Understanding Single Excited Motors

A motor is considered single excited if only one winding or part of the motor needs an external electrical supply to create the magnetic fields necessary for operation. In contrast, a double-excited motor typically requires separate external electrical supplies for both the stator and the rotor windings.

Three Phase Squirrel Cage Motor Excitation

The three-phase squirrel cage induction motor is a classic example of a single-excited motor. Here's why:

• The stator of the squirrel cage motor has three windings connected to a three-phase AC power supply. This supply directly creates a rotating magnetic field in the stator.
• The rotor of the squirrel cage motor consists of conducting bars short-circuited at the ends, resembling a squirrel cage.
• There is no external electrical connection or separate power supply to the rotor.
• Currents in the rotor bars are induced solely by the relative motion between the rotor and the stator's rotating magnetic field (due to electromagnetic induction).
• These induced rotor currents interact with the stator's magnetic field to produce torque and rotate the motor.

Since only the stator winding receives external electrical excitation from the power supply, the three-phase squirrel cage motor operates based on single excitation.

Comparing Excitation Types

Let's look at the other options provided:

• Synchronous motor: A synchronous motor typically requires a three-phase AC supply for the stator winding to create a rotating magnetic field and a DC supply for the rotor winding (field winding) to create a constant magnetic field. This is a double-excited motor.
• DC motor: A DC motor requires a DC supply for the armature (rotor) and a DC supply for the field winding (stator or permanent magnet). Even in a permanent magnet DC motor, the field is provided by the magnet, and the armature is electrically excited. Generally considered to involve excitation of both stationary and rotating parts through direct connection or induced fields requiring connection elsewhere. In the context of the AC motors listed, DC motors are distinct but involve exciting both field and armature circuits.
• Servo motor: A servo motor is a motor used in control systems. It can be a DC servo or an AC servo (like a brushless DC or induction motor). The excitation depends on the specific type of motor used as a servo. It's a classification based on application and control method, not purely on the excitation principle in the same fundamental way as synchronous or induction motors.

Based on the fundamental principle of requiring only one external electrical source connected to a winding (the stator) to initiate the magnetic field cycle leading to torque production, the three phase squirrel cage motor is accurately classified as a single excited motor among the given options.