The Construction of alternator or synchronous generator is very much similar to the DC machines. In the DC generator, the armature winding is placed in the rotor and the field winding is placed in the stator. This type of construction is used only for low power applications but it is unsuitable for high power applications.
The alternator is an AC generator that converts mechanical energy into electrical energy. It produces an output that is alternating in nature and is so-called ALTERN-ATOR. It also rotates at a constant speed, which is nothing but a synchronous speed and is hence called a Synchronous Generator.
The power required for industrial and domestic appliances is met by the power generated by these alternators from different power plants like thermal, hydro, nuclear, etc.
Alternators are the major source of electrical energy. If there are no giant alternators today, then the world will still be in the dark.
There are certain applications like mobile, computers, vehicles, charging devices, etc which need DC power to run. Since it is very difficult to generate the DC power, the required DC power is obtained from the Alternator with the help of rectifiers, which convert the AC power into DC power.
There are also DC generators and motors, where Commutator and brush arrangements are used to deliver the DC output.
In such DC machines, the field is stationary and the armature is rotating but in the case of alternators, the armature becomes stationary as the stator and the field are rotating. There are several advantages of the stationary armature.
Advantages of stationary armature
- Easier to insulate the stationary armature winding for high AC voltages.
- The output current can be easily taken to the load circuit from the fixed terminals.
- Field circuit requires low voltage and low power DC and hence rotating field winding be easily insulated.
- The armature winding can be more easily braced, so as to prevent deformation caused by the mechanical force due to high centrifugal force.
Ok. Now Let’s move on to the construction and working of alternator.
Construction of alternator or synchronous generator
Rotor and Stator are two main parts in the construction of an alternator.
The stator of the synchronous generator consists of a stator frame and stator core. The stator frame is used for holding the armature stampings and windings in position. Ventilation is maintained with the help of holes cast in the frame itself, which assist in cooling the alternator.
The armature core is supported by the stator frame and is built up of laminations of steel alloys or magnetic iron. The core is laminated to minimize the eddy current loss.
The laminations are insulated from each other and have spaces in between them for allowing the cooling air to pass through. The stator is made up of a number of slots on its inner periphery, as shown in the below figure. The slots are used for holding the armature winding.
There are two types of rotor, smooth cylindrical type and salient pole type.
Smooth Cylindrical type alternator
The rotor consists of a smooth solid steel cylinder, having a number of slots along its outer periphery for hosting the field coils. They do not have projected poles, instead, it has a uniform length in all directions, giving a cylindrical shape to the rotor.
The pole areas are un-slotted, as shown in the figure below. Here, the rotor has 4 poles. The pole areas are surrounded by field windings placed in slots.
The windings are placed so that the flux density will be maximum on the polar central line and gradually falls away on either side.
It has a very long axial length but small diameters. The construction of the rotor gives better balance, quieter operation and less windage loss.
This type of rotor is generally used for very high-speed operation (at 3600 rpm) and hence called turbo-generators, which are employed in alternators driven by a steam turbine. The machine is built with ratings from 10 MVA to over 1500 MVA.
Salient pole type alternator
The term salient means protruding or projecting. The poles are made of thick laminated steel sections pivoted together.
The poles are also laminated to minimize the eddy current losses. The salient pole type of rotors is characterized by their large diameters and relatively short axial lengths.
It is generally used for low and medium-speed operations, mainly employed in engine-driven alternators.
Role of damper winding
Most of the alternators have slots on the pole shoes, as shown in the above figure. Copper bars are placed in the slots, called damper winding. Both the end of the copper bars are shorted by the end ring. Damper winding in the alternator is similar to the construction of a squirrel cage rotor in an induction motor.
Damper winding is used to reduce hunting and in synchronous motors, they are used to provide the starting torque for the motor. At normal running conditions, the damper winding does not have any role, as the rotor runs at synchronous speed.
Working of Alternator
An alternator or AC generator works on the principle of electromagnetic induction as a DC generator. The mechanical energy to the alternator is given through the prime mover. Thus the rotor and hence the field magnets are rotated in the direction of the prime mover.
The field poles are magnetized by applying a voltage to the field coils through slip rings. The field windings are connected such that, alternate North and South poles are produced.
As the rotor rotates, the North and South poles of the magnetic flux is cut by the stationary armature, which induces an emf in the armature windings.
As the magnetic poles are alternately North and South Poles, the emf thus generated will be alternating in nature.
The three-phase alternating voltage generated by the alternator is drawn below.
The frequency of the induced em depends on the number of N and S poles moving past the armature conductor in one second. The direction of induced emf can be found by Fleming’s right hand rule and frequency is given by
where N = Speed of the rotor in rpm, P = Number of rotor poles