An Alternator also known as synchronous generators which converts mechanical energy into electrical energy in the form of alternating current (at a specific voltage and frequency). most alternators use a rotating magnetic field with a stationary armature.
History of Alternator
The Michael Faraday and Hippolyte Pixii gave the very first concept of alternator. Michael Faraday designed a rotating rectangular turn of conductor inside a magnetic field to produce alternating current in the external static circuit. After that in the year of 1886 J.E.H. Gordon, designed and produced first prototype of useful model. After that, Lord Kelvin and Sebastian Ferranti designed a model of 100 to 300 Hz synchronous generator. Monofindia.com Nikola Tesla in 1891, designed a commercially useful 15 KHz generator. After this year, poly phase alternators came into picture which can deliver currents to multiple phases.
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.
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
- Output Current with Speed of Alternator: The output of the current reduced or decreased when the alternator speed reduced or decreased.
- The efficiency with Speed of Alternator: Efficiency of an alternator is reduced when the alternator runs with low speed.
- Current Drop with Increasing Alternator Temperature: When the temperature of an alternator increased the output current will be reduced or decreased.
Types of Alternators
Alternators or synchronous generators can be classified in many ways depending upon their applications and designs.
The five different types of alternators include:
- Automotive alternators – used in modern automobiles.
- Diesel-electric locomotive alternators – used in diesel electric multiple units.
- Marine alternators – used in marine applications.
- Brushless alternators – used in electrical power generation plants as the main source of power.
- Radio alternators – used for low band radio frequency transmission.
We can categorize these AC generators (alternators) in many ways, but the two main categories depending on their design are:
- Salient Pole Type
- Smooth Cylindrical Type
Salient Pole Type
We use it as low and medium speed alternator. It has a large number of projecting poles having their cores bolted or dovetailed onto a heavy magnetic wheel of cast iron or steel of good magnetic quality.
Such generators get characterized by their large diameters and short axial lengths. These generators look like a big wheel. These are mainly used for low-speed turbine such as in hydel power plant.
Smooth Cylindrical Type
We use it for a steam turbine driven alternator. The rotor of this generator rotates at very high speed. The rotor consists of a smooth solid forged steel cylinder having certain numbers of slots milled out at intervals along the outer periphery for accommodating field coils.
These rotors are designed mostly for 2 poles or 4 poles turbo generator running at 36000 rpm or 1800 rpm respectively.
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
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