Electric Motor Fundamentals: Induction and Single-Phase Types

Induction and Asynchronous Motors

Induction motors, also known as asynchronous motors, operate with damped windings where voltage is induced in the rotor coils. They do not require a rotor excitation current to operate.

Internal Structure of Induction Motors

The stator of an induction motor is identical to that of a synchronous motor, but the rotor differs, with two primary types:

• Squirrel Cage Rotor: This type is formed by a series of conductive bars placed within the outer slots of the rotor. All ends of these bars are connected by a short-circuit ring.
• Wound Rotor: This rotor has three-phase windings, identical to the stator. The three-phase rotor winding is typically connected so that its endpoints are connected to slip rings. Three brushes are placed on the axis and make contact with these rings, which are then short-circuited.

Speed-Slip in Induction Motors

Slip (s) is defined as the difference between the synchronous speed and the rotor speed of an induction motor.

Starting Methods for Induction Motors

Starting methods for squirrel cage rotor motors include:

• Direct Online (DOL) Starting: Direct connection of the stator to the electric power supply.
• Resistor Starting: A resistor is connected in series with each phase of the stator. As the motor accelerates, these resistors are gradually removed from the circuit.
• Autotransformer Starting: The stator is connected to the power supply through an autotransformer, which reduces the initial voltage.
• Star-Delta (Y-Δ) Starting: The motor starts with the stator coils connected in a star (Y) configuration, so each coil receives a voltage equal to the line voltage divided by the square root of 3 (V_line/√3). In the second phase, the star connection is switched to a delta (Δ) connection, and the coils are fed with the full line voltage from the power supply.

Single-Phase and Special Electric Motors

• Universal Motor Principles

  A universal motor is one that can operate with both DC and single-phase AC power. It is essentially a DC series excitation motor where both the poles and the stator must be wound. However, magnetic losses in AC operation are very large.

• Single-Phase Induction Motors

  A single-phase induction motor is an induction motor with a similar rotor but with a single coil in the stator. Solutions for starting single-phase induction motors include:

  • Split-Phase Motor: The stator coil is divided into two parts: a main winding and an auxiliary winding. These are designed with a 90-degree electrical phase delay, where the auxiliary coil offers greater electrical resistance.
  • Capacitor Motor: The stator coil is also divided into two parts. An electrolytic capacitor is connected in series with the auxiliary coil, causing its current to lead the main coil's current by 90 degrees. Since the coils are mechanically displaced by 90 degrees, this current phase difference produces a unique, uniform rotating magnetic field, making the motor behave as if it were a multi-phase connection.