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Direct Current Motors and Generators
A.
Bhatia
Overview
A direct current generator, or DC generator, produces a voltage
of constant polarity, which means the voltage and current do not change over
time.
DC
generators are basically AC generators whose output voltage is switched the
other way around at the proper moment, so that the direction of the voltage is
always in a single direction. The AC generator uses slip rings to transfer the
current to the electrical circuit, while the DC generator uses a split-ring
commutator.
Generators can be very small or quite huge. Commercial DC generators are
commonly found in traction applications like subways and trains.
Factories that do electrolysis, electroplating and those that produce aluminum,
caustic soda, chlorine, and some other industrial materials need large amounts
of direct current and use DC generators.
An
electric motor is very similar to a generator, except that power is provided to
turn the rotors. They may, in fact, be described as generators "running
backwards". When current is passed through the armature of a DC motor, a torque
is generated by magnetic reaction, and the armature revolves. Adjustable speed
is difficult to obtain with motors whose armatures are connected to fixed
frequency power lines (AC motors). Here is where DC motors shine; their rotating
field speed depends on the rotor speed itself. The speed of DC series motors
varies with load, and torque varies inversely with speed. This makes them
particularly suitable to starting high inertia loads such as railway trains.
Starting a DC motor often requires an external resistor or
rheostat to limit the current. The value, in Ohms, of that resistor is reduced
in steps as the speed of the motor increases, until finally that resistor is
removed from the circuit as the motor reaches close to its final speed.
This 5-hr course discusses
salient features of DC motors and generators in detail. The course is
based
entirely on Naval Education and Training Materials (NAVEDTRA
14177),
Electricity and Electronic Training Series;
Module-5 and covers Chapter 1 and 2 titled “Direct Current
Generators and Direct Current Motors”.
The
student must take a multiple-choice quiz consisting of twenty-five (25) questions at the
end of this course to obtain PDH credits.
Specific Knowledge or Skill Attained
This course teaches the
following specific knowledge and skills:
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The
principle by which generators convert mechanical energy to electrical energy.
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The
rule to be applied when you determine the direction of induced emf in a coil.
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What
component causes a generator to produce direct current rather than alternating
current.
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How
field strength can be varied in a dc generator.
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The
three classifications of dc generators.
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The
term that applies to voltage variation from no-load to full-load conditions
and how it is expressed as a percentage.
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The
factors that determine the direction of rotation in a dc motor.
-
The
right-hand rule for motors.
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The
main differences and similarities between a dc generator and a dc motor.
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The
advantages and disadvantages of the different types of dc motors.
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The
means of controlling the speed and direction of a dc motor.
-
The
need for a starting resistor in a dc motor.
Course
Click on the following PDF attachment and review the document
prior to taking
a quiz for credit.
DC Motors and Generators (700 KB)
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credit. Click on the link below.
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