Automatic Voltage Control
Automatic voltage control may be used where load current variations exceed the built-in ability of
the generator to regulate itself. An automatic voltage control device "senses" changes in output voltage
and causes a change in field resistance to keep output voltage constant.
The actual circuitry involved in automatic voltage control will not be covered in this chapter.
Whichever control method is used, the range over which voltage can be changed is a design characteristic
of the generator. The voltage can be controlled only within the design limits.
PARALLEL OPERATION OF GENERATORS
When two or more generators are supplying a common load, they are said to be operating in parallel.
The purpose of connecting generators in parallel is simply to provide more current than a single generator
is capable of providing. The generators may be physically located quite a distance apart. However, they
are connected to the common load through the power distribution system.
There are several reasons for operating generators in parallel. The number of generators used may be
selected in accordance with the load demand. By operating each generator as nearly as possible to its
rated capacity, maximum efficiency is achieved. A disabled or faulty generator may be taken off-line and
replaced without interrupting normal operations.
Q21. What term applies to the use of two or more generators to supply a common load?
Amplidynes are special-purpose dc generators. They supply large dc currents, precisely controlled, to
the large dc motors used to drive heavy physical loads, such as gun turrets and missile launchers.
The amplidyne is really a motor and a generator. It consists of a constant-speed ac motor (the prime
mover) mechanically coupled to a dc generator, which is wired to function as a high-gain amplifier (an
amplifier is a device in which a small input voltage can control a large current source). For instance, in a
normal dc generator, a small dc voltage applied to the field windings is able to control the output of the
generator. In a typical generator, a change in voltage from 0-volt dc to 3-volts dc applied to the field
winding may cause the generator output to vary from 0-volt dc to 300-volts dc. If the 3 volts applied to
the field winding is considered an input, and the 300 volts taken from the brushes is an output, there is a
gain of 100. Gain is expressed as the ratio of output to input:
In this case 300 V ÷ 3 V = 100. This means that the 3 volts output is 100 times larger than the input.
The following paragraphs explain how gain is achieved in a typical dc generator and how the
modifications making the generator an amplidyne increase the gain to as high as 10,000.
The schematic diagram in figure 1-22 shows a separately excited dc generator. Because of the 10-
volt controlling voltage, 10 amperes of current will flow through the 1-ohm field winding. This draws 100
watts of input power (P = IE).