AC Motor Classification
The two primary classifications of AC motors are induction motors and synchronous motors. The choice between these types for a specific application depends on various factors such as speed and power requirements, voltage rating, load characteristics (including starting torque), efficiency requirements, maintenance needs, and operating environment conditions (e.g., underwater operation or temperature).
- Induction Motors:
- Named for the induction of a magnetic field in the rotor through electromagnetic induction, interacting with the stator field.
- No direct electrical connection to the rotor, eliminating the need for slip rings or brushes that wear out.
- Rotor current is induced by electromagnetic induction, akin to how transformers operate.
- Synchronous Motors:
- The rotor moves synchronously, at the same rate, as the rotating field of the stator.
- Used in applications where maintaining a constant speed is crucial.
- Not self-starting; requires starting torque from an external source or built-in starting windings.
- Like alternators, synchronous motors utilize slip rings and brushes to supply current to the rotor.
Rotating Stator Field
In both synchronous and induction AC motors, the stator windings are arranged in a similar manner, enabling the stator’s magnetic field to rotate. This rotating stator field mimics the effect of moving a magnet in a circle but is achieved electrically without any moving parts.
How can the magnetic field in the stator rotate if the stator itself does not move?
The rotation of the magnetic field is a result of the changing AC itself. Consider a three-phase stator, as depicted in Figure 43:
- At different times, one of the three phases “dominates” the current. For instance, when phase 1 is at 90°, the current in the phase 1 winding is at its maximum, and currents in the other windings are smaller.
- As a result, the stator magnetic field is oriented toward the phase-1 stator winding.
- As the phase-1 current decreases, the phase-2 current increases, causing the field to rotate toward the phase-2 winding, and so on.
- This rotation of the magnetic field occurs cyclically as the currents in the phases change, ultimately completing a full rotation as the field returns to the phase-1 winding.
The frequency of the applied voltage determines the rate at which the field rotates. A more detailed analysis shows that while the direction of the field changes, the magnitude of the field remains constant.
As the stator field moves, the rotor responds differently in synchronous and induction motors:
- Synchronous Motor: The rotor moves in sync with the stator field.
- Induction Motor: The rotor lags behind the rotating stator field.
The speed at which the stator field moves is known as the synchronous speed of the motor.