Construction of Alternator

An alternator consists of field poles placed on the rotating part of the machine. It has two main parts: a rotor and a stator. The rotor rotates inside the stator, and the field poles project onto the rotor body. Armature conductors are housed in the stator. An alternating three-phase voltage (aa’, bb’, cc’) is induced in these conductors, generating three-phase electrical power. Modern electrical power generating stations use this technology to generate three-phase power, making the alternator (also known as a synchronous generator) crucial for power engineers.

An alternator is basically a type of AC generator. The field poles are made to rotate at synchronous speed N_s = 120 f/P for effective power generation. Where, f signifies the alternating current frequency and the P represents the number of poles.

In most alternators, the armature winding is stationary, and the field rotates, unlike in a DC generator where it’s the opposite. This setup handles the high power (hundreds of megawatts) produced by AC generators. High power requires larger and heavier conductors for optimal performance. Therefore, it’s beneficial to use lighter field windings instead of heavy armature windings, reducing the centrifugal force needed to turn the rotor and allowing higher speeds.

There are mainly two types of rotors used in construction of alternator:

1. Salient pole type.
2. Cylindrical rotor type.

Salient Pole Type

The term salient means protruding or projecting. The salient pole type of rotor is generally used for slow speed machines having large diameters and relatively small axial lengths. The poles, in this case, are made of thick laminated steel sections riveted together and attached to a rotor with the help of joint.

An alternator as mentioned earlier is mostly responsible for generation of very high electrical power. To enable that, the mechanical input given to the machine in terms of rotating torque must also be very high. This high torque value results in oscillation or hunting effect of the alternator or synchronous generator. To prevent these oscillations from going beyond bounds the damper winding is provided in the pole faces as shown in the figure.

Damper windings are copper bars short-circuited at both ends, placed in holes along the pole axis. At steady speed, the relative velocity between the damper winding and the main field is zero. If the alternator deviates from synchronous speed, the relative motion between the damper winding and the main field induces a current. This current exerts torque on the field poles to bring the alternator back to synchronous speed.

The salient feature of pole field structure has the following special feature-

1. They have a large horizontal diameter compared to a shorter axial length.
2. The pole shoes covers only about ²/₃rd of pole pitch.
3. Poles are laminated to reduce eddy current loss.
4. The salient pole type motor is generally used for low-speed operations of around 100 to 400 rpm, and they are used in power stations with hydraulic turbines or diesel engines.

Salient pole alternators driven by water turbines are called hydro-alternators or hydro generators.

Cylindrical Rotor Type

Cylindrical rotors are used for high-speed operations and are found in steam turbine-driven alternators, like turbogenerators. These machines come in various ratings, from 10 MVA to over 1500 MVA. Cylindrical rotors have a uniform length and shape, allowing consistent flux cutting in all directions. The rotor is a smooth steel cylinder with slots on its outer edge for field coils.

The cylindrical rotor alternators are generally designed for 2-pole type giving very high speed of

Or 4-pole type running at a speed of

Where, f is the frequency of 50 Hz.

The cylindrical rotor synchronous generator does not have any projections coming out from the surface of the rotor. Rather, the central polar area is provided with slots for housing the field windings as we can see from the diagram above.

The field coils are so arranged around these poles that flux density is maximum on the polar central line and gradually falls away as we move out towards the periphery. The cylindrical rotor type machine gives better balance and quieter-operation along with lesser windage losses.