Alternators - Construction

Salient Pole Type Rotor

Non Salient Pole Type or Cylindrical Type Rotor

EMF EQUATION OF ALTERNATORS

EMF EQN. COND……….

ALTERNATOR ON LOAD

When the load on the alternator is increased (i.e., armature current

Ia is increased), the field excitation and speed being kept

constant, the terminal voltage V (phase value) of the alternator

decreases.

REASON FOR VOLTAGE DROP:

(i) Voltage drop IaRa where Ra is the armature resistance per phase.

(ii) Voltage drop IaXL where XL is the armature leakage reactance

per phase.

(iii) Voltage drop because of armature reaction (Xa)

Synchronous Reactance (Xs)

DETERMINATION OF VOLTAGE REGULATION

DETERMINATION OF OCC

DETERMINATION OF SHORT CIRCUIT CHARACHTERISTICS (SCC)

SYNCHRONOUS MOTORS

FEATURES OF SYNCHRONOUS MOTORS

• A synchronous motor runs at synchronous speed or not at all. Its speed is constant (synchronous speed) at all loads. The only way to change its speed is to alter the supply frequency (Ns = 120 f/P).

• Synchronous motor is that it can be made to operate over a wide range of power factors (lagging, unity or leading) by adjustment of its field excitation.

• Synchronous motor can be made to carry the mechanical load at constant speed and at the same time improve the power factor of the system.

• Synchronous motors are generally of the salient pole type.• A synchronous motor is not self-starting and an auxiliary means

has to be used for starting it.

Non-Self-Starting Action of Synchronous Motor

Making Synchronous Motor Self-Starting By External Means

Making Synchronous Motor Self-Starting by Dampers

Starting with Dampers contd..

• 3-phase supply is given to the stator winding while the rotor

Field winding is left unenergized.

The rotating stator field induces currents in the damper or squirrel cage winding and the motor starts as an induction motor.

• As the motor approaches the synchronous speed, the rotor is excited with direct current.

EQUIVALENT CIRCUIT