Alternator is an electrical dynamic device which converts mechanical energy to electrical energy form. According to faraday’s 1st law either magnet or armature must be in motion condition to induced emf. Alternator is also known as synchronous generator.
We can categorize Alternator in many ways, but according to the design of field system it can be divided into two types –
- Salient pole type
- Cylindrical pole type
Salient pole type –
In this type of field system, the no of poles is more than two. The pole bars are arranged in such a way that each section can be detached from the holes at any fault condition. Since it is an electromagnet the hole winding is provided in such a way that the consecutive opposite holes will develop.
This type of field structure is slow in sped. It’s speed doesn’t exceeds 1000 rpm indicates the no of pole varies 6 to 60.
It is usually made of cast iron or cast steel , and has a heavy central hub ,a no of radial arms the poles may be either solid or laminated to reduce eddy current loss. The pole faces are shaped that the radial gap length increases from the pole center to the pole tips.
Salient pole machines are frequently provided with damper windings on the rotor to reduce oscillation during transient condition and also provide stability on unbalanced load condition.
The damper winding consist of upper bars laid in a slots of the pole phase and connected each other at the end by means of short circuiting segment. Under this condition it is similar to as squirrel cage rotor.
The salient pole rotor has the following points-
- It is least expensive and provide more space for field ampere turns.
- It is cheaper than cylindrical rotor.
- It has large diameter and short axial length.
- This are operates with hydraulic turbine or diesel engines.
Cylindrical pole type –
It is used in very high speed alternator. To reduce the peripheral velocity, the diameter is reduced and the length is increase. Due to high peripheral speed, to rotating part of the turbo generator is used.
Chromium nickel steel or special chrome-nickel-molybdenum steel is used for the rotor of turbo alternators.
Normally 2/3rd path of rotor is slated for the field winding and 1/3 is left without slots. So as to form pole face. Each slot is provided with a ventilation hole are the buttom.
The Cylindrical pole rotor has the following points-
- They are of smaller diameter and long axial length.
- Robust construction and noise less operation.
- Less windage loss.
- Better in dynamic balancing.
- Highest operation speed 3000 rpm.
Also Read- Types of DC generators