Polyphase AC Motors: Synchronous and Induction
Polyphase AC Motors
Two basic types are designed to operate polyphase AC motors: synchronous motors and induction motors.
Synchronous Motors
The synchronous motor is essentially a three-phase alternator that works in reverse. The field magnets are mounted on a rotor and are excited by direct current. The armature coils are divided into three parts with three-phase alternating current. The variation of the three waves of current in the armature reaction causes a variable magnetic field, making the field rotate at a constant speed determined by the frequency of the AC power line. Constant speed synchronous motors are advantageous in certain applications. However, they may stall under heavy mechanical loads if the engine slows down and becomes out of phase with the current frequency. Synchronous motors can operate with a single-phase power source by including appropriate circuit elements to achieve a rotating magnetic field.
Induction Motors
The simplest type of electric motor is the induction motor with a three-phase supply, often using a squirrel cage design. The stator of this motor consists of three fixed coils, similar to the synchronous motor. The rotor comprises a core with high-capacity conductors positioned in a circle around the shaft and parallel to it. When they have no core, the rotor conductors resemble cylindrical cages used for squirrels.
Induction Motors Details
The key difference of an induction motor from other electric motors is that no current is directly led to one of its windings (usually the rotor). The current in the rotor winding is induced by the rotating field, hence the name induction motor.
Induction motors are also called asynchronous motors because the motor speed is not synchronized with the network frequency. Over 80% of electric motors used in industry are induction motors, typically operating at a nearly constant rate. However, with the development of power electronics (inverters and cycloconverters), variable speed induction motors are increasingly used.
The extensive use of induction motors is due to:
- Simple construction
- Low weight
- Minimum volume
- Low maintenance cost
- Lower cost compared to other electric motors
Types of Induction Motors
- Squirrel Cage Motors: The rotor winding consists of copper or aluminum bars, with ends shorted by two rings joined by welding or molding.
- Wound Rotor Motors: The rotor winding consists of a coil similar to the stator phase with an equal number of poles. A wound rotor motor is more expensive, less robust, and requires more maintenance than a squirrel cage motor. However, it offers two main advantages: the rotor’s electrical circuit characteristics can be changed externally, and the rotor voltage and current are directly accessible for measurement or electronic monitoring.