Electrical Drives And Controls Set 4

Mechanical Engineering

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This set of Electrical Drives and Controls EDC Multiple Choice Questions & Answers (MCQs) focuses on Electrical Drives And Controls Set 4

Q1 | Which winding is mostly used winding in the single phase induction motor?
  • circular winding
  • concentric winding
  • progressive winding
  • skein winding
Answer: concentric winding
Explaination:
the concentric winding is the most widely used winding. it is also the most flexible winding of the windings used in the single phase induction motor.
Q2 | What is/are the advantages of the skein winding?
  • low cost to wind
  • low cost to insert
  • permits some freedom of choice of distribution
  • low cost to wind, low cost to insert, permits some freedom of choice of distribution
Answer: low cost to wind, low cost to insert, permits some freedom of choice of distribution
Explaination:
the skein winding is the low cost to wind and to insert. it also permits some freedom of choice of distribution.
Q3 | What material is used in the tunnel of the rotor of the single phase induction motor?
  • aluminium
  • copper
  • steel
  • wood
Answer: aluminium
Explaination:
the rotor consists of a block of slotted laminations. the slots form a series of tunnels that are filled with aluminium in its molten state.
Q4 | What type of operations are used in the starting switches?
  • mechanical operation
  • electrical operation
  • centrifugal operation and mechanical operation
  • centrifugal operation
Answer: centrifugal operation and mechanical operation
Explaination:
the starting switch is used to cut the auxillary winding when the motor attains 75% of the full load speed. the switches operate in both the centrifugal as well as mechanical operation.
Q5 | The ac electrolytic capacitor is formed by winding two sheets of etched aluminium foil.
  • true
  • false
Answer: true
Explaination:
modern capacitor start motors employ ac electrolytic capacitors. the ac electrolytic capacitor is formed by winding two sheets of etched aluminium foil, separated by two layers of insulating paper, into a cylindrical shape.
Q6 | The electrolytic capacitor and insulator unit is impregnated using ethylene glycol or a derivative.
  • true
  • false
Answer: true
Explaination:
the electrolytic capacitor and insulator unit is impregnated using the
Q7 | What is the range of the power factor of electrolytic capacitors?
  • 2-4
  • 4-6
  • 6-8
  • 7-9
Answer: 6-8
Explaination:
the minimum power factor of the electrolytic capacitor is 6. the maximum power factor of the electrolytic capacitor is 8.
Q8 | Why starters are required in a DC motor?
  • back emf of these motors is zero initially
  • these motors are not self-starting
  • these motors have high starting torque
  • to restrict armature current as there is no back emf at starting
Answer: to restrict armature current as there is no back emf at starting
Explaination:
at the time of starting (n=0), the induced emf of a motor is zero such that current drawn by armature, from rated voltage supply would be ia= v/ra. since armature resistance is very low, armature current drawn is very high and will damage the machine.
Q9 | For which motors DOL starter can be used?
  • up to 5 h.p
  • up to 10 h.p
  • up to 15 h.p
  • up to 20 h.p
Answer: up to 5 h.p
Explaination:
dol starters are limited to the small rating motors where distribution system (mains supply) can withstand high starting currents without excessive voltage dips. for a large rating motor, ranging from 5 hp to 25 hp, oil immersed dol starters are used
Q10 | A three-point starter is used for  
  • shunt motors
  • shunt as well as compound motors
  • shunt, compound and series motors
  • not for dc motors
Answer: shunt as well as compound motors
Explaination:
3-point starters are used only for shunt and compound motors, they are not used for series motors. three-point starter is employed where motor field current can be varied in a narrow range and so does the motor speed.
Q11 | The starting resistance of a DC shunt motor is generally  
  • low
  • around 0.5 kΩ
  • around 5 kΩ
  • infinitely large
Answer: low
Explaination:
starting resistance of a dc shunt motor and dc compound motor is low. well, that’s the reason why we use starters in a dc motors, in order to limit the armature current flowing through the armature and to protect machine circuitry.
Q12 | without starter?
  • heavy sparking at brushes
  • it’ll start smoothly
  • will not start at all
  • depends on load
Answer: heavy sparking at brushes
Explaination:
it would cause intolerably heavy sparking at the brushes which may destroy the commutator and brush-gear. sudden development of large torque will cause mechanical shock to the shaft, reducing its life. such heavy current cannot be generally permitted to be drawn from the source of supply.
Q13 | Motor will start quickly when used without starter.
  • true
  • false
Answer: true
Explaination:
the only thing in favour of direct starting must be mentioned here. since the torque of the motor with direct start is much higher, the motor starts much more quickly. as a consequence, the joule input
Q14 | In shunt and compound motor starting the shunt field should be made on with full starting resistance in  
  • series with field
  • parallel with field
  • series with armature
  • parallel with armature
Answer: series with armature
Explaination:
in shunt and compound motors starting the shunt field should be switched on with full starting resistance in armature circuit. a short time delay in this position allows the field current to build up to the steady value of the inductive field transients.
Q15 | motor. The maximum allowable current is 55 A and the minimum current is about 35 A. The armature resistance of the motor is 0.4 Ω. What will be the number of sections of starter resistance required?
  • 5
  • 4
  • 6
  • 8
Answer: 6
Explaination:
i1=55 a, i2 =35 a
Q16 | γ is given as 1.585. Resistance at maximum allowable current is given equal to 4 Ω, what is the 5th step resistance?
  • 0.235
  • 0.370
  • 1.476
  • 2
Answer: 0.235
Explaination:
γ is given as 1.585. so γ-1 is equal to 0.631. r1 is provided and it is equal to 4 Ω.
Q17 | Four-point starter is used when
  • motor field current is varied in narrow range
  • motor speed is varied in small range
  • motor field current is varied over wide range
  • can be used anywhere
Answer: motor field current is varied over wide range
Explaination:
three-point starter is employed where motor field current can be varied in a narrow range and so does the motor speed while four-point starter is used when motor field current can vary over a wide range and so does the motor speed.
Q18 | starting in an induction motor as well as dc motor.
  • induction motor has to control starting torque whereas in dc motor, it is done to avoid large current
  • to limit starting current in both the machines
  • to limit starting speed
  • all of the mentioned
Answer: induction motor has to control starting torque whereas in dc motor, it is done to avoid large current
Explaination:
for both the machines the purpose is different.
Q19 | A 100 hp, 250 V, 350 A shunt dc motor with an armature resistance of 0.05 ohms. To limit maximum starting current to twice the rated of its value, what will be the number of stages of starting resistances?
  • 3
  • 2
  • 4
  • 5
Answer: 3
Explaination:
n = log(ra/rt)/log(imin/imax); rt = vt/imax = 250/700 = 0.357 ohms
Q20 | b)
  • starting resistance would burn
  • field winding would burn
  • speed will rise steeply
  • any of the mentioned
Answer: starting resistance would burn
Explaination:
due to the slow cut off the
Q21 | (iv) Burning of the field winding
  • (i), (ii), (iii), (iv)
  • (i), (iii)
  • (ii), (iii), (iv)
  • (i), (ii)
Answer: (i), (ii), (iii), (iv)
Explaination:
all the observations can be seen due to large starting current in dc motor.
Q22 | is/are  
  • 3-point and 4-point starter
  • 5-point starter
  • 4-point starter
  • 5-point and 3-point starter
Answer: 3-point and 4-point starter
Explaination:
both 3-point and 4-point starters can be used.
Q23 | The direct-on-line starter is used to start a small dc motor because it limits initial current drawn by armature circuit.
  • true
  • false
Answer: false
Explaination:
the direct-on-line starter can not limit the speed by limiting the current.
Q24 | over DOL starter due to  
  • lesser losses
  • controlled direction
  • least resistance offered
  • all of the mentioned
Answer: lesser losses
Explaination:
a thyristor controlled method
Q25 | full load efficiency of 85% has armature resistance of 0.25 ohms. Calculate the value of starting resistance in ohms for a current 1.5 times of the full load current.
  • 2.788
  • 3.038
  • 2.688
  • 2.588
Answer: 2.788
Explaination:
full load current = 7460/(200*0.85) = 43.88 a
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