What is An Alternator And How Does it Work?

 What is an Alternator?

An alternator is defined as a machine or generator which produces AC (Alternating Current) supply and it converts mechanical energy into electrical energy, so it is also called an AC generator or synchronous generator. There are different types of alternators based on applications and design. The Marine type alternator, Automotive type alternator, Diesel-electric locomotive types alternator, Brushless type alternator, and Radio alternators are the types of alternators based on the applications. The Salient Pole type and Cylindrical rotor type are the types of alternators based on the design.

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.

The stator word based on the stationary and the rotor word based on the rotating. The construction of the stator of an alternator is equal to the construction of the stator of an induction motor. So induction motor construction and synchronous motor construction are both are same. Thus the stator is the stationary part of the rotor and the rotor is the component that rotates inside of the stator. The rotor is located on the stator shaft and the series of the electromagnets arranged in a cylinder causing the rotor to rotate and create a magnetic field. There are two types of rotors they are shown in the below figure.

Salient Pole Rotor

The meaning of the salient is projecting outward, which means the poles of the rotor are projecting outward from the center of the rotor. There is a field winding on the rotor and for this field winding will use DC supply. When we pass the current through this field winding N and S poles are created. The salient rotors are unbalanced so the speeds are restricted. This type of rotor used in hydro stations and diesel power stations. The salient pole rotor used for low-speed machines approximately 120-400rpm.

Cylindrical Rotor

The cylindrical rotor is also known as a non-salient rotor or round rotor and this rotor is used for high-speed machines approximately 1500-3000 rpm and the example for this is a thermal power plant. This rotor is made up of a steel radial cylinder having the number of slots and in these slots, the field winding is placed and these field windings are always connected in series. The advantages of this are mechanically robust, flux distribution is uniform, operates at high speed and produces low noise.

An AC motor comes in many shapes and sizes, but we can’t have an AC without a rotor and stator. The rotor is made up of a cast iron and the stator is made up of silicon steel. The prices of the rotor and stator depend on the quality.

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. In the figure (a) rectangular coil is placed between the north and south magnetic poles. The position of the coil is such that the coil is parallel to the flux, so no flux is cutting and therefore no current is induced. So that the waveform generated in that position is Zero degrees.

If the rectangular coil rotates in a clockwise direction at an axis a and b, the conductor side A and B comes in front of the south pole and C and D come in front of a north pole as shown in figure (b). So, now we can say that the motion of the conductor is perpendicular to the flux lines from N to S pole and the conductor cuts the magnetic flux. At this position, the rate of flux cutting by the conductor is maximum because the conductor and flux are perpendicular to each other and therefore the current is induced in the conductor and this current will be in maximum position.

The conductor rotates one more time at 90⁰ in a clockwise direction then the rectangular coil comes in the vertical position. Now the position of the conductor and magnetic flux line is parallel to each other as shown in figure (c). In this figure, no flux is cutting by the conductor and therefore no current is induced. In this position, the waveform is reduced to zero degrees because the flux is not cutting.

 

In the second half cycle, the conductor is continued to rotate in a clockwise direction for another 90⁰. So here the rectangular coil comes to a horizontal position in such a way that the conductor A and B comes in front of the north pole, C and D come in front of the south pole as shown in the figure (d). Again the current will flow through the conductor that is currently induced in the conductor A and B is from point B to A and in conductor C and D is from point D to C, so the waveform produced in opposite direction, and reaches to the maximum value. Then the direction of the current indicated as A, D, C and B as shown in figure (d). If the rectangular coil again rotates in another 90⁰ then the coil reaches the same position from where the rotation is started. Therefore, the current will again drop to zero.

In the complete cycle, the current in the conductor reaches the maximum and reduces to zero and in the opposite direction, the conductor reaches the maximum and again reaches zero. This cycle repeats again and again, due to this repetition of the cycle the current will be induced in the conductor continuously.

This is the process of producing the current and EMF of a single-phase. Now for producing 3 phases, the coils are placed at the displacement of 120⁰ each. So the process of producing the current is the same as the single-phase but only the difference is the displacement between three phases is 120⁰. This is the working principle of an alternator.

 

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.

 

Applications

The applications of an alternator are

·         Automobiles

·         Electrical power generator plants

·         Marine applications

·         Diesel electrical multiple units

·         Radiofrequency transmission

 

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