 1,750) / 1,800 = 50 / 1,800 = 0.0278, or 2.78%.

Full-Load Current and Nameplate Data

Every induction motor carries a nameplate listing data essential for correct sizing and protection:

• Full-load amps (FLA) — the current the motor draws at rated voltage, frequency, and load; used to size branch-circuit conductors and running overload protection.
• Locked-rotor current — the much larger current drawn at standstill, typically identified on the nameplate by a NEMA (National Electrical Manufacturers Association) code letter; commonly six to eight times FLA. This value drives the selection of short-circuit protection, which must tolerate the inrush without nuisance tripping, and is the reason large motors need reduced-voltage starting, covered in the next section.
• Rated horsepower/kW, voltage, frequency, number of poles/rated rpm, service factor, and insulation class — all needed to confirm the motor is correctly matched to its application and supply.

Reading and applying nameplate data correctly — especially FLA versus locked-rotor current — is a frequent subject of RME exam calculation problems.