AC Induction Motor Design (2)

3. Induction Motor Construction
An induction motor consists of three basic components:

  • Stator: Houses the stator core and windings. The stator core consists of many layers of laminated steel, which is used as a medium for developing magnetic fields. The windings consist of three sets of coils separated by 120 degrees electrical.
  • Rotor: Also constructed of many layers of laminated steel. The rotor windings consist of bars of copper or aluminum alloy shorted, at either end, with shorting rings.
  • Endshields: Support the bearings which center the rotor within the stator.

4. Operating Principles

The basic principle of operation is for a rotating magnetic field to act upon a rotor winding in order to develop mechanical torque. The stator windings of an induction motor are evenly distributed by 120 degrees electrical. As the three phase current enters the windings, it creates a rotating magnetic field within the air gap (the space between the rotor and stator laminations). The speed that the fields travel around the stator is known as synchronous speed (Ns). As the magnetic field revolves, it cuts the conductors of the rotor winding and generates a current within that winding. This creates a field which interacts with the air gap field producing a torque. Consequently, the motor starts rotating at a speed N Ns in the direction of the rotating field.

The speed of the rotating magnetic field can be determined as:

Ns = (120 * f) / p — eq. 1

Where Ns is the synchronous speed, f is the line frequency, and p is the number of poles found as:

p = (# of groups of coils) / 3 — eq. 2

The number of poles is normally expressed as an even number.
The actual output speed of the rotor is related to the synchronous speed via the slip, or percent slip:

s = (Ns – N) / Ns — eq. 3

%s = s * 100 — eq. 4

to be continued……….

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