Three Phase Alternators: Introduction, Windings, Classification of Alternator, Construction of an Alternator, E.M.F. Equation, Maintenance of an Alternator.

 Introduction

The machine that generates an alternating emf is called as synchronous generators or A.C. generators. It Is also called as an Alternator.

An alternator can also be operated as a motor which is called synchronous motors.

Both these machines i.e. the alternator and synchronous motor work at a specific constant speed they are called synchronous speed.

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Windings

There are two windings of an alternator:

1. Field winding 2. Armature winding

The field winding is placed on the rotor i.e.. it is a rotary winding.

The armature winding is placed on the stator i.e. armature is a fixed winding.

Stator: Armature

Rotor: Field

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Classification of Alternator

Classification based on speed:

1. Low speed alternator (60- 500 RPM)
2. Medium speed alternator (600- 1000 RPM)
3. High speed alternator (1500- 3000 RPM)

Classification based on voltage:

1. Small capacity alternator - 250, 420,1200, 3300 Volts.
2. Medium capacity alternator - 3.3kv, 6.6kv.
3. Large capacity alternator - 6.6kv, 11kv, 33kv.

Classification based on construction:

1. Revolving armature type
2. Revolving field type

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Construction of an Alternator

Construction of a synchronous machine

Alternator is a synchronous type of machine.

Field windings are placed on the rotor and the armature winding is wound and the armature winding is placed in the stator slots of suitable shape.

The field windings (rotor) are connected to an external dc source called exciter, through a pair of slip rings.

The armature winding is a 3 phase winding placed on the stator and the induced voltage in this winding is applied to the load.

Armature winding produces a rotating magnetic field.

Prime mover is a motor or diesel generator which is mechanically coupled to the shaft of the alternator. The prime mover is used to rotate the armature at a speed called synchronous speed ( Ns).
The rotating field, fixed armature arrangement Preferred because it gives certain advantages.

E.M.F. Equation of an Alternator

Let Φ be the flux per pole in webers

Let f be the frequency of induced emf in Hz and Tph be number of turns of armature conductors per phase.

When a single conductor cuts flux of one pole the emf is induced in it

It is given by Lenz's law as follows

e_avg = Flux cut/Time taken

e_avg = dΦ/dt

This is the induced emf in one conductor cutting flux of one pole.

If N rpm is the speed of rotation then time required for 1 complete revolution = (60/N)seconds

Total flux cut by a single conductor = dΦ = d x P

Where P = Number of poles

e_avg per conductor = ΦP / 60/N Volts

e_avg per conductor = ΦPN / 60 Volts

But we know that f = PN/120

Eph = 4.44 Φf_Tph Volts

emf(line value) = √3E_ph

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Maintenance of an Alternator

Test stand:

For the maintenance purpose, an alternator used test stand is for testing alternators and constant speed motors in a repair facility.

Preventive maintenance:

1. General inspection.
2. Lubrication service.
3. Cooling system service.
4. Fuel system sevice.
5. Servicing and testing starting batteries.
6. Regular engine exercise.

General inspection:
The general periodic inspection and maintenance of a alternator involves the following
Check the wiring, the brushes, and the drive unit for faults.

Check the exciter for its residual magnetism.

If the residual magnetism is low, it can be restored by flashing the field.

Clean the alternator exterior with an approveed fiuid.
Smooth a rough or pitted exciter commutator a slip ring with 000 sandpaper, it then clean and with a clean, dry cloth.