DC Generator
1. THE DIRECTION OF CURRENT DUE TO INDUCED VOLTAGE: UNDERSTANDING FLEMING’S
RIGHT HAND RULE
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The direction of current flow in the conductor is given by Fleming’s right hand rule
Figure 1: Fleming’s right hand rule
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The directions are:
Thumb indicates: relative motion of conductor
Pointing finger indicates: flux
Middle finger indicates: current
Figure 2: Change in current direction
Key point : If the direction of relative motion of the conductor is reversed keeping the flux
direction same or if the flux direction is reversed keeping the direction of relative motion of
conductor same, the direction of induced emf and hence the direction of current is reversed.
KURRATUL AIN, Lecturer, Deptt. Of EEE,
Stamford University Bangladesh
Page 1
2. THE AMOUNT OF INDUCED EMF
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The induced emf in conductor depends on the position of plane of flux direction and plane of
conductor’s rotation.
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Mathematically it is found that if the plane of rotation is parallel to the plane of flux, there will no cut
of flux; hence there will be no induced emf.
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The flux cutting, and hence induced emf is maximum when the plane of rotation is perpendicular to
the plane of flux.
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In between these two extremes, for any angle between the plane of rotation and plane of flux, the
induced emf depends on the perpendicular projection of rotation to the plane of flux.
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The induced emf is given by E= BL(vsinα)
Where, B = magnetic field
L = active length of the conductor
v = relative velocity of the conductor
α = angle between the plane of rotation and plane of flux
Plane of
rotation
α
Plane of
flux
Figure 3: Flux induced in conductor
Key point: For an induced emf in the conductor, not only the relative motion between conductor and
flux is necessary but also the plane of rotation should not be parallel to the plane of flux.
3. OPERATION OF A SINGLE COIL DC GENERATOR
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A single coil dc generator is composed of three elements:
o
Armature: The moving part of a magnetic device consisting of one or more coils that are
electrically connected to create the rotatable section of a generator.
KURRATUL AIN, Lecturer, Deptt. Of EEE,
Stamford University Bangladesh
Page 2
o
Slip Rings: A slip ring is a method of making an electrical connection through a rotating
assembly, commonly found in electrical generators.
o
Brushes: conducts current between rotating and stationary parts of a generator.
Figure 4: Single coil dc generator
Figure 5: Operation of a single coil dc generator.
KURRATUL AIN, Lecturer, Deptt. Of EEE,
Stamford University Bangladesh
Page 3
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Initially the conductor is in the position A. In this position it has no induced emf in it, because the
plane of motion is parallel to the plane of flux.
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Next the rotor starts to rotate clockwise i.e. Leg 1 [darker portion of the conductor] is moving toward
the south pole and Leg 2 [lighter portion of the conductor] is moving toward the north pole [see
position B].
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As a result Leg 1 is moving downward and Leg 2 is moving upward.
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Emf starts to induce in the conductor, hence it causes to current flow in the direction given by
Fleming’s right hand rule.
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The induced emf and consequently the current is maximum when plane of motion is perpendicular to
the plane of flux [see position C].
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In the next quarter, the emf starts to reduce [see position D] until the conductor reaches the position E
and at this position the induced emf is zero again.
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Moving toward the next quarter [see position F] results in such a way that Leg 1 is now moving
upward and Leg 2 is in downward i.e. the exact opposite phenomenon is happening to the position B.
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According to Fleming’s right hand rule now, the current flow in the external circuit is opposite to that
of in the first half cycle.
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The current in opposite direction becomes maximum at the position G. In next quarter the current
reduces [see position H] and become zero after finishing one cycle [see position A].
4. M AKING THE CURRENT UNIDIRECTIONAL IN THE EXTERNAL CIRCUIT :
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The current produced from a dc generator using slip sing is alternating in every cycle in the external
circuit.
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To make this current unidirectional in the external circuit, slip ring is replaced by split ring. The split
ring has two parts. Each part is connected to the corresponding brushes.
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The split ring is allowed to rotate at the same velocity of the rotation of conductor while the brush
positions are remain unchanged
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As a result, after every half cycle the one part of split ring goes in touch of other brush form the first
one to whom it did touch in first half cycle.
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Consequently, current becomes unidirectional in the external circuit.
KURRATUL AIN, Lecturer, Deptt. Of EEE,
Stamford University Bangladesh
Page 4
Figure 6 : Making current unidirectional
References:
1. U. A. Bakshi, V. U. Bakshi, Electrical Technology, 4th Edition 2009, Technical Publication Pune.
2. whs.wsd.wednet.edu
KURRATUL AIN, Lecturer, Deptt. Of EEE,
Stamford University Bangladesh
Page 5