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 MIL-HDBK-1038
4.5.16
Resistors. There are three constructions for power resistors; namely,
wirewound, edgewound, and grid type.
The principal functions of resistors are to control the speed of motors
by limiting current flow and to dissipate regenerated energy as heat.
NEMA utilizes Class Numbers to designate the current ratings of
resistors. The Class 90 rating is the continuous rating of a resistor. Other
Class Numbers used on industrial (non-Navy) cranes are 150, 160, and 170. The
current rating of each of these other Class Numbers is based upon the resistor
being "on" for 15 seconds and being "off" for 45 seconds, 30 seconds, and 15
seconds, respectively. NEMA ICS 9, Part 2 addresses resistors.
4.5.16.1 Wirewound. Wirewound resistors consist of resistive wire wound on a
tubular dielectric. They are used for high resistance/low power applications.
Because the wire can break under vibration, NCC customarily does not permit their
use in those applications where the other types of resistors are available, that
is for segments less than 8 ohms.
4.5.16.2 Edgewound. Edgewound resistors are more substantially constructed than
wirewound resistors. A ribbon of stainless steel, rather than a wire, is wound on
a tubular dielectric. They have higher power ratings than the wirewound
resistors.
4.5.16.3 Grid Type. Grid type resistors are used for high power applications.
The resistor elements are stainless steel stampings, either continuous or welded
segments, having low resistance/high power ratings. Cast iron grids had been used
in the past and may still be found on the older cranes. Although subject to
cracking, they did have the advantage of excellent thermal characteristics in that
they were slow to heat-up.
Grid type resistors are also used in load banks. A resistor load bank is
used to establish a minimum load level for the diesel engine-generator set. A
resistor load bank is used to absorb regenerative power from the motors to prevent
back-driving of the diesel engine-generator set.
4.5.17
Electronic Voltage Conversion Units. Electronic voltage conversion units
are used to change alternating current to simulated direct current. By utilizing
thyristors to conduct selectively between the input's phases, the waveform of the
output current is relatively flat, but it does contain ripples. The output
voltage is varied by changing the firing rate of the thyristors. This is done in
response to a continuous comparison of a feedback signal from a tachometer or
encoder with a reference signal from the operator's master switch or pushbutton so
as to maintain a desired speed.
4.5.18
Transformers. Transformers are used primarily to change the voltage of
an AC electrical system. However, the coupling effect of the process also
provides isolation against undesired influences between sections.
4.5.18.1 Step-Down Transformers. Step-down transformers are used to change the
power system voltage, usually 480 VAC, to a lower voltage. For control circuits,
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