Working Principle Of A DC Motor
An Electric DC motor is a machine which
converts electric energy into mechanical energy.
The principle of working of a
DC motor is that "whenever a current carrying conductor is
placed in a magnetic field, it experiences a mechanical force".
The direction of mechanical force is given by
Fleming’s Left-hand Rule and its magnitude is given by F = BIl Newton.
Fleming's
left hand rule: If we stretch the first finger, second
finger and thumb of our left hand to be perpendicular to each other AND
direction of magnetic field is represented by the first finger, direction of
the current is represented by second finger then the thumb represents the
direction of the force experienced by the current carrying conductor.
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Ø There is no basic difference in the construction of a DC
generator and a DC motor. In fact, the same d.c. machine can be used
interchangeably as a generator or as a motor. Like generators DC motors are
also classified in to shunt-wound, series-wound and compound-wound.
Ø DC motors are seldom used in ordinary applications because
all electric supply companies furnish alternating current.
Ø However, for special applications such as in steel mills,
mines and electric trains, it is advantageous to convert alternating current
into direct current in order to use dc motors. The reason is that
speed/torque characteristics of d.c. motors are much more superior to that of a.c.
motors. Therefore, it is not surprising to note that for industrial drives,
d.c. motors are as popular as 3-phase induction motors.
Consider a
part of a multipolar d.c. motor as shown in
Animation side . When
the terminals of the motor are connected to an external source of d.c.
supply:
(i) the field magnets are excited developing alternate N and S poles;
(ii) the armature conductors carry currents. All conductors under N-pole carry currents in one direction while all the conductors under S-pole carry currents in the opposite direction.
(i) the field magnets are excited developing alternate N and S poles;
(ii) the armature conductors carry currents. All conductors under N-pole carry currents in one direction while all the conductors under S-pole carry currents in the opposite direction.
Suppose the
conductors under N-pole carry currents into the plane of the paper and
those under S-pole carry currents out of the plane of the paper as shown
in Figure. Since each armature conductor is carrying current and is placed
in the magnetic field, mechanical force acts on it. On
applying Fleming’s left hand rule, it is clear that force on each
conductor is tending to rotate the armature in anticlockwise direction.
All these forces add together to produce a driving torque
which sets the armature rotating.
When the conductor moves from one side of a brush to the other, the current in that conductor is reversed and at the same time it comes under the influence of next pole which is of opposite polarity. Consequently, the direction of force on the conductor remains the same.
When the conductor moves from one side of a brush to the other, the current in that conductor is reversed and at the same time it comes under the influence of next pole which is of opposite polarity. Consequently, the direction of force on the conductor remains the same.
Back EMF: According
to fundamental laws of nature, no energy conversion is possible until there
is something to oppose the conversion. In case of generators this opposition
is provided by magnetic drag, but in case of dc motors there is back emf.
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Types Of DC Motors
DC motors are usually classified of the basis of their
excitation configuration, as follows -
§ Separately excited
(field winding is fed by external source)
§ Self excited -
§ Series wound (field
winding is connected in series with the armature)
§ Shunt wound (field
winding is connected in parallel with the armature)
§ Compound wound -
§ Long shunt
§ Short shunt
MCQ
Principle
of working of a DC motor - MCQs with Answers
Q1. The generating action and motoring action in
d.c. motor is determined by
a. Fleming’s left hand rule, Fleming’s right hand rule
b. Both by Fleming’s left hand rule
c. Both by Fleming’s right hand rule
d. Fleming’s right hand rule, Fleming’s left hand rule
ANSWER:
d. Fleming’s right hand rule, Fleming’s left hand rule
Q2. In a practical motor, to reverse the
direction of rotation
a. Reverse the direction of main field produced by the field
winding
b. Reverse the direction of current passing through the
armature is reversed
c. Either (a) or (b)
d. None of these
ANSWER:
c. Either (a) or (b)
Significance of Back EMF in D.C Motor - MCQs with
Answers
Q1.
A 220 V, d.c. motor draws an armature current of 20 A. Its armature resistance
is 0.6 ohm. Then the induced emf in the motor will be
a. 195 V
b. 202 V
c. 208 V
d. 215 V
ANSWER: c. 208 V
Q2. If the
flux is increased by 50% and speed is reduced by 50% of a d.c. motor (keeping
the other parameters constant), then its back emf will become
a. 50 % of the original back emf
b. 0.75 % of the original back emf
c. 100 % of the original back emf
d. 150 % of the original back emf
ANSWER: b. 0.75 % of the original back emf
Q3. If Eb is the back emf of d.c. motor and V is
the terminal voltage, then the condition for maximum power is
a. Eb = V
b. Eb = 2V
c. Eb = ( V / 2 )
d. Eb = V ∧ 2
ANSWER: c. Eb =
( V / 2 )
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