According to Faraday's laws of electromagnetic induction, whenever a conductor is placed in a varying magnetic field (OR a conductor is moved in a magnetic field), an emf (electromotive force) gets induced in the conductor.
2. 4-pole D.C. machines
D.C. machine is a highly versatile
energy conversion device. It can meet the demand of loads requiring high
starting torques, high accelerating and declerating torques. At the same time,
D.C. machine is easily adaptable for drives requiring wide-range speed control
and quick reversals. These inherent characteristics can further be modified, if
desired, by feedback ‘ circuits. In view of these outstanding features, D.C.
machine possesses a high degree of flexibility. These are therefore widely used
in industry, particularly for tough jobs as are encountered in steel-mill
drives-inspite of their higher initial cost.
D.C. machines discussed in this
chapter have hetropolar field system (alternate N and S poles) and
armature-commutator system. In normal D.C. machines, stator core is not
laminated ; armature core is, however, always laminated to reduce eddy-current
losses. Direct-current machines used in control systems have their magnetic
circuit completely laminated. This is done to minimise the effect of
eddy-current damping on the fast response required in DC. machines employed in
controlled systems.
At present, the annual production of
DC. machines is about 40% of the rupee volume in electrical-machine production
and sales. This is on account of the fact that most highway vehicles use
batteries for the storage of electric energy. In these vehicles and
automobiles, DC. motors are used as starter motors, windshield-wiper motors,
fan motors and for driving other accessories in the vehicles. For these
purposes, almost millions of DC. motors are built each year. In industrial
applications requiring accurate control of speed and/or torque, DC. motor is
unrivalled. Therefore, DC. motors are almost universally employed in steel and
aluminium rolling mills, power shovels, electric elevators, railroad
locomotives and large earth-moving equipment.
DC machines - Label all its parts and mention the matertial
The constructional features of DC.
machines have already been described in Chapter 3, where it has been stated
that field winding is a concentrated winding on salient poles bolted to the
stator frame and armature Winding is a distributed winding housed in the Slots
around the periphery of the cylindrical rotor. Basic principles underlying the
torque production and e.m.f. generation in DC. machines are also outlined in
Chapter 3. The object if this chapter is to present the physical concepts
regarding the steady state behaviour of DC. machines. ‘
The object of this example is to
supplement the constructional details already described in Art 3.2.3. The
reader must go through this article before studying the following
presentation.
(a) Sketch of a 6-pole
D.C. machine is shown in Fig.1. In this figure, iron from the bottom of
armature teeth to the shaft diameter is the armature core. The flux paths for
the six poles are also shown. It is observed from this figure that
1. Flux paths in a 6-pole D.C. machines
(i) each pole carries
a flux ɸ (say),
(ii) yoke handles half
of the pole flux, i.e. ɸ/2,
(iii) armature core
also handles a flux ɸ/2.
Reveals that main flux 4) starts from a
north pole, crosses the air gap and then travels down to the armature core.
There, it divides into two equal (¢/2) halves, each half enters the nearby
south pole, each half then passes through the yoke and reaches the starting
point of north pole so as to complete the flux path. Each flux line crosses the
air gap twice. Some flux lines may not enter the armature ; this flux, called
the leakage flux, is not shown in Fig.1.
(b) Various parts of a 6-pole D.C. machine are
shown in Fig.1. commutator forms the most important component of a DC. machine.
In Fig.2, various parts of a 4ole D C machine along with its commutator are
labelled.
Stator of a DC. machine consists of yoke
(or frame), field windings, interpoles, compensating Winding, brushes and end
covers. Rotor consists of armature core, armature winding, commutator and
shaft. Stator components are described first.
Yoke -
It has two functions : -
(i) it provides path for the pole flux ɸ and
carries half of it, i.e ɸ/2
(ii) it prevides mechanical support to the
whole machine. Since the flux carried by yoke is stationary (i.e. constant), it is not laminated. As
stated before, case iron is used for small, D.C.machines and fabricated steel
for large D.C. machines. In case DC. motor is to be operated through a
power-electronics converter, the yoke is laminated to reduce the eddy/ current
losses.
Field poles -
Field
pole consists of pole core and pole shoe. The pole core is made from cast steel
but the pole shoe is laminated and fixed to the pole core appropriately.
The present-day practice, however, favours
laminated pole. Thus, both pole core and pole shoe are made from thin
laminations of sheet steel to reduce the eddy-current losses. The laminated
pole is welded 0r bolted to the yoke.
Field winding -
The pole is excited
by a winding wound around the pole core. This winding, called field or exciting
Winding, is prepared from copper. The number of turns and cross-section of
field Winding depend upon the type of DC. machine as under :
' (i) For D.C. shunt
machine, large number of turns of small cross-section are used.
(ii) For D.C. series
machine, small number of turns of large cross-section are used.
(iii) For D.C.
compound machine, both shunt (thin Wire) and series (thick wire) field windings
are used.
Interpoles -
These
are fixed to the yoke in between the main poles of a DC. machine. These are
usually tapered With sufficient sectional area at the root to avoid magnetic
saturation. The interpole Winding, consisting of a few turns of thick wire, is
connected in series With the armature so that its magnetomotive force is
proportional to armature current.
Compensating windings -
These windings are placed in the slots cut in
the pole faces of a DC. machine. Compensating winding is also connected in
series With the armature circuit. This winding is, however, used in large D.C.
machines only.
Brushes -
Brushes are housed in box-type brush holders
attached to the stator end cover, Fig.2 , or the stator yoke. A small spring
keeps the brushes pressed on to the commutator surface. The brush pressure on
the commutator surface must be carefully adjusted. Too small a brush pressure
may lead to excessive arcing between the brushcommutator contact. If the brush
pressure is too high, it may cause excessive wear of the commutator surface and
the brushes. Brushes are made of carbon for small D.C. machines,
electrographite for all D.C. machines and copper-graphite for low-voltage
high-current D.C. machines. Rotor components are now described below.
Armature core -
It serves the twin purpose of (i) housing the
armature coils in the slots and (ii) providing the low-reluctance path to the
magnetic flux ¢l2. It is made from 0.35 to 0.50 111m thick laminations of
silicon steel to keep down the iron losses. For larger sizes of DC. machines,
the armature core is placed on the spider as shown in Fig.2.
Armature
winding -
The armature winding is made from copper. It
consists of large number of insulated coils, each coil having one or more turns.
The coils are usually former Wound. These are placed in slots and appropriately
connected in series and parallel depending “Poll the type of winding required.
There are basically two winding types : (i) lap Winding and (”Wave winding. Two
coil ends of each coil are then connected to the riser of segments of a
cmmutator, Fig.2.
Commutator -
It 1s of cylindrical structure. It is built up
of wedge-shaped segments of
high conductivity
hard-drawn ccpper to reduce its wear and tear. Segments. are insulated from
each other by 0.8 mm thick mica sheets. The segments are tapered as shown in
Fig.3. so that their assemblyresults in circular shape. Hub H and ring R are
insulated from commutator segments by mica sheet M and V-shaped so as to
prevent the segments from flying out due to centrifugal forces, Fig.2. Each commutator
segment, Fig. 3, has a riser where conductors from the armature winding are
connected.
3. One commutator segment
Shaft -
On armature shaft are mounted
(i)
hub H of commutator,
(ii) spider in big machines or armature
core in smallmachines and (iii) bearings. End covers are connected to the yoke
on one side and to the bearings and shaft on the other side,Fig.2.
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