Alternator Complete Guide for Diploma & ITI Students – Working, Parts, Types & Uses

2. Rotor (Rotating Part)

The rotor is the rotating part that produces the magnetic field.

Components of Rotor:

• Field winding

• Rotor core

• Slip rings

• Shaft

Functions:

• Produces rotating magnetic field

• Rotates using mechanical input

• Induces voltage in stator winding

Types of Alternator Rotor

Alternator rotors are mainly classified into two types:

1. Salient Pole Rotor

• Large diameter

• Short axial length

• Poles project outward

• Used in low speed machines

• Suitable for hydroelectric plants

Features:

• More number of poles

• Used at low speed

• Large size

2. Non-Salient Pole Rotor (Cylindrical Rotor)

• Smooth cylindrical shape

• Small diameter

• Long axial length

• Used in high speed machines

• Suitable for steam turbine plants

Features:

• Fewer poles

• Used at high speed

• Strong mechanical construction

Construction of Alternator

Stator Construction

• Made of laminated silicon steel sheets

• Slots are cut on inner surface

• 3-phase armature winding placed in slots

• Windings are connected in star or delta

Rotor Construction

• Rotor carries field winding

• DC supply is given through slip rings and brushes

• When DC flows through rotor winding → magnetic field is produced

Working of Alternator

Step-by-step working:

1. DC supply is given to rotor field winding

2. Rotor produces a magnetic field

3. Prime mover rotates the rotor (turbine/engine)

4. Rotating magnetic field cuts stator conductors

5. EMF is induced in stator winding

6. Output is obtained as AC voltage

Alternator Frequency Formula

Frequency of generated voltage:

f=P×N120f = \frac{P \times N}{120}f=120P×N

Where:

• f = frequency (Hz)

• P = number of poles

• N = speed in RPM

Example:
If P = 4 and N = 1500 RPM

f=(4×1500)/120=50Hzf = (4 × 1500) / 120 = 50 Hzf=(4×1500)/120=50Hz

EMF Equation of Alternator

Generated EMF per phase:

E=4.44×f×Φ×T×Kp×KdE = 4.44 × f × Φ × T × Kp × KdE=4.44×f×Φ×T×Kp×Kd

Where:

• f = frequency

• Φ = flux per pole

• T = number of turns

• Kp = pitch factor

• Kd = distribution factor

Types of Alternators Based on Application

1. Automotive Alternator

• Used in vehicles

• Charges battery

• Supplies power to lights and systems

2. Power Plant Alternator

• Used in power stations

• Large size

• High voltage output

3. Portable Alternator

• Used in small generators

• For home backup supply

Excitation System of Alternator

Excitation means supplying DC current to rotor field winding.

Types of Excitation:

1. DC Excitation

• External DC source

• Battery or DC generator used

2. Brushless Excitation

• No brushes or slip rings

• More reliable

• Used in modern alternators

Advantages of Alternator

• High efficiency

• Rugged construction

• Less maintenance

• Can generate large power

• Better cooling system

• Suitable for high voltage generation

Disadvantages of Alternator

• Requires DC excitation

• Synchronization required before connecting to grid

• Complex control system

• Higher initial cost

Alternator vs DC Generator

Losses in Alternator

1. Copper Loss

Due to resistance of windings

2. Iron Loss

• Hysteresis loss

• Eddy current loss

3. Mechanical Loss

• Friction

• Windage

Voltage Regulation of Alternator

Voltage regulation is the change in terminal voltage when load changes from no-load to full-load.

Voltage Regulation=Eno−load−Vfull−loadVfull−load×100\text{Voltage Regulation} = \frac{E_{no-load} – V_{full-load}}{V_{full-load}} × 100Voltage Regulation=Vfull−loadEno−load−Vfull−load×100

Synchronization of Alternator

Before connecting an alternator to grid, these conditions must match:

• Same voltage

• Same frequency

• Same phase sequence

• Same phase angle

Methods:

• Lamp method

• Synchroscope method

Applications of Alternator

Power Generation

• Thermal power plants

• Hydro power plants

• Nuclear plants

• Wind turbines

Automotive

• Car charging system

• Truck electrical system

Industrial

• Standby generators

• Backup power systems

Renewable Energy

• Wind energy systems

• Micro hydro plants

Alternator in Power Plant

In power plants:

• Turbine rotates alternator rotor

• Rotor produces magnetic field

• Stator produces AC voltage

• Voltage stepped up by transformer

• Sent to transmission lines

Cooling Methods of Alternator

• Air cooling

• Hydrogen cooling (large machines)

• Water cooling

Large alternators use hydrogen because:

• Better cooling

• Low friction

• Higher efficiency

Maintenance of Alternator

Important maintenance points:

• Check insulation resistance

• Clean dust and moisture

• Inspect bearings

• Check brush condition

• Monitor temperature

• Tighten connections

Common Faults in Alternator

• Overheating

• Low output voltage

• Bearing failure

• Field winding open

• Insulation breakdown

Safety Precautions

• Never run without load control

• Proper grounding required

• Avoid overloading

• Check excitation system

• Use protective relays