GENERATOR SELECTION CONTENTS - Diesel Generator

Voltage.
The relationships of the machine governing voltage level are more complex.
SECTION 4121
January, 1990
Page 3
Output Voltage (V) depends on 1) Driven Speed (rev/min)
2) Number of turns of copper wire in output winding.
3) Strength of magnetic field produced by the main field excitation
magnetic poles.
The number of turns of copper wire in the output winding is fixed by the machine manufacturer. Voltage is also
affected by driven speed. In fact with a constant magnetic field strength, voltage as well as frequency would be
directly proportional to driven speed.
As the speed is fixed to obtain the correct frequency, the only variable left can be used to change and fix the machine
voltage level is the magnetic field strength. This is exactly the parameter that the machine’s control system does
adjust in order to set the voltage level, and compensate for both speed and load current changes. Let us now take a
look at the control systems that are readily available.
Control System.
These are two popular methods of voltage control system for a.c. generators, firstly a closed loop electronic system
and secondly, an open loop transformer system.
a) Electronic System.
This system continually monitors the output voltage an compares it with a reference voltage level set by the user.
Once the reference voltage level is set, the automatic voltage regulator (AVR) will automatically compare the actual
output voltage with the reference voltage level and if they are different will adjust the magnetic field strength to make
the output voltage the same as the reference voltage. This system is therefore an accurate closed loop control by
using static magnetic amplifiers (combinations of transformers of transformers and controlled reactors). This system
is very reliable although slow in operation and very bulky compared to the electronic systems which have now virtually
superseded it.
b) Transformer Control.
This system is really in two parts, firstly adjustment for no load voltage level and, secondly, compensation for load
current. This system once set up is not normally adjusted. It is an open loop control system; there is no continuous
monitoring to adjust the output voltage. It can only provide the amount of magnetic field strength it has been set up to
provide.
Machine Types
Previously we have stated that rotational speed must be maintained between the output winding (the ‘wire’) and the
field winding, (the ‘magnet’). Also we have assumed that the field winding is rotated with the output winding remaining
stationary. This type of machine is logically called a rotating field a.c. generator and an example of such a machine is
the Newage Stamford Brushless range. In fact all brushless machines are rotating field types, but as we see in figure
2, not all rotating field type machines are brushless.
The alternative is to rotate the output winding whilst keeping the field winding stationary. This still complies with all the
basic requirements for voltage generation and such a machine is called a rotating armature a.c. generator. An
example of such a machine was the Newage Stamford ‘D’ range.
Operation.
In the next part the combined operation of the machine with the control system will be examined in more detail.