GENERATOR SELECTION CONTENTS - Diesel Generator

General
BASIC DESIGN THEORY
SECTION 4121
January, 1990
Page 1
One of the simplest ways to remember how every a.c. generator works is to imagine a magnet and a piece of wire.
Move one of them with respect to the other, whilst keeping them close together, and a measurable voltage will be
induced at the ends of the wire.
Immediately then we see that all a.c. generators must have the following, before an output voltage can be generated:-
a) a magnet - to produce the magnetic field excitation.
b) a piece of wire - usually coils of copper wire.
c) relative movement between these tow - usually a constant rotational speed.
Theory
In its simplest form an a.c. generator is diagrammatcially shown in figure 1. All three criteria stated above are met and
a voltage output will be produced. In this case the magnetic field produced is at a constant level from a permanent
magnet. This type of machine does have practical applications, particularly when supplying a constant load; for
example, a pedal bicycle dynamo.
Figure 1
There are two conditions to consider at this point, the no load condition and the on load condition. The no load output
voltage level is sustained by the constant magnetic field strength produced and fixed by the permanent magnet. On
load, current is drawn from the machine which will cause the output voltage to fall, since the permanent magnet
cannot produce a change in the magnetic field strength. The typical relationship between the output voltage and the
load current is shown in the graph in figure 1. Typically the relationship is nearly linear between no load voltage (V) at
zero current and the short circuit current (Isc) at zero voltage. In order to maintain the output voltage whilst supplying
current, the magnetic field strength must be increased as load is applied. It is this requirement that brings us to the
next stage of a.c. generator design incorporating the electrically produced magnetic field system. (Figure 2).