Induction Motors Overview

Types of Induction Motors

• Squirrel Cage: Rotor consists of copper or aluminum bars shorted at ends, like a cage
• Slip Ring: Rotor has windings with slip rings and brushes for external resistance control

Rotating Magnetic Field

• Generated by applying AC current to stator windings
• Creates a constantly changing magnetic field that rotates
• Rotational speed determined by stator frequency and number of poles

Principle of Operation

• Rotating magnetic field induces current in rotor windings
• Induced current creates own magnetic field
• Interaction between stator and rotor fields produces torque, rotating the rotor

Torque-Slip Characteristics

• Slip: Difference between synchronous speed and rotor speed
• Torque: Force that causes rotation
• Torque increases with slip, reaching maximum torque at a specific slip (pull-out torque)
• As slip increases further, torque decreases

Starting and Maximum Torque

• Starting Torque: Torque generated at starting (high slip)
• Maximum Torque: Highest torque achievable (pull-out torque)

Effect of Parameter Variation on Torque-Slip Characteristics

• Rotor Resistance: Increasing resistance increases starting torque but reduces maximum torque
• Stator Voltage: Decreasing voltage reduces both starting and maximum torque
• Frequency: Lower frequency increases starting and maximum torque, but also reduces speed

Equivalent Circuit

• Represents induction motor as a combination of impedances
• Allows analysis of performance characteristics
• Includes stator impedance, rotor impedance, and magnetizing reactance

Phasor Diagram

• Visual representation of currents and voltages in the motor
• Helps analyze power relationships and identify efficiency

Performance Evaluation

• Direct Testing: Measuring motor characteristics under actual load conditions
• Indirect Testing: Using circle diagrams to estimate performance parameters

Circle Diagram

• Graphical representation of motor performance at various loads
• Constructed using data from indirect testing
• Provides information like efficiency, power factor, and torque-slip characteristics

Construction

• Squirrel Cage Rotor: Consists of a cylindrical core with slots that house a series of aluminum or copper bars, shorted together at the ends.
• Slip-Ring Rotor: Consists of a cylindrical core with slots that house a winding connected to slip rings, allowing for external resistance to be added to the rotor circuit.

Rotating Magnetic Fields

• Created by the interaction of alternating currents flowing in the stator windings.
• The magnetic field created by each winding rotates, resulting in an overall rotating magnetic field.
• The speed of the rotating magnetic field is called synchronous speed.

Principle of Operation

• Squirrel Cage: The rotating magnetic field induces currents in the rotor bars, creating a magnetic field that interacts with the stator field.
• Slip-Ring: The rotating magnetic field induces currents in the rotor winding, and external resistance can be used to control the rotor current and speed.

Torque-Slip Characteristics

• Squirrel Cage: As the rotor speed increases, the slip decreases, and the torque increases until it reaches a maximum value.
• Slip-Ring: The torque-slip curve is more adjustable due to the external rotor resistance, allowing for a wider range of operating speeds.

Starting and Maximum Torque

• Squirrel Cage: The starting torque is limited by the rotor resistance, often requiring a reduced voltage starting technique to avoid high starting currents.
• Slip-Ring: The starting torque can be increased by adding external resistance to the rotor circuit, providing a higher starting torque with lower starting current.

Effect of Parameter Variation on Torque-Slip Characteristics

• Rotor Resistance: Increasing rotor resistance increases starting torque but decreases maximum torque and efficiency.
• Stator Voltage: Lowering stator voltage reduces the torque-slip curve proportionally.
• Frequency: Lowering frequency reduces synchronous speed and shifts the torque-slip curve to the left.

Equivalent Circuit

• Squirrel Cage: A simplified model representing the stator and rotor as resistances and reactances.
• Slip-Ring: An equivalent circuit similar to the squirrel cage but with additional elements representing the external rotor resistance.

Phasor Diagram

• A graphical representation of the relationships between the voltages and currents in the motor under various operating conditions.

Performance Evaluation

• Direct Testing: Measuring the motor's performance directly using a dynamometer or other load devices.
• Indirect Testing: Using no-load and blocked-rotor tests to determine motor parameters, which can then be used to predict performance under different conditions.

Circle Diagram

• A graphical representation of the motor's performance characteristics, allowing for the visualization of power, torque, efficiency, and power factor at various operating points.