Electrical Drives And Controls Set 2

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 2

Q1 | -Φ fully controlled rectifier is  
  • 5th
  • 7th
  • 3rd
  • 2nd
Answer: 7th
Explaination:
only 6k±1 order harmonics are present in 3-Φ fully controlled rectifier. the second lowest order harmonic is 7th harmonic for k=1. third order harmonics are absent in
Q2 | to protect them from dust and contamination.
  • enclosures
  • open environment
  • moisturizing environment
  • hot environment
Answer: enclosures
Explaination:
electric motors are placed in enclosures to protect them from dust and contamination. there are seven most common types of enclosures. for example- open drip proof.
Q3 | voltage of a dc machine is a   function of its magneto-motive force.
  • non-linear
  • linear
  • constant
  • inverse
Answer: non-linear
Explaination:
the flux and induced emf are non linear function of its mmf.
Q4 | compound dc motor is shown below. What conclusions can be made?
  • this is an unstable machine
  • there is regenerative increment in the speed
  • this is impractical to be used
  • all of the mentioned
Answer: all of the mentioned
Explaination:
this is basically an unstable scenario which makes the machine unable to be used.
Q5 | It is impossible to start a differential compounded dc motor.
  • true
  • false
Answer: true
Explaination:
it is so because of the unstable speed and torque characteristics. at starting, armature current and series field current are high. since the series flux subtracts from shunt flux, series flux actually reverses the magnetic polarity. this motor will typically be still or turn slowly in wrong direction while burning up because of excessive current.
Q6 | compound motor and he has been asked to make it start. How will he try?
  • by shorting series field at start
  • to run as shunt motor at start
  • by making rated current at start
  • all of the mentioned
Answer: all of the mentioned
Explaination:
at starting, the flux should be rated so that there is no abnormal situation arise.
Q7 | with compensating winding has a field current of 5 A to produce a voltage of 250 V at 1200 rpm. What will be the shunt field current of this machine at no load?
  • 5 a
  • 5.6 a
  • 4 a
  • 0 a
Answer: 5 a
Explaination:
at no load, armature current is zero, so the internal generated voltage is 250
Q8 | b) position, then the  
  • motor will draw large current
  • motor will not start
  • motor will burn
  • all of the mentioned
Answer: all of the mentioned
Explaination:
as the starter is placed very rapidly from off to on the resistance seen by the machine will be very less and it will draw a large starting current.
Q9 | many types?
  • three
  • two
  • four
  • five
Answer: two
Explaination:
load torques can have two types. they are active and passive load torques. active load torques are able to run the motor under equilibrium conditions and their sign remains the same even if the motor rotation changes but passive load torques always opposes the motion by changing their sign with the change in rotation of the motor.
Q10 | Rolling mills exhibit what type of load torque characteristics?
  • constant torque characteristics
  • linearly rising torque characteristics
  • non-linearly rising torque characteristics
  • non-linearly decreasing torque characteristics
Answer: non-linearly decreasing torque characteristics
Explaination:
rolling mills are an example of non-linearly decreasing torque characteristics because torque and speed exhibits inversely proportional relationships and power are constant.
Q11 | What is the relationship between torque and speed in constant type loads?
  • torque is independent of speed
  • torque linearly increases with increase in speed
  • torque non-linearly increases with an increase in speed
  • torque non-linearly decreases with an increase in speed
Answer: torque is independent of speed
Explaination:
speed hoist is a perfect
Q12 | Torque inversely varies with the speed in the windage load torque component.
  • true
  • false
Answer: false
Explaination:
torque varies with a square of speed in the windage load torque component whereas in coulomb torque component torque is constant.
Q13 | What type of force handles for active torques?
  • strong nuclear forces
  • weak nuclear forces
  • gravitational forces
  • electrostatic forces
Answer: gravitational forces
Explaination:
gravitational forces are responsible for active torques. active torques due to gravitational forces can be obtained in the case of hoists, lifts or elevators and railway locomotives operating on gradients.
Q14 | Passive torques always oppose the motion of the driven machine.
  • true
  • false
Answer: true
Explaination:
passive torques are due to friction or shear and deformation in elastic bodies. they always oppose the motion, restricting the motion of the machine.
Q15 | Among the following which one exhibits linearly rising load torque characteristics?
  • elevators
  • rolling mills
  • fan load
  • separately excited dc generator connected to the resistive load
Answer: separately excited dc generator connected to the resistive load
Explaination:
separately excited dc generator connected to the resistive load is an example of linearly rising load torque characteristics as the torque increases linearly with an increase in speed.
Q16 | What is the condition for the steady-state operation of the motor?
  • load torque > motor torque
  • load torque <<<< motor torque
  • load torque = motor torque
  • load torque < motor torque
Answer: load torque = motor torque
Explaination:
according to the dynamic equation of motor, load torque must be equal to motor torque so that motor should run at a uniform speed. if load torque is greater than motor torque, the motor will fail to start and if load torque is less than motor torque, the motor will run at a higher speed which can damage the shaft of the motor.
Q17 | Choose the correct one. (* stands for multiplication, J represents the moment of inertia, w represents angular speed).
  • j*d(w)/dt = load torque – motor torque
  • j*d(w)/dt = load torque + motor torque
  • j*d(w)/dt = motor torque – load torque
  • j*d(w)/dt = load torque * motor torque
Answer: j*d(w)/dt = motor torque – load torque
Explaination:
j*d(w)/dt = motor torque – load torque is the dynamic equation of the motor. motor torque will try to aid the motion of the motor, but load torque will oppose the motion of motor that’s why it subtracts in the equation.
Q18 | Which of the following is the best braking method?
  • friction
  • electromechanical action
  • eddy-currents
  • electric braking
Answer: electric braking
Explaination:
braking methods based on friction, electromechanical action, eddy- currents, etc. are independent of the motor but sometimes electric braking is better justified owing to its greater economy and absence of brake wear.
Q19 | DC motor is still widely used in tractions due to its excellent braking properties.
  • true
  • false
Answer: false
Explaination:
dc motor is used in tractions because of its excellent braking characteristics and ability of smooth transition from the motor to the generator
Q20 | Which of the following is not the method of electrical braking?
  • plugging or counter-current
  • dynamic or rheostatic
  • regenerative
  • eddy current
Answer: eddy current
Explaination:
eddy current is the electrical effect or response of the system, which is reflected mechanically at brakes to reduce the speed of the motor. thus, eddy current is not an electrical brake, it is mechanical one.
Q21 | Which of the following is the plugging method of braking?
  • reversal of field connections
  • reversal of armature connections
  • addition of equal and opposite field
  • removal of field circuit from current machine circuit
Answer: reversal of armature connections
Explaination:
plugging is method where
Q22 | Which of the following is correct formula for braking torque in plugging?
  • n (ka2/rb)
  • n2 (ka2/rb)
  • n-1 (ka2/rb)
  • (ka2/rb)
Answer: n (ka2/rb)
Explaination:
braking torque is equal to braking power divided by speed of the motor. t= [(nka )2/rb]/n. as, braking power is equal to ea2/rb/n. by solving for the braking torque from the above equation, we get n (ka2/rb).
Q23 | Electrical braking of any variety becomes less effective as  
  • speed increases
  • speed decreases
  • independent of speed
  • depends on supply voltage
Answer: speed decreases
Explaination:
braking torque of the dc machine is given by n (ka2/rb). here, braking torque is directly proportional to the speed of the motor, so as the speed decreases the efficiency of electrical brakes which is dependent on braking torque decreases.
Q24 | Plugging is applied in a motor, if we don’t make the switch OFF what will happen?
  • motor will come to rest as a result of plugging
  • motor will come to rest and will start rotating in another direction
  • motor will burn
  • nothing will happen
Answer: motor will come to rest and will start rotating in another direction
Explaination:
if the switch is kept on near to zero speed, motor will have braking torque acting in opposite direction greater than the electromechanical torque. thus, motor will come to rest and for the next instant motor will start rotating in opposite direction.
Q25 | Plugging is used in  
  • small motors only
  • small and medium powered
  • only in large heavy machines
  • everywhere
Answer: small motors only
Explaination:
plugging is used in small scale applications only. the large initial current and high mechanical stress restrict the application of plugging in large machines. so, in order to balance stress this method is used in small machines only.
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