What is Alternator?

     

    An alternator is a machine that generates AC (Alternating Current) by converting mechanical energy into electrical energy. It is also known as an AC generator or synchronous generator. There are different types of alternators based on their applications and design, including marine, automotive, diesel-electric locomotive, brushless, and radio alternators. Based on design, alternators can be either salient pole type or cylindrical rotor type.

    Construction of an Alternator

    The main components of an alternator or synchronous generator are rotor and stator. The main difference between rotor and stator is, the rotor is a rotating part and stator is not a rotating component means it is a stationary part. The motors are generally run by rotor and stator.

    An alternator has two main parts: the rotor and the stator.

    • Stator: The stationary part, similar to the stator in an induction motor.
    • Rotor: The rotating part, located inside the stator. It has a series of electromagnets arranged in a cylinder to create a magnetic field when it rotates.
    • There are two types of rotors they are shown in the below figure.

    1. Salient Pole Rotor:

      • Poles project outward from the center.
      • Uses DC supply for field winding to create North (N) and South (S) poles.
      • Used in low-speed machines (120-400 rpm), like in hydro and diesel power stations.
      • Not balanced, hence limited to lower speeds.

    2. Cylindrical Rotor:

      • Also known as non-salient or round rotor.
      • Used in high-speed machines (1500-3000 rpm), such as in thermal power plants.
      • Made of a steel cylinder with slots for field windings, which are connected in series.
      • Advantages: mechanically strong, uniform flux distribution, high-speed operation, and low noise.

    Both the rotor and stator are essential for the operation of an alternator, with the rotor creating a magnetic field as it spins inside the stationary stator.

    Working Principle of Alternator

     

    All the alternators work on the principle of electromagnetic induction.According to this law, for producing the electricity we need a conductor, magnetic field and mechanical energy. Every machine that rotates and reproduces Alternating Current. To understand the working principle of the alternator, consider two opposite magnetic poles north and south, and the flux is traveling between these two magnetic poles. 

    1. Setup:

      • Imagine two magnets, one with a north pole and the other with a south pole.
      • Between these magnets, there’s a rectangular coil of wire.

    2. Initial Position:

      • At first, the coil is positioned parallel to the magnetic field lines between the north and south poles.
      • Since there’s no cutting of magnetic flux, no electricity is generated. The waveform is at zero degrees.

    3. Rotation and Induction:

      • As the coil rotates clockwise, sides A and B of the coil face the south pole while sides C and D face the north pole.
      • Now, the coil cuts across the magnetic field lines, inducing a maximum amount of current because the coil and the flux lines are perpendicular to each other.
      • This maximum current position corresponds to a waveform at its peak.

    4. Further Rotation:

      • Continuing the rotation, the coil reaches a position where it’s vertical, parallel to the magnetic field lines.
      • In this position, no cutting of magnetic flux occurs, resulting in zero induced current and a waveform back to zero degrees.

    5. Completing the Cycle:

      • In the second half of the cycle, the coil continues rotating clockwise until sides A and B face the north pole and sides C and D face the south pole.
      • Current flows again but in the opposite direction, reaching another peak in the waveform.
      • After completing a full rotation (360 degrees), the cycle repeats.

    6. Three-Phase Generation:

      • For three-phase generation, multiple coils are placed with a 120-degree displacement between them.
      • Each coil follows the same principle of induction, but with a phase shift, resulting in a three-phase alternating current (AC) output.

    So, the alternator’s working principle is all about rotating coils to cut through magnetic fields, inducing a current that alternates in direction to produce AC electricity.

     

    Characteristics

    The characteristics of an alternator are

    1. Output Current with Speed of Alternator: The output of the current reduced or decreased when the alternator speed reduced or decreased.
    2. The efficiency with Speed of Alternator: Efficiency of an alternator is reduced when the alternator runs with low speed.
    3. Current Drop with Increasing Alternator Temperature: When the temperature of an alternator increased the output current will be reduced or decreased.

    Advantages

    The advantages of an alternator are

    • Cheap
    • Low weight
    • Low maintenance
    • Construction is simple
    • Robust
    • More compact

    Disadvantages

    The disadvantages of an alternator are

    • Alternators need transformers
    • Alternators will overheat if the current is high

    Applications

    The applications of an alternator are

    • Automobiles
    • Electrical power generator plants
    • Marine applications
    • Diesel electrical multiple units
    • Radiofrequency transmission