[Q:-01]
What is a d.c. motor ?
Ans
:
It is a d.c. machine which converts is
electrical energy into mechanical energy.
[Q:-02]
What is the difference between the action of a d.c. generator and a d.c. motor
?
Ans
:
Though they are similar in construction
but their functions are entirely different. A d.c. generator generates voltage
when conductors are moved in a magnetic field while a d.c. motor develops a
twisting effort or a torque when a current is passed through the conductors
under the influence of a magnetic field.
[Q:-03]
What is the working principle of a d.c. motor ?
Ans
:
The working principle of a d.c. motor is
that when a current carrying conductor is placed in a magnetic field a force is
exerted on the conductor, The direction of force acting on the conductor is
determined by Flemming's Left hand rule.
[Q:-04]
What Is back e.m.f. ?
Ans
:
When a current is passed through the
armature of a d.c. machine and its field coils excited, torque is developed and
the armature rotates.
When the armature continues to rotate, the
armature conductors cut the magnetic lines of force and therefore e.m.f's are
induced in the armature, conductors. According to Lenz's law the direction of
the induced e.m.f is opposite to that of the applied voltage and that is why it
is called back e.m.f or counter e.m.f.
[Q:-05]
What will be the armature current in a d.c. motor ?
Ans
:
Let V be the applied voltage and Eb
be the back e.m.f, then the effective e.m.f acting round the armature circuit
is V-Eb.
Therefore, the
armature current. Ia = V-Eb / Ra
where Ra
is the armature resistance.
[Q:-06]
What Is the voltage equation of a d.c. motor ?
Ans
:
V = Eb + IaRa
[Q:-07]
A 440 volt d.c. machine has an armature resistance of 0.25 ohm and load current
of 60 amps. What will be the difference in the Induced voltages when the
machine Is running as a motor and as a generator ?
Ans
:
30 volts.
[Q:-08]
What is the back e.m.f at stand still condition and what will happen if there
is no back e.m.f In the d.c. motor ?
Ans
:
At stand still condition the back e.m.f is
zero and the motor will burn if there is no back e.m.f in the armature.
[Q:-09]
What is mechanical power at the shaft ?
Ans
:
Mechanical power available in watt EbIa
= VIa - Ia2 Ra
where VIa
represents power in watt supplied to the armature and Ia2Ra
is the armature copper loss in watt.
[Q:-10]
What is the condition of maximum power ?
Ans
:
Maximum mechanical power is available when
Eb = V/2
[Q:-11]
What is torque of a d.c. motor ?
Ans
:
Torque is a mechanical couple tending to
produce rotation which is equal to the product of the force in newton acting on
the armature conductor and its perpendicular distance in metre from the axis of
rotation.
[Q:-12]
Where is the torque maximum ?
Ans
:
At right
angles to the magnetic flux.
[Q:-13]
What is the torque relation in a d.c. motor ?
Ans
:
The torque in a d.c. motor is proportional
to the flux per pole and the armature current i.e T Ia.
[Q:-14]
What change will be observed if the applied voltage of a d.c. shunt motor is
halved with load torque doubled ?
Ans
:
Armature current will be four times.
[Q:-15]
For a given percentage increase in the load torque which motors will have least
or maximum percentage increase of current ?
Ans
:
Shunt motor will have maximum percentage
increase of current and series motor will have least percentage increase of
current.
[Q:-16]
On what factors does the torque developed by a d.c. motor depend ?
Ans
:
Torque developed by a d.c. motor depends
upon the current flowing through the conductor, magnetic field, active length
of the conductor, number of conductors and radius of armature.
[Q:-17]
What factors control the magnitude of the torque ?
Ans
:
Armature current and magnetic field
control the magnitude of the torque.
[Q:-18]
What is called b.h.p ?
Ans
:
The horse power obtained from the shaft
torque is called the brake horse power (b.h.p).
[Q:-19]
What is the speed relation in a d.c. motor ?
Ans
:
The speed of a d.c. motor is directly
proportional to the back e.m.f. and inversely proportional to the flux per pole
i.e
N Eb
/ or N (V – IaRa ) /
[Q:-20]
On what factors does the speed of a d.c. motor depend?
Ans
:
The speed of a d.c. motor depends upon
applied voltage, field flux, armature current and armature resistance.
[Q:-21]
What will be the result if the field circuit tit a d.c. motor is opened ?
Ans
:
Due to weakening of field flux the
armature will race up to abnormally high speeds and the armature windings may
got permanently damaged duo to the severe centrifugal forces.
[Q:-22]
What is meant by speed regulation of a d.c. motor ?
Ans
:
The speed regulation of a d.c. motor is defined
as the charge in speed when the load on the motor is reduced from rated load to
zero expressed in percent of the rated load speed.
∴ % speed regulation = (No load
speed - Full load speed)/Full load speed X 100
[Q:-23]
What are the characteristics of a d.c. motor ?
Ans
:
There are two main characteristics e.g.
electrical and mechanical characteristics.
(a) Electrical
characteristics are
(i) Speed (N)
vs Armature current (1n)
(ii) Torque
(T) vs Armature current (1n)
(b) Mechanical
characteristics is Torque (T) vs Speed (N)
[Q:-24]
What are the types of d.c. motors ?
Ans
:
(i) Series
wound,
(ii) Shunt
wound,
(iii) Compound
wound.
[Q:-25]
What do you mean by shunt motor ?
Ans
:
The d.c. motor in which the field winding
is connected across the armature is known as shunt motor.
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