Understanding AC Motor Types: Synchronous & Induction
AC Motor Types: Synchronous and Induction
Synchronous Motors
Two basic types of polyphase AC motors exist: synchronous and induction. The synchronous motor operates like a three-phase alternator in reverse. Its field magnets, mounted on a rotor, are energized by direct current. The armature coils are divided into three parts and receive three-phase alternating current. The varying current waves in the armature reaction create variable magnetic poles, causing the field to rotate at a constant speed, determined by the power line frequency. This constant speed is advantageous in specific applications. However, if the motor’s load becomes excessive, causing it to slow down, it may fall out of phase with the current frequency and stall. Synchronous motors can also function with a single-phase power source using appropriate circuit elements to generate a rotating magnetic field.
Induction Motors
The simplest electric motor is the squirrel cage induction motor, typically used with a three-phase supply. Its stator, similar to a synchronous motor, consists of three fixed coils. The rotor comprises a core with several high-capacity conductors arranged in a circle around and parallel to the core. Without a core, these conductors resemble cylindrical squirrel cages.
Key Differences and Advantages
Induction motors, also known as asynchronous motors, differ from other electric motors because no current is directly supplied to the rotor winding. Instead, the current is induced by the rotating magnetic field. The motor’s speed is not synchronized with the network frequency, hence the name “asynchronous.” These motors constitute over 80% of industrial electric motors due to their simple construction, light weight, compact size, low maintenance, and cost-effectiveness. Recent advancements in power electronics have enabled variable speed operation, further expanding their applications.
Types of Induction Motors
- Squirrel Cage Motors: The rotor winding consists of copper or aluminum bars shorted by two rings, typically joined by welding or molding.
- Wound Rotor Motors: The rotor winding is similar to the stator phase, with an equal number of poles. While more expensive, less robust, and requiring more maintenance than squirrel cage motors, wound rotor motors offer two key advantages: the rotor’s electrical circuit characteristics can be externally modified, and the rotor voltage and current are directly accessible for monitoring or control.