Electrical Drives And Controls Set 6

Mechanical Engineering

Practice Mode Previous Next
On This Page

This set of Electrical Drives and Controls EDC Multiple Choice Questions & Answers (MCQs) focuses on Electrical Drives And Controls Set 6

Q1 | The torque limit of speed for a shunt motor  
  • remains constant till base speed
  • remains constant after base speed
  • varies linearly after base speed
  • varies inversely till base speed
Answer: remains constant till base speed
Explaination:
the torque limit of the shunt motor remains constant till the base speed.
Q2 | A laboratory group was working with a set of 3-hp shunt DC motor. But there was a mistake that it was fused with a 0.3A fuse instead 3A fuse. Then it was started
  • a flash occurred instantly
  • it ran for 3s and fuse was blown
  • it ran normal
  • none of the mentioned
Answer: it ran for 3s and fuse was blown
Explaination:
due to the wrong usage of the fuse wire, it could not sustain the current and in few seconds it was blown off.
Q3 | avoided by using  
  • a turn or two of cumulative compounding
  • compensating winding
  • stablizer winding
  • all of the mentioned
Answer: compensating winding
Explaination:
the runaway in large machine is avoided by using compensating winding.
Q4 | fast when under-loaded?
  • due to weak field
  • due to high line voltage
  • due to brush-shifted to neutral
  • all of the mentioned
Answer: due to weak field
Explaination:
all of the reasons cause the dc motor to increase in speed when under loaded.
Q5 | Concentrated winding differ from distributed winding with the concern of
  • identical magnetic axis
  • two magnetic axis
  • no magnetic axis
  • physical spacing
Answer: identical magnetic axis
Explaination:
concentrated winding have all turns on one magnetic axis.
Q6 | We can place closed windings in
  • ac commutator machine
  • stepper motor
  • ac machine
  • dc machine
Answer: ac commutator machine
Explaination:
closed windings are placed only in commutator as it does not need any reconnections later.
Q7 | Which of the following machines can be used to place open slot winding?
  • ac machine
  • ac commutator machine, ac machine
  • dc machines
  • all of the mentioned
Answer: ac machine
Explaination:
only ac machines are used for open type of winding inside the machine.
Q8 | The simplex lap winding has the range of winding pitch of  
  • (-2,2)
  • (-1,1)
  • more than 2
  • less than 1
Answer: (-2,2)
Explaination:
for a progressive winding, forward pitch – backward pitch = winding pitch
Q9 | A 200V dc machine has 4 poles and 40 coils, having simplex lap winding. The number of commutator segments which required in the given machine will be?
  • 40
  • 20
  • 80
  • 26
Answer: 40
Explaination:
it is a simplex winding and so one segment of commutator will be needed for each coil.
Q10 | Commutator segments in a DC shunt machine is equal to the number of  
  • coil sides
  • turns
  • coils
  • slots
Answer: coil sides
Explaination:
commutator segments are placed as per the coil sides in the machine.
Q11 | We can employ dummy coils in a DC machine to  
  • compensate reactance voltage
  • reduce armature reaction
  • provide mechanical balance to the armature
  • improve the waveforms generated inside the commutator
Answer: compensate reactance voltage
Explaination:
dummy coils are placed inside so that to provide mechanical support to dc machine and they do not take part in the emf development.
Q12 | In AC machines we should prefer double layer winding over single layer windings because  
  • it requires identical coils
  • it is economical to use
  • it offers lower leakage reactance
  • all of the mentioned
Answer: it requires identical coils
Explaination:
double layer winding enables more usage of the space and lesser air gaps so the leakage flux also reduces and efficiency improves.
Q13 | For an electrical machine with C number of coils and P poles, the distance between the coils connected by an equalizer ring is?
  • c/p
  • c/2
  • 2c/p
  • c/2p
Answer: 2c/p
Explaination:
backward pitch for an equalizer ring = number of coils/pole pairs = c/p/2 = 2c/p.
Q14 | With a P-pole DC machine with armature current Ia, the current per brush arm for a lap connected windings is?
  • ia/p
  • ia/2p
  • 2ia/p
  • ia/4p
Answer: 2ia/p
Explaination:
current per brush (lap winding)
Q15 | unused component in the machine, then we must  
  • s/p/2 should not be an integer
  • s/p/2 should be integer
  • s/p should be integer
  • s/p should not be an integer
Answer: s/p/2 should not be an integer
Explaination:
if slots per pole pair is not integer than all the winding will be completed without adding any dummy coil.
Q16 | While doing regular checks on the dc machine with the lap connected winding, it is reported to have ammeter fluctuations, this can be due to  
  • different air gaps under poles
  • variable reluctances in the core
  • irregular design deformations
  • all of the mentioned
Answer: all of the mentioned
Explaination:
in a lap winding configuration, if the machine as air gap variations due to the design aspects, it will give circulating current in the windings.
Q17 | with the remaining brushes, voltage and power that could be obtained from the machine are  
  • e, p
  • e, 2p/3
  • e/p, 2p/3
  • e, p/3
Answer: e, p
Explaination:
taking kirchoff’s law into study here, the voltage across the winding will remain same.
Q18 | Ward Leonard method is  
  • armature control method
  • field control method
  • combination of armature control method and field control method
  • totally different from armature and field control method
Answer: combination of armature control method and field control method
Explaination:
ward leonard method is the combination of armature control method and field control method, which can also be called as voltage control method. this is the most efficient method of speed control over wide range.
Q19 | Reducing the armature voltage will give us
  • variable torque speed control
  • constant torque speed control
  • variable and constant both can be achieved
  • cannot comment on torque
Answer: constant torque speed control
Explaination:
as seen from speed torque characteristics, reducing armature voltage
Q20 | b) DC generator
  • reduce armature voltage
  • increase armature voltage
  • increase field current
  • decrease field current
Answer: decrease field current
Explaination:
in ward leonard speed control method, speed can be increased above base value by weakening of the field, which can be done by lowering field current value. by increasing armature voltage speed can be increased but this is not employed in ward leonard method.
Q21 | Reducing the field current will give us
  • constant torque and variable power speed control
  • constant torque speed control
  • variable power speed control
  • constant power speed control
Answer: constant torque speed control
Explaination:
as seen from speed torque characteristics, reducing field current will increase the speed of the motor above base value but power will remain same. thus, reducing armature voltage will give constant power speed control, with variable torque.
Q22 | Speed-power characteristic for Ward Leonard speed control method
  • will start from origin
  • will start from some positive value on power axis
  • will start from some positive value on speed axis
  • depends on other parameters
Answer: will start from origin
Explaination:
speed power characteristic of dc motor is plotted when, ward leonard speed control method is employed. for speed equal to zero, which is less than base speed, we get constant torque but variable power operation. thus, power will start increasing from origin.
Q23 | Efficiency of Ward Leonard method is
  • higher than rheostatic control method but lower than shunted field control method
  • lower than rheostatic control method
  • higher than rheostatic control method and shunted field control method
  • depends on load
Answer: higher than rheostatic control method and shunted field control method
Explaination:
unlike all other methods, external resistance is not added in the circuit of control system. thus, efficiency of ward leonard control method is always highest at various different speeds.
Q24 | Ward Leonard method is an ideal choice for motor which undergoes frequent starting, stopping, speed reversal.
  • true
  • false
Answer: true
Explaination:
absence of external resistance increases efficiency. also, when the generator emf becomes less than the back emf of the motor, electrical power flows back from motor to generator, is converted to
Q25 | Starting gear used in Ward Leonard method 
  • is of small size
  • is of large size
  • size depends on application
  • is absent
Answer: is absent
Explaination:
no special starting gear is required in ward leonard method of speed control. as the induced voltage by generator is gradually raised from zero, the motor starts up smoothly. speed reversal is smoothly carried out.
Previous Next