1) Alternators generate electricity through the interaction of a magnet and coils moving relative to each other. 2) The simplest form of alternator is a bicycle dynamo, which uses a permanent magnet rotor. The output voltage depends on speed and magnet strength. 3) Early alternators used brushes that caused disadvantages, while later models replaced the permanent magnet with an electromagnet that allows adjustable output voltage.

1. Alternator Basic Theory
For generating electricity/induction we require
Magnet
Relative motion between the two
Coil
N S

2. Bicycle Dynamo
Simplest & most basic form of alternator is
bicycle dynamo.
Rotor is a Two Pole Permanent Magnet.
Voltage ∝ Speed.
Voltage ∝ Strength of the magnet. (Can not
be adjusted).
Alternator Basic Theory
Principle

3. Disadvantages
Two Carbon brushes are required to be
used.
Alternator Basic Theory
Brush System
Permanent magnet used earlier can be
replaced by an electromagnet.
Output voltage can be adjusted at the same
speed by changing DC supply.
Principle

4. Alternator Basic Theory
Brushless Separate excitation System
In order to avoid use of brushes we have to
use a small alternator named ‘Exciter’.
Main Field, Rotating Rectifiers & Exciter
Armature are mounted on the same shaft.
Excitation supply comes from battery to
exciter field instead of main field.
In this case the DC power required for
excitation is much lesser.
Disadvantages
This is an open loop system & input to
exciter does not vary for variation in output
voltage.
Principle

5. Alternator Basic Theory
Self Excited & Regulated Shunt System
In order to give correction in the exciter input
according to the voltage output we have to use
AVR.
Different Types of AVR’s are being used in DG
sets. The one that is being used currently is
Basler DECS
Principle

6. Main
Armature
Main
Field
Exciter
Armature
Rotating
Rectifiers
Exciter
Field
Surge
Suppressor
Short Circuit
Back Up CT’s
Rotor
AVR

7. AVR
Booster
Unit
End

8. Alternator Basic Theory
Stator
Winding
N
S
Main
Field
V
AC
Back

9. Alternator Separate Excitation
Brush System
AC
DC
Slip
Rings
Back
Stator
Winding
V
Main
Field
S
N
+
-

10. Alternator Separate Excitation
Brushless System
Stator
Winding
Main
FieldN
SV
AC
DC
Exciter
Rotating
Rectifiers
Back
+
-

11. Alternator Self Excitation
Brushless System
AC
DC
Rotating
Rectifiers
Back
ExciterStator
Winding
Main
FieldN
SV

12. SCM CT’S
N
S
Phase Terminals
Neutral Terminals
AC
DC Back