Question 1

The condition for maximum efficiency for a D.C. generator is
A)eddy current losses = stray losses
B)hysteresis losses = eddy current losses
C)copper losses = 0
D)variable losses = constant losses

Accepted Answer

The condition for maximum efficiency in a D.C. generator is when variable losses (which include copper losses) are equal to constant losses (which include eddy current and hysteresis losses). This balance ensures that the generator operates at its most efficient point.

Question 2

D.C. generators are normally designed for maximum efficiency around
A)full-load
B)rated r.p.m.
C)rated voltage
D)all of the above

Accepted Answer

D.C. generators are typically designed for maximum efficiency at full-load conditions to ensure optimal performance during normal operating conditions.

Question 3

In a D.C. generator, the iron losses mainly take place in
A)yoke
B)commutator
C)armature conductors
D)armature rotor

Accepted Answer

Iron losses, which include hysteresis and eddy current losses, mainly occur in the armature rotor of a D.C. generator because it is made of iron and rotates in the magnetic field.

Question 4

D.C. generators are installed near the load centres to reduce
A)iron losses
B)line losses
C)sparking
D)corona losses

Accepted Answer

Installing D.C. generators near load centers primarily reduces line losses. This is because the energy has to travel a shorter distance through the conductors, thus minimizing the energy lost as heat due to the resistance of the conductors.

Question 5

The purpose of retardation test on D.C. shunt machines is to find out
A)stray losses
B)eddy current losses
C)field copper losses
D)windage losses

Accepted Answer

Stray losses in a DC shunt machine are not directly measurable through conventional methods. The retardation test allows for the determination of these losses by observing the machine's deceleration when disconnected from the supply, hence providing an indirect way to calculate stray losses.

Question 6

Which of the following tests will be suitable for testing two similar D.C. series motors of large capacity ?
A)Swinburne's test
B)Hopkinson's test
C)Field test
D)Brake test

Accepted Answer

Field test is suitable for testing two similar D.C. series motors of large capacity because it allows for testing under actual load conditions, which is important for large machines.

Question 7

Hopkinson's test on D.C. machines is conducted at
A)no-load
B)part load
C)full-load
D)overload

Accepted Answer

Hopkinson's test on D.C. machines is conducted at full-load conditions. This test, also known as the back-to-back test or regenerative test, is used to determine the efficiency of two identical machines by running them as a motor-generator set.

Question 8

During rheostat braking of D.C. series motors
A)motor is run as a generator
B)motor is reversed in direction
C)motor is run at reduced speed
D)none

Accepted Answer

During rheostat braking of D.C. series motors, the motor is run as a generator. This process involves disconnecting the motor from its power supply and connecting it to a variable resistor (rheostat). The kinetic energy of the motor (and the connected load) is then converted into electrical energy, which is dissipated as heat in the rheostat, effectively slowing down the motor.

Question 9

For which types of D.C. motor, dynamic braking is generally used ?
A)Shunt motors
B)Series motors
C)Compound motors
D)All of the above

Accepted Answer

Dynamic braking can be used with all types of DC motors, including shunt, series, and compound motors. This method involves using the motor as a generator to slow down the load.

Question 10

Which method of braking is generally used in elevators ?
A)Plugging
B)Regenerative braking
C)Rheostatic braking
D)None of the above

Accepted Answer

Plugging is a method of braking used in elevators by reversing the direction of the motor, causing it to act as a generator and slow down the elevator.

Question 11

In variable speed motor
A)a stronger commutating field is needed at low speed than at high speed
B)a weaker commutating field is needed at low speed than at high speed
C)same commutating field is needed at low speed than at high speed
D)none of the above is correct

Accepted Answer

At low speeds, a weaker commutating field is required compared to high speeds because the induced current in the armature (which the commutating field must counteract to prevent sparking at the brushes) is lower due to the lower speed of rotation. At higher speeds, the induced current is higher, necessitating a stronger commutating field to effectively counteract it and ensure smooth operation.

Question 12

When the armature of a D.C. motor rotates, e.m.f. induced is
A)self-induced e.m.f.
B)mutually induced e.m.f.
C)back e.m.f.
D)none of the above

Accepted Answer

The e.m.f. induced when the armature of a D.C. motor rotates is known as back e.m.f. It acts against the direction of the current that powers the motor, hence the term "back."

Question 13

Where D.C. motor of H.P. 12 or more requires frequent starting, stopping, reversing and speed control
A)drum type controller is used
B)three point starter is used
C)four point starter is used
D)all above can be used

Accepted Answer

Drum type controllers are specifically designed for applications that require frequent starting, stopping, reversing, and speed control, making them suitable for high-power D.C. motors.

Question 14

If a D.C. shunt motor is working at full load and if shunt field circuit suddenly opens
A)this will make armature to take heavy current, possibly burning it
B)this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
C)nothing will happen to motor
D)motor will come to stop

Accepted Answer

If the shunt field circuit suddenly opens, the field current will drop to zero and the motor will lose its excitation. This will cause the armature current to increase dramatically, possibly burning out the armature.

Question 15

D.C. motor is to drive a load which has certain minimum value for most of the time and some peak value for short duration. We will select the
A)series motor
B)shunt motor
C)compound motor
D)any of the above

Accepted Answer

The answer of D.C. motor is to drive a load...

Question 16

D.C. motor is to a drive a load which is almost nil for certain part of the load cycle and peak value for short duration. We will select this
A)series motor
B)shunt motor
C)compound motor
D)any of the above

Accepted Answer

The answer of D.C. motor is to a drive a...

Question 17

Which D.C. motor has got maximum self relieving property ?
A)Series motor
B)Shunt motor
C)Cumulatively compounded motor
D)Differentially compounded motor

Accepted Answer

Series motors have maximum self-relieving property because their torque decreases with an increase in speed, which helps prevent over-speeding under light load conditions.

Question 18

In the D.C. motor the iron losses occur in
A)the field
B)the armature
C)the brushes
D)the commutator

Accepted Answer

Iron losses in a D.C. motor occur in the armature due to the alternating magnetic field as it rotates.

Question 19

The speed of a D.C. shunt motor is required to be more than full load speed. This is possible by
A)reducing the field current
B)decreasing the armature current
C)increasing the armature current
D)increasing the excitation current

Accepted Answer

Reducing the field current in a DC shunt motor decreases the flux, which inversely affects the speed according to the formula $N \propto \frac{V - I_aR_a}{\Phi}$, where $N$ is the speed, $V$ is the applied voltage, $I_a$ is the armature current, $R_a$ is the armature resistance, and $\Phi$ is the magnetic flux. Therefore, with reduced flux, the speed increases.

Question 20

One D.C. motor drives another D.C. motor. The second D.C. motor when excited and driven
A)runs as a generator
B)does not run as a generator
C)also runs as a motor comes to stop after sometime
D)none

Accepted Answer

When one DC motor drives another DC motor, the second motor, if excited properly, will run as a generator. This is because the mechanical energy from the first motor is converted into electrical energy by the second motor, functioning as a generator.

Quiz 5: D.C. Generator and Motor Efficiency, Testing and Braking Techniques

The condition for maximum efficiency for a D.C. generator is

D.C. generators are normally designed for maximum efficiency around

In a D.C. generator, the iron losses mainly take place in

D.C. generators are installed near the load centres to reduce

The purpose of retardation test on D.C. shunt machines is to find out

Which of the following tests will be suitable for testing two similar D.C. series motors of large capacity ?

Hopkinson's test on D.C. machines is conducted at

During rheostat braking of D.C. series motors

For which types of D.C. motor, dynamic braking is generally used ?

Which method of braking is generally used in elevators ?

In variable speed motor

When the armature of a D.C. motor rotates, e.m.f. induced is

Where D.C. motor of H.P. 12 or more requires frequent starting, stopping, reversing and speed control

If a D.C. shunt motor is working at full load and if shunt field circuit suddenly opens

D.C. motor is to drive a load which has certain minimum value for most of the time and some peak value for short duration. We will select the

D.C. motor is to a drive a load which is almost nil for certain part of the load cycle and peak value for short duration. We will select this

Which D.C. motor has got maximum self relieving property ?

In the D.C. motor the iron losses occur in

The speed of a D.C. shunt motor is required to be more than full load speed. This is possible by

One D.C. motor drives another D.C. motor. The second D.C. motor when excited and driven