INTRODUCTION
A motor is a mechanism by which electrical
energy is converted into mechanical energy. Its operating principle is the
reverse of a DC generator. When a coil, carrying current, is placed in a
magnetic field, it experiences forces, which turn it about in a direction
perpendicular to both the field and current. Thus the motor armature placed
inside the magnetic field gets motion, converting electrical energy to
mechanical.
DC MOTORS – Principle of operation
A motor is a mechanism by which electrical
energy is converted into mechanical energy. Both in principle and design, a DC
motor is the reverse of a DC generator.
A steady current is passed through the
armature coil from the commutator and the brushes are so arranged as to reverse
the current every half revolution. When a coil, carrying a current, is placed
in a magnetic field, it experiences forces, which turn it about in a direction
perpendicular to both the field and the current. Due to the rotating torque the
motion of rotation will not be continuous, unless the direction of the current
is reversed each half revolution with the help of a split ring commutator (in a
2-pole machine).
The electric motor is fundamentally similar
to the primitive form of D.C. generator described earlier and is based on the
fact that, if a "loop of wire". If it is supplied, through its
commutator, with electric current from a battery or any other source of direct
current (D.C.) supply, the loop will rotate.
If the brushes of the machine were
connected to the terminals of a primary cell, instead of being connected to
load R, the "loop of wire "would rotate. A greatly enhanced
performance would be obtained by having an iron core on this loop, a further
improvement would be to have many loops, another to have increased pole area,
and a still further improvement would be obtained by having electromagnets
instead of permanent magnets.
When used for traction, the direct current
electric motor is usually of the series wound type, that is, the current, which
passes through the armature also, passes through the field coils. The reason
for this is that a motor having this particular type of winding has
characteristics eminently desirable for traction work, its torque being
proportional to the current flow, multiplied by the magnetic strength of the
field system. The series wound motor is capable; therefore, of producing a high
torque when the vehicle is started, and also has the advantage that as the load
increases its speed drops.
The direct current traction motor can be
considered as having the following major parts;
1. The electro-magnetic system consisting of the frame with pole
pieces, the field winding's and brush gear.
2. The reduction gears between the
armature shaft and the road wheels, together with the gear case, which protects
the gear wheels and holds the gear lubricant.
3. The axle bearing where the
traction motor frame rests on the axle of the vehicle, this arrangement
maintains a constant.
4. The nose suspension arrangement prevents
the frame of the motor from rotating round the axle of the vehicle, The nose is
spring borne on a bogie cross member.
BACK EMF
Due to the rotation of the armature coil
(i.e. a conductor) in the magnetic field, the motor works as a DC generator and
induced e.m.f acts in the circuit, which opposes the current. This induced
e.m.f is called back e.m.f.
TYPES OF D.C.MOTORS
Like DC generators, DC motors are also of 3
types-
(i) Series wound motor,
(ii) Shunt wound motor and
(iii) Compound wound motor.
SPEED EQUATION
We know that back e.m.f. is produced by the
generator action of the motor
Hence back e.m.f. E = φZNP/60A, where symbols have their usual meanings.
Let V be the applied voltage and Ia
and Ra is the armature circuit current and resistance respectively.
Then E =V-IaRa
SPEED CONTROL OF DC MOTOR
We know that, N = (V-Ia Ra)/φ
From this formula it follows that the speed
of a D.C. motor can be regulated by:
(i) varying the supply mains voltage V
(ii) Varying the voltage drop in the
armature circuit Ia Ra
(iii) Varying the field flux
Methods
(ii) & (iii) are possible in any
installation with constant supply voltage. But the first method is possible
only in special installation; that permits the control of the supply voltage.
CHARACTERISTICS OF D.C.MOTORS
There are three important characteristics
of a D.C motor, which are given below: -
(Ref. Attached figures)
(i)
Torque - Armature current characteristics
This shows the relation between mechanical
torque developed and armature current.
(ii)
Speed- Armature current characteristics
As the name indicates, it relates speed
with armature current.
(iii)
Speed - Torque characteristics
The characteristics curve gives the
relation between speed and torque of a DC motor.
CONSTRUCTIONAL
DETAILS OF DC MOTOR
INTER POLES
In addition to the main field coils of a
motor being in series with the armature, there are also the coils of a smaller
system of field magnets known as inter-poles. On generators with separately
excited main fields, the inter-pole coils are in series with the armature.
The inter-poles are smaller than the main
poles of either a generator or motor, but are the same length and positioned
alternatively with the main poles. In a generator the polarity of an inter-pole
is the same as the main pole ahead of it according to the rotational direction
of the armature. The polarity of an inter-pole in a motor is the same as the
main pole proceeding it. An electrical machine with no inter-poles would have
some magnetically neutral regions between its pole-pieces. When a coil of the
armature reaches a position during its rotation in the neutral region, its
connections are short-circuited with the connection of the armature coil in
advance, because in this position the commutator brushes will be in contact
with both of their corresponding commutator segments. The purpose of the
inter-poles, being situated in the neutral regions, is to induce a current in
the short-circuited armature wingdings, which flows in the same direction as the
current, which will flow when it has left the neutral region.
The use of inter-poles also serves to
prevent the distortion of the main field of the generator by the reaction of
the armature field, and thereby prevents the induction of Electromotive forces
into coil sides, which are being short- circuited by the brushes.
In small machines the need for inter-poles
is not important but on large generators and motors the net effect of the
inter-poles is to improve the commutation. Ideally there should be no sparking
of the brushes on the commutator surface, although this is often difficult to
achieve in practice.
HEATING AND COOLING
Every electrical machine is a power (or
energy) conversion device. During these power conversion some of the energy is
wasted. In electrical machines the loss in energy occurs in electrical circuits
and in portions of magnetic circuits also. There are also frictional losses in
the dynamic parts of the machines. These losses are converted in the form of
heat energy, which increases, or tends to increase the temperature of iron and
copper above that of the ambient temperature, which in turn effects the winding
insulation. In addition to the effect it has on the insulation, an excessive
temperature rise may also adversely influence the mechanical operating
conditions of a given machine part. Thus, for example the original dimension of
the commutator may change. Solder between the commutator and winding may get
washed out. So to avoid all these, it is very essential to provide a cooling
system on machines.
In most cases, the cooling is done by air
currents. The cooling of machines by air streams is called ventilation. The
ventilation employed depends on the environmental conditions of the place where
the machine is to operate.
According to the method of ventilation
employed, the following types of machines are distinguished: -
(i) Machines with natural ventilation.
(ii) Machines with internal
self-ventilation.
(iii) Machines with external self-ventilation.
(iv) Machines with independent ventilation.
Enclosures have got the direct bearing with
the ventilation. The following are the main types of enclosures: -
(i)
Open pedestal
Rotor and stator windings are in free
contact with the surroundings.
(ii)
Open end Bracket
Rotors and stator windings are in contact
with surrounding through openings.
(iii)
Protected (formerly called semi-enclosed)
Openings in the frame are protected with
wire, perforated covers etc.
(iv)
Drip proof
Opening so constructed that no solid or
liquid particles falling at an angle greater than 150 will enter the machine.
(v)
Splash proof
Similar to drip proof but the angle of
approach is 100O from vertical.
(vi)
Duct or pipe ventilated
Air for ventilation enters and emerges
through a pipe through the openings.
(vii)
Totally enclosed
Exchange of air throughout side and inside
of the machine is prohibited.
(viii)
Water proof
The machine is totally enclosed so as to
exclude water applied as a stream as specified.
(ix)
Flame proof
It is designed normally for mines.
(x)
Resistant
Machine is so constructed, that it will not
be harmed easily by moisture fume, alkali etc.
(XI)
Submersible
So constructed that it will work when
submerged in water under specified condition of pressure and time.
RATINGS
There are three types of ratings as
specified.
(i)
Continuous Rating: This is an output, which a
machine delivers continuously without exceeding the permissible temperature. It
can deliver 25% overload for two hours.
(ii)Continuous
maximum Rating: This is similar to continuous
rating but not allowing overload.
(iii)
Short time ratings: This is an output which a
machine can deliver for a specified period (say 1 hr 1/2 hr, 1/4 hr etc)
without exceeding the maximum temperature rise limit.
SUMMARY
Information regarding operating principle,
construction, characteristic and selection of carbon brushes for DC motors have been given in this unit. These
information will help in maintaining the motors to ensure reliability and
their trouble free functioning. Sketches and diagrams have been included in
this unit to understand the unit with more practical and systematic approach.
SELF-ASSESSMENT EXERCISES
1.
Describe and mention the speed
equation of a dc motor.
2.
What are the different types of
cooling arrangements of DC motors?
3.
What do you mean by rating of a
motor? What are the types of ratings?
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