Synchronous Motor Notes

UNIT V: Synchronous Motor Notes

Syllabus

  • Synchronous Motor:
    • Synchronous motor principle and theory of operation
    • Effect of excitation on current and power factor
    • Synchronous condenser
    • V and inverted V curves
    • Expression for power developed
    • Hunting and its suppression
    • Methods of starting
    • Applications

Introduction

  • A synchronous motor is a device that converts electrical energy into mechanical energy, operating at synchronous speed.
  • It works only at synchronous speed, which is constant regardless of load changes, although speed may vary temporarily upon loading.

Construction of Three Phase Synchronous Motor

  • Comprises two main parts:
    1. Stator:
    • Contains a three-phase winding (star or delta).
    • Powered by a three-phase AC supply.
    1. Rotor:
    • Features a field winding (can be salient or non-salient type).
    • Commonly uses salient pole construction, excited by a separate DC supply through slip rings.

Principle and Theory of Operation

  • Operates on the principle of magnetic locking.
  • When opposite poles of magnets are close, they experience a strong attractive force.
  • If one magnet rotates (e.g., rotor), the other must also rotate in the same direction and at the same speed due to magnetic locking.
  • In a synchronous motor, both stator and rotor poles interact, affecting the motor's rotation.

Key Aspects of Magnetic Locking

  • Requires two unlike poles (N and S) to be near each other for locking.
    • AC supply induces a rotating magnetic field (synchronous speed = determined by supply frequency $f$ and pole number $P$).
    • For an AC supply of frequency $f$ and $P$ poles, synchronous speed ($Ns$) is given by: Ns = rac{120f}{P}

Magnetic Field Dynamics

  • The three-phase AC supply generates a rotating magnetic field that dictates synchronous speed.
  • The rotor, initially stationary, will be pulled into synchronism by the rotating stator field, provided that magnetic locking occurs.
  • Synchronous motors are not self-starting due to the need for this locking.

Effect of Excitation on Current and Power Factor

  • Load changes while maintaining constant excitation increase the current drawn.
  • Changing excitation while maintaining load alters the motor's power factor, a unique feature of synchronous motors.

Operation Scenarios

  • Normal Excitation: Induced emf $E_b = V$.
  • Under Excitation: $E_b < V$; current increases, leading to a lagging power factor.
  • Over Excitation: $E_b > V$; current increases again, switching to a leading power factor.
  • Critical Excitation: $E_b = V$; power factor is unity with minimum current drawn.

Synchronous Condenser

  • An over-excited synchronous motor, running on no load, acts similarly to a capacitor, drawing leading power factor current.
  • Utilized in power factor correction, functioning as a synchronous condenser.

V and Inverted V Curves

  • V-Curves: Plot armature current ($Ia$) against field current ($If$); demonstrates how $I_a$ varies with excitation.
  • Inverted V-Curves: Relate power factor to field current; shows power factor response to changes in excitation.

Expression for Power Developed

  • In a synchronous motor, the armature resistance is often negligible compared to synchronous reactance.
  • Key parameters include:
    • Input power: $P = VI ext{cos } heta$
    • Mechanical output power and losses to be calculated depending on the armature losses.

Hunting and its Suppression

  • Hunting: Oscillation of the rotor due to inertia when sudden load changes occur, causing periods of instability.
  • The load angle $ heta$ experiences fluctuations, affecting current draw and stability.
  • Damper Windings: Installed in pole face slots, provide induced EMF opposing rotor oscillations, minimizing hunting.

Methods of Starting Synchronous Motor

  • Synchronous motors are not self-starting; various methods include:
    1. Pony Motors: Use an external device to bring the rotor to synchronous speed.
    2. Damper Windings: Enable induction characteristics during start-up.
    3. Slip Ring Induction Motor: Employing external rheostats for resistance at start.
    4. Coupled DC Machine: Starts the motor and acts as an exciter post start.

Applications of Synchronous Motors

  • Used in constant speed applications like:
    • Machine tools, generators, synchronous clocks, compressors, pumps, and mills.
  • Effective in power factor correction and voltage regulation in transmission lines.

Disadvantages of Synchronous Motors

  • Higher costs, maintenance needs, and reliance on an external DC source for excitation.
  • Overall initial setup is costly.