Synchronous Machine Introduction

Unit Overview

  • Unit Topic: Synchronous Machines

  • Key Areas Covered:

    • Construction and working of synchronous machines

    • Types: salient pole and non-salient pole

    • Characteristics and performance evaluation

    • Parameters: regulation, efficiency, testing, parallel operation, applications.

Synchronous Machine Types

  • Synchronous Generator (Alternator):

    • Converts mechanical power from steam, gas, or hydraulic turbines to AC electrical power.

    • Primary source of electrical energy in large AC power networks.

  • Synchronous Motor:

    • Operates at constant speed, larger in size compared to induction motors.

    • Used for constant speed industrial drives.

Definition of Synchronous Machine

  • A machine that operates at a constant speed and frequency regardless of load conditions, defined by the equation:

    • N = 120f/P

      • N: speed in revolutions per minute (RPM)

      • f: frequency in Hertz (Hz)

      • P: number of poles.

Operating Principle

  • Based on Faraday’s Law of Electromagnetic Induction.

  • Armature winding is located on the stator; field winding is on the rotor and excited by DC current.

Construction Details

Synchronous Generator Construction

  • Conversion: Mechanical energy to AC electric energy.

  • Rotor Winding: Energized by DC to generate a magnetic field.

  • Induced Voltage: 3-phase voltages generated in stator windings.

Rotor Types

1. Salient Pole Rotor

  • Characteristics:

    • Projected pole type, large diameter, small axial length.

    • Mechanical strength is lower, suitable for low-speed alternators (125-500 RPM).

    • Applications include water turbines and internal combustion engines.

2. Cylindrical (Non-Salient Pole) Rotor

  • Characteristics:

    • Smooth solid steel cylinder with slots for field coils.

    • Small diameter and large axial length, high mechanical strength.

    • Preferred for high-speed applications (1500-3000 RPM).

    • Common in steam turbines and electric motors.

Rotor and Stator Construction

  • Stator:

    • Similar to induction motor construction with laminated low silicon steel rings.

    • Example: 36 slot stator with 3 coil conductors per slot and 12 slots per phase.

  • Rotor:

    • Two types: Salient Pole and Cylindrical (Non-Salient Pole).

    • Specific designs are implemented for varying application speeds.

Slip Rings and Brushes

  • Slip Rings: Connect rotor windings to DC supply while rotating.

    • Insulated metal rings providing electrical connection.

  • Brushes: Conductive blocks that apply DC voltage to field windings, can cause power losses if maintenance is neglected.

Synchronous Generator Operation

  • Induces electromagnetic force (EMF) in the stator coils as the rotor turns.

  • Delayed phase differences of 20/3 = 6.667ms between phases due to differences in the EMF.

Synchronous Motors

Introduction and Principles

  • Rotates at synchronous speed determined by the rotating magnetic field (RMF).

  • Stator and rotor construction similar to the induction motor, but rotor is locked to RMF.

Load Changes

  • Increased load results in rotor lag but maintains synchronous speed.

  • Overloading can cause loss of synchronization (pull out torque).

Starting Methods

  1. External Prime Mover: An external motor drives the synchronous motor initially.

  2. Damper Windings: Utilizes copper bars in rotor to allow initial operation as an induction motor.

Operational Characteristics

  • Must achieve synchronous speed for operation.

  • Can adjust power factor, beneficial in industrial settings.

Parallel Operation of Alternators

  • Advantages:

    • Enlarged load capacity by connecting multiple generators.

    • Maintains power supply during generator failures.

    • Facilitates maintenance without total power loss.

Conditions for Parallel Operation

  • Exactly matched RMS line voltages are critical.

  • Same phase sequence and equal phase angles must be maintained.

  • The incoming generator's frequency must be slightly higher during connection.

General Procedure for Paralleling Generators

  1. Adjust field current to match voltage.

  2. Confirm phase sequences using an induction motor test.

  3. Match frequency before connecting to the system.

  4. Use synchroscopes or light bulb methods to ensure in-phase connection and minimize damage risk.

Synchronizing Techniques

  • Three Dark Lamps: Indicate synchronization readiness.

  • Two Bright and One Dark Lamp: Signal frequency comparisons to ensure correct connections.

  • Synchroscope Method: Measures the frequency difference and assists in timing the closure of the switch.