Principle
of DC motor
When a
current carrying conductor is placed in the magnetic field, it experiences a
mechanical force whose direction is given by Fleming’s left hand rule and
magnitude is given by
F = BILsinθ
F = Force in Newton
B = Maximum flux density in Wb / m2
I = Current in Ampere
L= Length of conductor in meter
θ = Angle between position of conductor and
direction of magnetic field
State the
Fleming’s left hand rule.
Fleming’s
left hand rule
If first
finger shows a direction of magnetic field, middle finger shows direction of
current then thumb indicates direction of force.
Describe
the function of commutator in the DC motor.
Function of
commutator
The
function of the commutator is to convert alternating current into continuous
and unidirectional current and also torque.
Explain:
Magnetic drag or backward force in the DC generator
Magnetic
drag
When the
armature rotates in the influence of magnetic field of poles, it experiences a
force whose direction is opposite to that rotation of the armature. This
opposition force is known as backward force or magnetic drag in the DC
generator.
What is
opposition force produces in the motor and generator?
The force
of opposition in the
DC motor: Back emf
DC
generator: Magnetic drag
Define:
Back emf
Back emf
When supply
is given to the DC motor, dynamically induced emf is produced in the armature
conductors. The direction of the
dynamically induced emf is opposite to the main supply voltage therefore it is
called as back emf.
The
armature torque in the DC motor is due to force on the armature conductors -
True or False. Comment on the above statement
False, The
torque in the DC motor is not due to mechanical force on the armature conductor
but actually it is due to tangential pull on the armature teeth.
Describe
the effect on armature current due to change of speed in the DC motor.
The
armature current is given by
Ia
= V – Eb / Ra
The back
emf is given by
Eb
= ФZNP / 60 × A
From the
above equation
When speed
of the DC motor increases, Eb increases and armature current
decreases. When speed of the DC motor decreases, Eb increases and
armature current increases.
Explain the
term: Motor efficiency
Motor
efficiency
The motor
efficiency is given by ratio of power developed in the armature to the input
power to the DC motor.
Describe
the condition for maximum power transfer in the DC motor.
Condition
for maximum power
When the
back emf is equal to half of the supply voltage, armature develops maximum
power in the DC motor.
Explain the
term: Lost torque
Lost torque
The
difference between armature torque and shaft torque is known as lost torque.
Explain:
Speed regulation
Speed
regulation
It is
defined as the change in the speed of motor from no load to the full load at a
percentage of full load speed.
Speed regulation = ( N0 – Nf )
/ N0 × 100 %
Where N0 = No load speed
Nf = Full
load speed
Describe
the relation between armature torque and armature current for DC shunt motor
and DC series motor.
Torque in
the DC motor is Ta α ФIa
DC shunt
motor
The flux is
practically constant if supply voltage is constant therefore armature torque is
directly proportional to armature current.
Ta α Ia
DC series
motor
The series
field winding also carries armature current therefore flux is directly
proportional to armature current up to point of saturation Ф α Ia
Therefore
Ta α Ia2
After
saturation Ta α Ia
The speed
in the DC motor depends upon torque and vice versa - True or False
The speed
in the DC motor depends upon torque, but torque does not depend upon the speed.
Give
reason: The DC series motor never start at no load.
DC series
motor at no load
The speed
of DC motor is inversely proportional to field flux at constant voltage and at
any given armature current.
N α Eb / Ф
As the
armature winding and field winding both are in series, the field flux is only
due to residual flux in the field winding which is very small at no load.
Therefore the DC series motor runs at dangerously high speed particularly at no
load. Hence the DC series motor never runs at a load.
What is
relation between torque and armature current at light load and heavy load in
the DC series motor?
Torque –
armature current characteristic
The field
winding of the DC series motor also carries the armature current therefore the
torque is directly proportional to square of the armature current before
saturation at light load. The flux in the DC series motor independent armature
current after saturation therefore the torque is directly proportional to
armature current. The torque – armature current characteristic is parabola at
light load and straight line at heavy load.
The flux in
the DC shunt motor is constant. - comment on above statement
Flux in the
DC shunt motor
The field
winding of the DC shunt motor is connected to the parallel of the supply
voltage. If the supply voltage is kept
constant, the field flux is also constant. However the value of field flux
slightly decreases at higher load condition due to de – magnetizing effect of
armature reaction.
The DC
shunt motor is constant speed motor. – comment on above statement
DC shunt
motor – Constant speed motor
The speed
of DC shunt motor depends upon back emf and field flux.
N α Eb / Ф
However the
back emf and flux both decrease with increase in the load. But the back emf
decrease slightly more than that of field flux. Therefore there is some
decrease in the speed at higher load condition.
The drop in
the speed from 5% to 10% of full load condition and it depends upon armature
reaction, saturation and brush position.
Explain:
Cumulative compound generator, Differential compound generator
Cumulative
compound generator
If the
direction of the series flux is in the same direction that of shunt flux, the
DC motor is called as cumulative compound motor. As the both fluxes are in the
same direction, the total flux increases with increase in the load and vice
versa.
Differential
compound generator
If the
direction of the series flux is opposite to that of shunt flux, the DC motor is
called as differential compound motor.
As the both fluxes are in the opposite direction, the total flux
decreases with increase in the load and vice versa.
Which type
of DC motor is suitable for pulsating load? Why?
Pulsating
load
The load on
the DC motor which is not constant with respect to time is known as pulsation
load. The cumulative compound motor is used for pulsating load. When load is
entirely cut off, due to shunt field winding, the speed of the motor does not
become dangerously high. The motor will able to take heavy load due to series
field winding.
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