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A simple guide to the main components and maintenance of a generator – Part 2: The alternator

A simple guide to the main components and maintenance of a generator – Part 2: The alternator
Last week we started a new blog series describing the key parts of a generator, and the maintenance that a unit requires to keep it functioning. In last weeks post we looked at the importance of the engine and its role. This week we take a look at the alternator.
Last week we started a new blog series describing the key parts of a generator, and the maintenance that a unit requires to keep it functioning. In last weeks post we looked at the importance of the engine and its role. This week we take a look at the alternator.

Alternator
“The alternator is the next key part of the generator. Typically this will be a 4-pole electrical machine running at 1500rpm. 

“One cycle of alternating current is produced each time a pair of field poles on the rotor passes over a point on the stationary winding on the stator. The output frequency of an alternator therefore depends on the number of poles and the rotational speed. An engine speed of 1500rpm gives 25 revolutions per second (1500/60), making a rotational frequency of 25Hz. The electrical output frequency is then derived by multiplying the pairs of poles by the frequency, giving an output of 50Hz. Where a different mains frequency is required, the engine speed or numbers of poles need to be altered. For example, USA mains require engines to run at 1800 rpm to give a mains frequency of 60Hz. 

“What we call the alternator is effectively composed of two alternators built end-to-end on one shaft. The larger of the two alternators is the main alternator and the smaller is the exciter. The exciter has the stationary field coils and a rotating armature. The main alternator uses the opposite configuration with a rotating field and stationary armature. A bridge rectifier, called the rotating rectifier, is mounted on the rotor. Varying the amount of current through the stationary exciter field coils, varies the output from the exciter. This output is rectified and the resultant DC supplies the rotating field of the main alternator, and hence alternator output. The result of all this is that a small DC exciter current controls the output of the main alternator.”

Author: Petya Asgarova

29/06/2015 09:30:00
Listed in : Key parts of a generator  
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