T1
CR1
CR2
18
CR3
CR4
F1
A
9
4
15
8
13
CR5
CR6
16 10
7
F2
4
B
2
14
1
3
F3
17
C
CR8
CR7
15
CR9
CR10
18
17
20
12
5
CR11
CR12
11
6
16
ASf07036
Figure 7-36.--A dc power supply circuit.
The three phases of the input voltage are applied to
seconds OFF. If this is practiced for 30 minutes,
operate the unit for 30 minutes at no load to allow the
the primary winding of transformer T1. The input
generator to cool.
voltage is stepped down by mutual induction. The
adjustable output of the secondary winding is 22 to 39
The operating speed of a generator must be
Vac. The output is controlled by a dc voltage regulator
maintained to produce the rated voltage and current
and a magnetic amplifier. The six-phase output must be
output. The required operating speeds vary widely.
rectified to dc. This is done by the full-wave rectifier
The speed range of the NC-2A is 6,000 rpm; the
network of CR1 through CR12. Each phase has a
NC-8A is 2,400 rpm. The high-speed generator can
full-wave rectifier.
produce more power than a low-speed generator of the
same size. This is possible because less wire in series is
The maximum output of the rectifier circuit can be
required to produce a given voltage in the high-speed
as high as 750 amperes. A detailed description of the
generator. Since a shorter wire is required, you can use
circuit operation is found in the intermediate
a larger wire. This greatly improves the
maintenance section of the applicable manufacturer's
current-producing capability of a generator.
manual.
The large amounts of current delivered create a
CURRENT RANGE
heat problem. Most MEPP generators are cooled by
internal fans (fig. 7-37). You must be sure the airflow is
The generators on MEPPs are designed for a wide
not restricted and the generator is not overheating. The
range of current applications. Large units, such as the
amount of heat that can be removed by air is
NC-2A and NC-8A, use dc generators rated as high as
determined by the volume and density of the air. The
500 amperes continuous duty. They may supply as
warmer the air becomes, the less dense it becomes and
much as 750 amperes intermittently. Consult the
the less heat it can remove. If the temperature of a
applicable technical manual for the maximum current
generator is allowed to rise too high, the insulation may
range. Exercise caution when you operate a generator
be damaged. The nameplate attached to a generator
at a high current level. Specific procedures are
identifies the voltage, current rating, speed range, and
established by the manufacturer. Usually, the use of
temperature rise. If the plate cannot be found, refer to
750 amperes must not exceed 20 seconds ON and 40
the manufacturer's manual.
7-28
