3. THREE-PHASE ALTERNATORS notes in english

 

3. THREE-PHASE ALTERNATORS:

3.1 Principle of Working:

A three-phase alternator works on the principle of electromagnetic induction. As the rotor (which is typically a magnet or electromagnet) rotates inside the stator (the stationary part of the alternator), a rotating magnetic field is created. This field induces an alternating current (AC) in the stator windings.

In a three-phase alternator, there are three sets of stator windings, spaced 120 degrees apart, producing three-phase electrical output. The alternator generates a three-phase AC voltage that is typically used in power generation, transmission, and distribution systems.

3.2 Moving and Stationary Armatures:

• Moving Armature: In some alternators, the armature is attached to the rotor, which rotates within the magnetic field. These are called rotating-armature alternators.
• Stationary Armature: In most modern alternators, the armature (the part where the voltage is induced) is stationary and fixed to the stator. The rotor rotates to produce the magnetic field, and the armature remains fixed to generate the AC power.

3.3 Constructional Details:

• 3.3.1 Parts and Their Functions:

  • Rotor: The rotating part of the alternator, which generates the magnetic field. The rotor can be of two types: cylindrical or salient pole, depending on the design and application.
  • Stator: The stationary part of the alternator, which consists of windings where the current is induced. It is usually made of laminated steel to reduce eddy current losses.
  • Slip Rings: These are used in some alternators to supply current to the rotor windings (in rotating field alternators).
  • Bearings: Support the rotor and allow it to rotate smoothly within the stator.
  • Exciter: Provides the DC current to the rotor to create the magnetic field.

• 3.3.2 Rotor Constructions:

  • Salient Pole Rotor: This rotor has large projecting poles and is used in low-speed alternators. The poles are energized with DC current to create a magnetic field.
  • Cylindrical Rotor: This rotor has a smooth surface and is used in high-speed alternators. It is commonly found in turbo-alternators and operates at high rotational speeds.

• 3.3.3 Windings: Single and Double Layer:

  • Single Layer Winding: This type of winding consists of one coil per slot in the stator. It is simpler and used in smaller machines.
  • Double Layer Winding: This type has two coils per slot, which makes it more efficient, as it allows for higher voltage and current handling. It is commonly used in larger alternators for higher power outputs.

3.4 Alternator Loading:

• 3.4.1 Factors Affecting the Terminal Voltage of Alternator: The terminal voltage of an alternator depends on several factors:

  • Load on the alternator: As the load increases, the voltage may drop due to internal impedance.
  • Speed of the rotor: If the rotor speed changes, it affects the frequency and magnitude of the output voltage.
  • Excitation: The level of excitation current (DC current supplied to the rotor) affects the output voltage. Higher excitation increases the terminal voltage.
  • Power factor of the load: A lagging power factor (inductive load) can cause a voltage drop, whereas a leading power factor can cause a rise in voltage.

• 3.4.2 Armature Resistance and Leakage Reactance Drops:

  • Armature Resistance: This is the resistance of the stator windings, and the current flowing through these windings produces a voltage drop (I * R). The higher the current, the higher the voltage drop across the armature resistance, reducing the terminal voltage.
  • Leakage Reactance: This is the reactance (opposition to current flow due to inductance) caused by the stator windings. It contributes to a voltage drop in the alternator when current is drawn through the machine.

3.5 Armature Reaction at Various Power Factors:

Armature reaction refers to the effect of the magnetic field produced by the current in the stator windings (armature) on the distribution of the magnetic field in the alternator. The reaction varies with the power factor of the load:

• Lagging Power Factor: In this case, the current in the stator windings is delayed with respect to the voltage, creating a magnetic field that weakens the main field of the rotor. This can lead to a decrease in voltage and efficiency.
• Leading Power Factor: When the power factor is leading, the stator current's magnetic field aids the rotor's magnetic field, which can improve the alternator's voltage.
• Unity Power Factor: At unity power factor, the armature reaction is minimal, and the alternator operates efficiently with minimal voltage drop.

3.6 Maintenance of Alternator:

The maintenance of an alternator is essential to ensure it operates efficiently and has a long operational life. Key maintenance tasks include:

• Cleaning: Regular cleaning of the alternator’s components, especially the stator and rotor, to prevent dust, dirt, and debris buildup that could cause overheating.
• Inspection: Routine inspection of the rotor, stator, slip rings (if applicable), and bearings for any wear or damage.
• Lubrication: Ensuring that the bearings are properly lubricated to prevent friction and wear.
• Exciter Checking: Regularly checking the exciter system to ensure the correct level of excitation current is provided to the rotor.
• Temperature Monitoring: Monitoring the temperature of the alternator to avoid overheating. Overheating can cause insulation breakdown and other mechanical failures.
• Testing Voltage and Frequency: Periodically testing the alternator’s output voltage and frequency to ensure they meet required specifications.