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MCQ
861. 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.
862. 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.
863. 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.
864. 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.
865. The efficiency of the DC motor at maximum power is-
90%
100%
Around 80%
Less than 50%
866. 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.
867. When the speed of DC motor is increased-
Back E.M.F increases and current drawn decreases
Back E.M.F decreases and current drawn increases
Back E.M.F and current drawn both increases
Back E.M.F and current drawn both decreases
868. 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.
869. 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
870. The ratio of starting torque to full load torque is least in-
Differential compound motor
Shunt motor
Series motor
Cumulative compound motor
871. What will happen when the DC motor is connected to A.C supply?
DC motor will run at rated speed
DC motor will burn
DC motor will run at slow speed
Both b & c
(i) In case of series connection of armature winding and field winding, it may run. (ii) But, In case of parallel connection, it won't rotate at all and will start humming and will create vibrations, us a torque produced by positive and negative cycle will cancel out each onther D.C motor will be heated up.
872. The starting resistance of DC motor is generally-
High
Infinite
Low
None of the above
873. 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.
874. 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.
875. 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.
876. The direction of rotation of DC series motor can be changed by-
Interchanging field terminal
Interchanging supply terminal
Both a & b
None of the above
The direction of rotation of DC series motor may be reversed by changing the polarity of either field or the armature winding.
877. 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.
878. Which DC motor is preferred for machine tools?
Series motor
Cumulative compound motor
Differentially compound motor
Shunt motor
Shunt characteris is the speciality of DC shunt motor The speed of the motor almost constant on various loads, thus it suits perfectly to the application, where speed requirement is constant like in machine tools.
879. Which DC motor is preferred for elevator?
Differentially compound motor
Series motor
Shunt motor
Cumulative compound motor
DC cumulative compound motor having high starting torque up to 450% depending upon the degree of compounding. The speed regulation is varying up to 25 -30% That's why these motors are used in elevators.
880. 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