Transcript armature - Study Channel

DC Generators
Simple Loop Generator
Construction
• Single turn rectangular copper coil ABCD
rotating about its axis in magnetic field.
• Magnetic field provided by either permanent
magnets or electro magnets.
• Ends of coil are joined by two slip-rings ‘a’ and
‘b’.
• Two collecting brushes press against slip-rings.
Function is to collect the current induced in the
coil and to convey to external load R.
• Rotating coil is “ARMATURE” and the magnets
as “FIELD MAGNETS”
Working
• Let coil rotate in clock-wise direction.
• As coil assumes successive position in the field,
the flux linked with it changes.
• Hence EMF is induced in it proportional to rate
of change of flux linkages.
• When the plane of coil is at right angles to the
lines of flux, that is at position 1, the flux linked
with the coil is maximum but the rate of change
of flux linkage is minimum. Coil sides AB and CD
do not cut the flux, rather they slide along them,
that is, they move parallel to them.
EMF Induced
Working
• As the coil continues rotating, the rate of
change of flux linkage increases. At
position 3, theta =90. Coil plane is
horizontal, that is, parallel to the lines of
flux.
• The flux linking the coil is minimum but the
rate of change of flux linkage is maximum.
Hence EMF induced in the coil is
maximum.
EMF Induced
Working
• In the next quarter, from 90-180 degree, flux linking the
coil gradually increases, but the rate of change of flux
linkage decreases. Hence, induced EMF decreases till
position 5. Here the EMF induced is zero. The current
flows from A to B and C to D. Flow of current is from
ABMLCD. In load resistance current flows from M to L
during 1st half revolution of the coil.
• In the next half revolution that is, from 180-360 degree
variations in the magnitude of the EMF are similar to
those in first half revolution.
• EMF value is maximum in position 7 and minimum in
position 1.
• The direction of induced current is from D to C and A to
B. Path of flow of current is DCLMBA which is just
reverse of previous current direction.
EMF Induced
Flow of Current
• Thus we find that current from a simple
generator alternates its direction every half
cycle. Hence we have Alternating Current.
• For making the flow of current unidirectional in
the external circuit, the slip rings are replaced by
split rings.
• Split rings are made up of conducting cylinder
which is cut into two halves or segments
insulated from each other by thin sheet of mica
or other insulating material.
Unidirectional Flow
Operation of Split Ring
• In the first half revolution current flows from
ABMLCD that is brush 1 is in contact with
segment ‘a’. Acts as positive and ‘b’ as negative.
• In the next halve revolution the direction of
induced current has reversed. But the position of
segments ‘a’ and ‘b’ have also reverses with the
result that brush No1 comes in touch with that
segment which is positive.
• Hence, current in load resistance flows from M
to L.
• This current is unidirectional but not continuous
like pure direct current.
Parts of DC generator
1.
2.
3.
4.
5.
6.
7.
Magnetic Frame or Yoke
Pole Cores and Pole Shoes
Pole coils or Field Coils
Armature Core
Armature winding or Conductors.
Commutator
Brushes and Bearings
Parts of Generator
Yoke
Pole Shoe
Armature & Commutator
Armature Core
Consists of slotted soft iron laminations.
Purpose of laminations is to reduce eddy current
loss.
 Laminations are slotted to accommodate and
provide mechanical security to armature winding
and give shorter air gap flux to cross pole face
and the armature teeth.
Armature Winding
• Slot of armature core hold insulated conductors
that are connected in suitable manner, known as
armature winding.
• Here EMF is induced.
• The armature conductors are connected in
series-parallel; the conductor being connected in
series so as to increase the voltage and in
parallel so on to increase the current.
• Windings are classified as
• i) Lap winding and
• ii) Wave Winding.
Lap Winding
Here connection starts from conductors in first slot
then connections overlap each other as winding
proceeds, till starting point is reached again
Number of Parallel Paths = Number of Poles
Wave Winding
Here the winding always travels a head avoiding
overlap.
Number of Parallel Paths = 2
Commutator
• Acts as mechanical rectifier that converts
AC induced EMF in the armature winding
into DC across the brushes.
• Made of copper segments insulated from
each other by Mica sheets.
• Mounted on shaft of the machine.