AC Motors: Construction and Characteristics Quiz

Shaded Pole Motor

• A shaded pole motor has a slight component of rotation in the field, causing the rotor to rotate.
• The direction of rotation can be reversed by altering the direction of the rotating magnetic field across the pole face.
• Shaded pole motors are simple and inexpensive, but have low efficiency and a very low starting torque.
• Speed regulation is poor, and they must be fan-cooled during normal operation.
• They are used in shaft-mounted fans and blowers, small pumps, toys, and intermittently used household items.

Synchronous Motor

• A synchronous motor works by rotating a magnetic field, but it cannot be started from standstill by applying three-phase AC power to the stator.
• The rotor does not have a chance to get started due to the high-speed rotating magnetic field.
• A synchronous motor has no starting torque, but it has torque only when it is running at synchronous speed.
• A squirrel-cage winding is added to the rotor to cause it to start.
• The squirrel cage is a series of heavy copper bars shorted together by copper rings, which induces a magnetic field that interacts with the rotating field of the stator.

AC Motor Theory

• AC motors have advantages, including wide availability of AC power, lower cost, and fewer maintenance problems.
• Most types of AC motors do not use brushes and commutators, eliminating problems of maintenance and wear.
• AC motors are well suited for constant-speed applications because their speed is determined by the frequency of the AC voltage applied to the motor terminals.
• Industry builds AC motors in different sizes, shapes, and ratings for many different types of jobs.

Induction Motor

• An induction motor derives its name from the fact that AC voltages are induced in the rotor circuit by the rotating magnetic field of the stator.
• Induction motors are probably the simplest and most rugged of all electric motors.
• There are only two main components: the stator and the rotor.
• The stator contains a pattern of copper coils arranged in windings, which form a moving magnetic field that induces a current in the rotor.
• The interaction of the fields produces a torque on the rotor.

Induction Motor Rotor

• The induction rotor is made of a laminated cylinder with slots in its surface.
• The windings in these slots are of two types: squirrel-cage winding and auxiliary winding.
• The squirrel-cage winding is the most common, made up of heavy copper bars connected together at each end by a metal ring.

Other Types of Induction Motors

• Split-phase motors have a starting winding in addition to the main winding and are used for small applications where high starting torque is not required.
• Capacitor start motors have a high starting torque and are used for applications where high starting torque is required.
• Permanent-Split Capacitor Motors have a capacitor left in series with the starting winding during normal operation and are used for low-inertia loads.
• Resistance-Start Motors have a starting winding in addition to the main winding, switched in and out of the circuit, and are used for applications where high starting torque is required.