PHYSICS: D.C. MACHINE 05/08 Online Test

The value of the back e.m.f., when the D.C motor is at rest is
Less than the supply voltage
Greater than the supply voltage
Equal to the supply voltage
Zero
Commutator segments of a d.c. motor are made of
Mica
Copper
Cast Iron
Stainless steel
Which of the following D.C generator will offer constant voltage at any load?
Level compounded generator
Self-excited generator
Separately excited generator
All of the above
Which of the following effects is observed for armature reaction in a D.C. generator?
Magnetising the center of all poles
Demagnetising the center of all poles
Demagnetisation of leading pole tip and magnetisation of trailing pole tip
Demagnetisation of trailing pole tip and magnetisation of leading pole tip.
In a D.C shunt motors if the load is increased the speed of the motor
Reduces slightly
Increases slightly
Remains unaltered
Increases in proportion to the increase in load.
For which of the following application, a D.C motor is preferred over an A.C motor?
High speed operation
Low speed operation
Fixed speed operation
Variable speed operation
In a D.C shunt generator, terminal voltage drops on load because of all of the following reasons except
Commutation
Armature reaction
Armature resistance drop
Field weakening due to armature reaction
A shunt generator can self-excite provide
Irrespective of the value of the resistance in the field circuit
Provided dummy coils are present in pole shoe
Only if the resistance of the field circuit is greater than critical value
Only 'if the resistance of the field circuit is less than critical value
After saturation the torque-current characteristic curve of a D.C series motor becomes a
Parabola
Straight line
Rectangular hyperbola
None of the above
On loading, the speed of a D.C series motor
Increases
Decreases
Remains same
Becomes zero
In a separately excited generator supplying rated load-the arn'1ature reaction
May be sometimes present
Is always present
Is always absent
None of the above
A series generator can self-excite provided
Speed is low
Inter pole is present
The load current is zero
The load current is not zero
If the field flux of D.C shunt motor approaches zero
The motor will stop
The motor speed will approach zero
Motor speed will remain unchanged
The motor speed will tend to rise at infinite speed
Ward Leonard method of speed controls provides speed control
Above rated speed
Below rated speed
In one direction only
In both directions
Which of the following statements is incorrect in D.C shunt generators?
hysteresis loss depends upon the supply frequency
hysteresis loss is independent of lamination thickness
hysteresis loss can be minimized by laminating the armature
In D.C generators, the brushes remain in
The inter-polar axis
Under south pole
Under north pole
None of the above position
The function of the brushes and the commutator in a D.C motor is to
Reduce sparking
Produce unidirectional torque
Help in changing the direction of rotation of the armature
Produce unidirectional current in the armature
Which of the following motor gives high starting torque
Series motor
Shunt motor
Compound motor
All of these
Change of D.C excitation of shunt motor changes
Motor speed
Direction of rotation of the motor
Applied voltage to the motor
None of these
Which one of the following types of generators does not need equalizes for satisfactory parallel operation?
Series
Over-compound
Under-compound
Flat-compound
Description:

This is test is a part of a series of eight test which helps students to check the level of knowledge in Electricity, specifically in the DC Machine in Physics.

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