DC GENERATOR
41. Write the expressions for the
synchronous speed.
The speed
of rotating magnetic field is called synchronous speed.
43. Write the mmf equation of dc
machine.
The
fundamental component of mmf wave is given by
Where θ =
electrical angle measured from the magnetic axis of the coil which coincides
with the positive peak of the fundamental wave.
44. What is meant by
electromagnetic torque?
When the
stator ad rotor windings of the machine both carry currents, they produce their
own magnetic fields along their respective axes which sinusoidally distributed
along the air-gaps. Torque results from the tendency of these two fields to
align themselves.
45.
State the
torque equation for round rotor machine.
Where P = No. pole
D =
Average diameter of air gap
l= Axial
length if air gap
μo = Permeability of free space = 4π x 10 -7 H/m
g = air
gap length
F1 = Peak
value of sinusoidal mmf stator wave
F2 = peak
value of sinusoidal mmf rotor wave
Α
= Angle between F1 and F2 called torque angle
46.
Define
rotating magnetic field.
When a
balanced three phase winding with phase distributed in space so that the
relative space angle is 120o is fed with balanced 3 phase current,
resultant mmf rotates in air gap at speed.
47. What is prime mover?
The basic
source of mechanical power, which drives the armature of the generator, is
called prime mover.
48.
Give the
materials used in machine manufacturing
Three materials are used in machine manufacturing.
(i)steel – to conduct magnetic flux
(ii)copper
– to conduct electric current
(iii)Insulation
49.
How will
you change the direction of rotation of a d.c motor?
Either
the direction of the main field or the direction of current through the
armature conductors is to be reserved.
50. What is back emf in d.c
motors ?
As the
motor armature rotates , the system of conductor come across alternate North
and South pole magnetic fields causing an emf induced in the conductors. The
direction of the emf induced in the conductors . The direction of the emf
induced is in the direction opposite to the current .As this emf always opposes
the flow of current in motor operation it is called back emf.
51.
Under
what condition the mechanical power developed in a dc motor will be maximum?
Condition
for mechanical power developed to be maximum is
Eb
= Ua /2
or Ia=
Ua / 2Ra
52.
What is
the function of a no-voltage release coil provided in a dc motor starter?
As long as the supply voltage is on healthy
condition the current through the NVR coil produce enough magnetic force of
attraction and retain the starter handle in the ON position against spring
force. When the supply voltage fails or becomes lower than a prescribed value
the electromagnet may not have enough force and the handle will come back to
OFF position due to spring force automatically. Thus a no-voltage or under
voltage protections given to the motor.
53.
Name the
two types of automatic starters used for dc motors.
Back emf type starter
Time
delay type starter
54. Enumerate the factors on
which the speed of a dc motor depends.
N = 1/CE
(Ua-IaRm)/ф
The speed
of dc motor depends on three factors.
Flux in
the air gap
Resistance
of the armature circuit
Voltage
applied to the armature
55. List the different methods of
speed control employed for dc series motor
Field
diverter method
Regrouping
of field coild\s
Tapped
field control
Armature
resistance control
Armature
voltage control for single motor
Series
parallel control for multiple identical motors
57. Name the different methods of electrical breaking of dc motors.
(i) Dynamic braking
(ii) Regenerating braking
(iii) Counter current braking or plugging
58. Under what circumstances does a dc shunt generator fail to build
up?
Absence
of residual flux.
Initial
flux set up by the field winding may be in opposite direction to residual flux
Shunt
filed circuit resistance may be higher than its critical field Resistance
Load
circuit resistance may be less than its critical load resistance.
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