Page 96 - Offshore Electrical Engineering Manual

CHAPTER
6
Direct Current Generators

INTRODUCTION
The author is not aware of any direct current (DC) generators being used on offshore
installations, but there are some older ships with DC systems.
The DC output current is generated in the same way as in the alternating current
(AC) machine by conductors in the armature cutting lines of magnetic flux. However,
the AC generated is mechanically switched to a unidirectional current by means of
the commutator, a drum made up of copper segments that are held together by clamp-
ing rings and keyed to the armature shaft. DC generators for supplying ship services
are usually compound wound; i.e., there are field winding connected to the output
terminals in both parallel and series to provide a suitable generator load/voltage out-
put characteristic.
Although compounding of field windings should maintain reasonably con-
stant generator output voltages at a constant ambient temperature, DC genera-
tor output voltage is fairly sensitive to ambient temperature changes, and field
rheostats (variable resistor field regulators) are used to compensate for changes in
shunt field resistance due to these variations in temperature. When tested during
manufacturing in temperate ambient conditions (e.g., around 15°C), the voltage
across the field regulator should not be less than 14% of the generator rated ter-
minal voltage to achieve rated output voltage with cooling air at tropical ambient
temperatures.

COMPOUND-WOUND GENERATORS
These have both series and shunt field windings (Fig. 2.6.1) and their terminal volt-
age is a combination of both series and shunt generator characteristics. Standard
practice is to connect the series field to the negative pole; both the generator and
switchgear manufacturers need to consult, as uniformity is essential.
Fig. 2.6.2 shows the typical voltage characteristics, in which curve A represents
the voltage due to the series field and curve B represents the voltage due to the shunt
field. The series field will produce additional ampere turns proportional to the load
and thus offsets the drooping characteristic of a shunt machine. The resultant (com-
pound) output voltage is approximately the sum of the ordinates of the two separate
curves, with any difference being because of the fact that, as a compound machine,
the excitation voltage of the shunt winding is almost nearly constant.

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