Normally Closed-(Symbol N.C.) -
As applied to a magnetically operated device such as a valve or the contacts of
a relay, Normally Closed indicates that the valve or relay contacts are closed
when no power is supplied to the operating magnet.
Normally Open- (Symbol N.O.) - As
applied to a magnetically operated device such as a valve or the contacts of a
relay, Normally Open indicates that the valve or the relay contacts are open
when no power is supplied to the operating magnet.
Ohm - The unit of measure of
electrical resistance. One ohm of resistance will allow a current of one ampere
to flow with an applied potential of one volt.
Ohmmeter - An instrument for
measuring electrical resistance.
Ohm's Law - A fundamental law
expressing the relationship between voltage, current and resistance in
electrical circuits. The law states that E = IR or voltage is equal to current
times resistance.
Open Circuit - 1) A circuit that
does not provide a complete path for the flow of current.2) A condition of an
electrical circuit caused by the breaking of continuity of one or more
conductors of the circuit; often an undesired condition. 3) An arrangement of
conductors and equipment that depends upon lack of continuity for operation, as
open circuit telegraphy; a circuit in which continuity is incomplete or
interrupted.
Oscillogram - The permanent
record produced by an oscillograph, or a photograph of the trace produced by an
oscilloscope.
Oscillograph - A measurement
device for determining waveform by recording the instantaneous values of a
quantity such as voltage as a function of time.
Oscilloscope - An instrument
primarily for making visible the instantaneous value of one or more rapidly
varying electrical quantities as a function of time or of another electrical or
mechanical quantity.
Oscilloscope, Dual Beam -- An
oscilloscope in which the cathode-ray tube produces two separate electron beams
that may be individually or jointly controlled.
Parallel Connection - An
electrical connection in which the input of one element is connected to the
input of another element. The output connections are similarly connected
together thus providing two or more paths for current to flow.
Parallel Operation - The method
by which generators having the same voltage and frequency characteristics are
connected to a common load.
Permanent Magnet Generator - A
generator which uses permanent magnets to create a magnetic field. As applied
to Cummins Generators; a permanent magnet generator, within the main generator,
supplies the power to the voltage regulator which in turn supplies power to the
exciter field. The permanent magnet generator still supplies power during a
short circuit of the main generator, This permits the main generator to sustain
high short circuit currents. The output of the permanent magnet generator is
electrically isolated from the main generator output and is not adversely
affected by main generator output waveform distortion such as from SCR type
loads.
PMG - See Permanent Magnet
Generator
Polarity - An electrical term
which indicates the direction in which current flows in a conductor. In a
direct current system, current flows in the same direction. In an alternating
current system, current changes direction as the polarity of the A.C. voltage
source changes.
Potential - Another term to
indicate voltage.
Potential Difference - The
difference in voltage between two points in a circuit.
Power - Defined as the rate of
performing work, Mechanical power is measured in horsepower or kilowatts. Electrical
power is measured in kilowatts.
Power Factor - The ratio of the
true power (KW) requirements of a load to the apparent power (KVA) requirement
of a load. The combined loads applied to a power source may have a resistive,
capacitive, or inductive effect.
Resistive loads such as
incandescent lighting or resistive heating elements cause no shift in the
voltage/current wave-forms and are therefore unity (1.0) power factor loads.
Inductive and capacitive loads,
however, cause the voltage and current wave-forms to shift with respect to one
another. These types of loads are commonly called power factor loads (i.e., the
power factor is less than l.0),
See INDUCTANCE and CAPACITANCE
for more information.
Power Factor is frequently
multiplied by 100 and expressed as a percentage.
R – Symbol used to represent
resistance
Reactive Differential(Cross
Current) Compensation - A circuit arrangement for operating generators in
parallel with no generator voltage droop. Reactive Differential Compensation is
accomplished by a differential interconnection of the secondaries of the
generator current transformers used for Reactive Droop Compensation. This
arrangement is not often used and is very difficult to set up when paralleling
generators of different capacities.
Reactive Droop Compensation - A
circuit arrangement that creates a droop in generator output voltage that is
proportional to the reactive load current. This circuit arrangement makes
paralleled generators share the reactive component of the load current in
proportion to the generator ratings. It also can essentially eliminate the
reactive component of circulating current which may flow between paralleled
generators. The Reactive Droop Compensation arrangement is commonly called the
Generator Paralleling Module.
Relay -- An electro-mechanical
device having a magnetic coil which, when energized, opens or closes several
sets of contacts.
Resistance - Opposition to the
flow of current.
Resistor-(Symbol R) - A device or
component designed to have a definite amount of resistance. Resistors are used
in circuits to limit current flow or to provide a voltage drop.
Resistor, Fixed - A component
which is used to control voltage and current by introducing a constant
resistance of known value into the circuit.
Resistor, Variable - Similar to a
fixed resistor except the resistance value can be adjusted. The voltage
adjust-rheostat is a variable resistor.
Rheostat - A variable resistor.
Series Connection - An electrical
connection in which the input of one element is connected to the output of
another element providing only one path for current to flow.
Short Circuit - An abnormal
connection (including an arc) of relatively low resistance, whether made
accidentally or intentionally, between two points of different potential in a
circuit. Equipment, not protected by fuses or circuit breakers, may be damaged
by a short circuit.
Shunt Trip - An
electro-mechanical device which, when energized, trips the main line circuit
breaker.
Single Pole Switch - A switch
which has only one of contacts.
Single Throw Switch - A switch
which opens a set of contacts in one switch position and closes the contacts
the other position.
Speed Droop - A decrease in
steady-state speed of an engine due to
an increase in applied load. This decrease in speed from the no load speed is
expressed as a percentage of full load speed or:
Solenoid - An electro-mechanical
device which, when energized, acts on a movable core or plunger in the center
of the energizing coil to perform mechanical work.
Solid-State - A term which
applies to electronic devices which are constructed using semiconductor
material. Such devices include transistors, SCR's, and diodes.
Stability - Freedom from
undesired variations or oscillation. In generator set systems, the generator
set is stable if it can maintain or
quickly re-establish steady state conditions of voltage and frequency after a
sudden load change.
Surge Suppressor - A device
capable of conducting current in either direction in the presence of high
transient voltages thus protecting devices that could otherwise be damaged.
Synchronism - The condition where
two or more generators are operating at the same frequency and output voltage
and are in phase with one another.
Synchronizing - The procedure of
matching the frequency, phase rotation, voltage and phase relationship of one
generator to another for purposes of paralleling the units onto a common bus.
Synchronous - A generator whose
output frequency is directly related (proportional) to the speed of the engine
driving the generator.
Tachometer -- An instrument for
measuring the rate of rotation expressed in revolutions per minute.
Temperature Drift - A change in
the value of a component caused by changes in operating temperature.
Terminal - A mechanical device
for making electrical connections.
Transformer - Frequently, a
device using a laminated iron core with a primary and secondary winding. A
transformer changes some level of input (primary) voltage or current to a
proportional output (secondary) voltage or current. See current transformer and
potential transformer.
Transient - A temporary change
from the steady-state conditions.
V - Symbol used to represent
voltage.
Volt - The unit of measure of
electrical potential (voltage). One volt of electrical potential will force a
current of one ampere to flow through a resistance of one ohm.
Voltage - Electrical potential or
pressure which forces current to flow through a circuit.
Voltage Dip - A temporary
reduction in voltage generally resulting from an increase in load.
Voltage Droop - A decrease in
steady-state voltage of a generator due to an increase in load applied. This
decrease in voltage from the no load voltage is expressed as a percentage of
the full load voltage or:
Voltage Droop Compensation - A
system which permits paralleled generators to share the currents of a load in
proportion to the generator set ratings.
Voltage Regulator - A device
which controls the voltage output of a generator at a preset value regardless
of the amount of load applied.
Voltmeter - An instrument
designed to measure electrical potential or voltage.
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