1
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At what principle the DC motor operates?
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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 = BIL Sin θ
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
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2
|
State the Fleming’s left hand rule.
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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.
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3
|
Describe the function of commutator in
the DC motor.
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Function
of commutator
- The function of the commutator is to convert
alternating current into continuous and unidirectional current and also
torque.
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4
|
Explain : Magnetic drag or backward
force in the DC generator.
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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.
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5
|
What is opposition force produces in
the motor and generator?
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The force of opposition in the
DC motor
DC generator
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6
|
Define : Back emf
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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.
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7
|
The armature torque in the DC motor is
due to force on the armature conductors - True or False. Comment on the above
statement
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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.
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8
|
Describe the effect on armature
current due to change of speed in the DC motor.
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The armature current is given by
The back emf is given by
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.
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9
|
Explain the term : Motor efficiency
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Motor
efficiency
- The motor efficiency is given by ratio of power
developed in the armature to the input power to the DC motor.
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10
|
Describe the condition for maximum
power transfer in the DC motor.
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Condition
for maximum power
- When the back emf is equal to half of the supply
voltage, armature develops maximum power in the DC motor.
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11
|
Explain the term : Lost torque
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Lost
torque
- The difference between armature torque and shaft
torque is known as lost torque.
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12
|
Explain : Speed regulation
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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
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13
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Describe the relation between armature
torque and armature current for DC shunt motor and DC series motor.
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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
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14
|
The speed in the DC motor depends upon
torque and vice versa - True or False
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The speed in the DC motor depends upon torque, but
torque does not depend upon the speed.
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15
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Give reason : The DC series motor
never start at no load.
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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.
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16
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What is relation between torque and
armature current at light load and heavy load in the DC series motor?
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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.
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17
|
The flux in the DC shunt motor is
constant. - comment on above statement
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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.
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18
|
The DC shunt motor is constant speed
motor. – comment on above statement
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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.
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19
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Explain : Cumulative compound
generator, Differential compound generator
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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.
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20
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Which type of DC motor is suitable for
pulsating load? Why?
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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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