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MCQ
21. Which part of DC motor can sustain maximum temperature rise?
Armature winding
Field winding
Slip ring
Commutator
Field winding in a DC motor is not a rotating part. It is present away from the motating part of the machine. So, temperature rise in the machine will not produce any effect on the machine rotation. Thuse field winding can sustain the maximum temperature rise.
22. The application of differentially compound motor is-
Frequent on-off cycle
Low-starting torque
High-starting torque
Variable speed
Differential compound motor gives low starting torque, examined by torque current characteristic.
23. The ratio of starting torque to full load torque is least in-
Differential compound motor
Shunt motor
Series motor
Cumulative compound motor
24. Ampere-hour is the unit of-
quantity of electricity
strength of current
power
energy
Power is measured in watt. Joules. Strength of current measured in Amperes. Ampere-hour is the unit of quantity of electricity of electric charge.
25. The direction of induced e.m.f can be found by -
Lenz's law
Fleming's Right hand rule
Kirchoff's law
Ohm's law
The direction of the induced emf is always such as to result in opposition to the change producing it. This law is known as Lenz's law
26. Why is the speed of DC shunt motor dependent on Back EMF?
Because flux is proportional to the armature current
Because armature drop is negligible
Because Back EMF is equal to armature current
Because flux is constant in DC shunt motor
The field winding is connected across the armature. The resistance offered by the field is constant so the current is constant. Therefore the speed of the DC shunt motor depends on the back EMF.
27. Increase of load, the motion of DC shunt motor will-
remain unchanged
increase a little bit
increase tremendously
decrease a little bit
As increased load means that it requires more torque to turn at that speed. The motor is not producing that torque. So motor will slow or decrease a little bit.
28. The basic electrical requirement is art. Welding is that there should be-
coated electrodes
high open-circuit voltage
no arc blow
de power supply
The art of welding refers to a fabrication process that causes coalescence between materials such as metals, thermoplastics and others. In welding, heat is used to melt the workpieces and a filler materials is used to form a weld pool or molten material.
29. The current drawn by a 120 volts d.c. motor of armature resistance 0.50 and back e.m.f. 110 --- volts is ampere.
20
240
220
5
E = V- laRa ⇒ 110 = 120-la x0.5 la = 20 A
30. The synchronous speed of a 4-pole, 50Hz machine is-
1500 rpm
1800 rpm
1200 rpm
1000 rpm
N₁= 120f/P = 120x50/4 = 1500 грm
31. In the DC machine, the fractional pitch winding is used-
To reduce the harmonic in generated EMF
Improve cooling
Increase EMF
To reduce the copper losses
They improve the waveform of generated EMF Le. generated EMF can be made to approximate to sine wave more easily and the distorting harmonics can be reduced or totally eliminated.
32. The efficiency of the DC motor at maximum power is-
90%
100%
Around 80%
Less than 50%
33. The energy is stored in a lead acid battery in the form of
Electromagnetic energy
Electrostatic Energy
Charged ions
Chemical energy
34. A steel mill requires a motor having high-speed control. Which one of the following motors will you choose?
d.c shunt motor
synchronous motor
dic series motor
slip-ring induction motor
Slip ring induction motor is a type of motor where its external circuit is connected using some rings. This rings attached with high resistance acts as an external circuit for the rotor. This external circuit helps in developing a high starting torque with low current
35. The DC series motor should never be switched on at no load because-
the field current is zero
the machine does not pickup
the speed becomes dangerously high
it will take too long to accelerate
When the DC motor is connected across the supply without load then it will draw a small amount of current form supply. As low current will flow through the field winding as well as armature widing so main flux produced will be lesser. As, main flux is lower. speed will tend to increase. As speed increases back em also increases and thus current drawn from the supply will become more and more lower and thus main flux will also become more and more lower and thus speed will tend to increase more and more.
36. If the back EMF of DC motor vanishes then-
The motor continues to run
Motor will stop
Armature will burn
The motor continues to run in slow speed
If back emf vanishes suddenly, motor circuit will try to retain emf by drawing more current from supply. If supplying unit didn't trip down by this time, excess current in armature may heat up the armature.
37. Counter EMF of the DC motor is-
Less than the applied voltage
More than the applied voltage
Equal to the applied voltage
None of the above
Counter emf or back emf makes the DC motor a self- regulating machine. Le. It makes the DC motor to draw armature current which is just enough to develop the required torque. Thus it helps in energy conversion and it is always less than applied voltage.
38. Which DC motor has the least percentage increase in input current for the same percentage increase in torque?
Separately excited motor
Series motor
Shunt motor
Compound motor
39. For which of the following protection from negative sequence currents is provided?
Generators
Motors
Transmission line
Transformers
Negative sequence over current relays are used to detect the unbalanced load on a generator which may cause excessive rotor heating. The relay is also used to detect unbalanced load currents in motors.
40. The starting winding of a single phase motor is placed in the-
rotor
stator
armature
field
The starting winding also known as the auxiliary winding is used to create the torque needed to start a single phase motor. The starting winding is placed in the stator and also parallel of rotor of the motor.