Electrical Drives And Controls Set 3

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 3

Q1 | Which of the following is dynamic braking?
  • reversal of field connections
  • reversal of armature connections
  • addition of equal and opposite field
  • removal of armature circuit from current machine circuit
Answer: removal of armature circuit from current machine circuit
Explaination:
reversal of the connections of armature is the method called plugging. in dynamic braking we remove the armature circuit and connect it to different resistor, with field circuit still connected to the external supply.
Q2 | Braking time in the dynamic braking is the function of  
  • system inertia
  • load torque
  • motor rating
  • all- system inertia, load torque and motor rating
Answer: all- system inertia, load torque and motor rating
Explaination:
in dynamic braking, when
Q3 | In dynamic braking, when braking is applied system acts as  
  • freely running machine
  • motor with slow speed
  • generator
  • motor with same speed in opposite direction
Answer: generator
Explaination:
the armature is disconnected from the supply and then a braking resistor rb is immediately connected across it. the motor acts as a generator, driven by the inertia and stored kinetic energy dissipating power in rb. this is a simple method of bringing a motor nearly to a standstill.
Q4 | In which of the following electrical braking method, energy is supplied back to the supply?
  • plugging
  • dynamic braking
  • regenerative braking
  • in all electrical braking
Answer: regenerative braking
Explaination:
in plugging energy is wasted in braking resistance which is equal to starting resistance while running as a motor. in dynamic braking energy is generated but it is not fed back to supply. in regenerative method energy is sent back for reuse.
Q5 | connected across AC supply?
  • will run at normal speed
  • will not run
  • will run at lower speed
  • burn due to heat produced in the field winding
Answer: burn due to heat produced in the field winding
Explaination:
in case of parallel field connection, it won’t rotate at all and will start humming and will create vibrations, as a torque produced by positive and negative cycle will cancel out each other. dc motor will be heated up and it may burn.
Q6 | What will happen if the back emf of a DC motor vanishes suddenly?
  • the motor will stop
  • the motor will continue to run
  • the armature may burn
  • the motor will run noisy
Answer: the armature may burn
Explaination:
if back emf vanishes suddenly, motor circuit will try to retain back 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.
Q7 | What will happen, with the increase in speed of a DC motor?
  • back emf increase but line current falls.
  • back emf falls and line current increase.
  • both back emf as well as line current increase.
  • both back emf as well as line current fall.
Answer: back emf increase but line current falls.
Explaination:
in case of dc motor, the speed
Q8 | is DC motor and not an AC type?
  • winding
  • shaft
  • commutator
  • stator
Answer: commutator
Explaination:
all other parts except brushes and commutator are same in ac machine when outer looks are only taken in consideration. commutator is used only in dc machine for providing mechanical rectification and not in ac machine.
Q9 | can sustain the maximum temperature rise?
  • field winding
  • commutator
  • slip rings
  • armature winding
Answer: field winding
Explaination:
maximum temperature rise can be sustained by field winding, as it is not involved in rotary parts. field winding is present away from rotary parts of the machine, so temperature rise in the machine will not produce any effect in machine rotations.
Q10 | Direction of rotation of motor is determined by  
  • faraday’s law
  • lenz’s law
  • coulomb’s law
  • fleming’s left-hand rule
Answer: fleming’s left-hand rule
Explaination:
flemings laws can be summarized as whenever, a current carrying conductor comes under a magnetic field, there will be a force acting on the conductor and on the other hand, if a conductor is forcefully brought under a magnetic field, there will be an induced current in that conductor.
Q11 | of a motor?
  • gross power
  • power drawn in kva
  • power drawn in kw
  • output power available at the shaft
Answer: output power available at the shaft
Explaination:
name plate of the motor shows rated values i.e. rated speed, rated current, rated voltage. it also shows output power available at shaft when all other quantities are set to rated values.
Q12 | An electric motor is having constant output power. So, motor will have a torque speed characteristic  
  • circle about the origin.
  • straight line parallel to the speed axis.
  • straight line through the origin.
  • rectangular hyperbola
Answer: rectangular hyperbola
Explaination:
in case of dc motor for the constant output power, ea×ia = t×ω. as t×ω
Q13 | In which of the following case we will get maximum power?
  • ea = 2 x supply voltage
  • ea = supply voltage
  • supply voltage = 2 x ea
  • supply voltage = 4 x ea
Answer: supply voltage = 2 x ea
Explaination:
for a motor, from power equation it is known that,
Q14 | Sometimes motor has to be de-rated.
  • true
  • false
Answer: true
Explaination:
derating refers to the operation of equipment at reduced capacity/power or speed. derating in motors can be caused due to the following reasons- frequency, voltage, ambient temperature, altitude.
Q15 | able to withstand  
  • bending moment due to weight of the armature.
  • any unbalanced magnetic pull on the armature core.
  • twisting stains due to transmission of torque.
  • bending moment, unbalanced magnetic pull and twisting stains
Answer: bending moment, unbalanced magnetic pull and twisting stains
Explaination:
the armature shaft must be able to withstand any unbalanced magnetic pull on the armature core, bending moment due to weight of the armature and commutator, twisting stains due to transmission of torque, for a good and long run application of motor.
Q16 | present. The order of residual magnetism is
  • 2 to 3 per cent
  • 10 to 15 per cent
  • 20 to 25 per cent
  • 50 to 75 per cent
Answer: 2 to 3 per cent
Explaination:
in a dc machine residual magnetism is present and it plays very important role in starting of any dc machine. it’s present because of the previous application on the same motor, it also provides some no-load voltage.
Q17 | Sparking is discouraged in a DC motor.
  • true
  • false
Answer: false
Explaination:
sparking at brushes and commutator segments lead to damage of commutators, which is the main distinguishable component in a dc machine. so, it is advisable to fasten the commutation speed and avoid sparking.
Q18 | construction of single phase induction motor?
  • 3
  • 4
  • 5
  • 6
Answer: 5
Explaination:
there are 5 steps in the construction of the single phase induction motor. they are stator, stator windings, rotor, starting switches, electrolytic capacitor.
Q19 | What is the lamination used for the stator?
  • cast iron
  • die cast aluminium alloy frame
  • cast iron or die cast aluminium alloy frame
  • cast iron and die cast aluminium alloy frame
Answer: cast iron or die cast aluminium alloy frame
Explaination:
the stator is made up of a block of laminations. the block of laminations are made up of cast iron or die cast aluminium alloy frame.
Q20 | What type of coils are used for winding the single phase induction motor generally?
  • rectangular coils
  • square coils
  • cruciform coils
  • circular coils
Answer: circular coils
Explaination:
the slots house the starting and running windings. the single phase induction motors are generally wound with concentric coils.
Q21 | How many kinds of single phase windings are present?
  • 2
  • 3
  • 4
  • 5
Answer: 3
Explaination:
there are basically 3 kinds of single phase windings. they are concentric, progressive and skein.
Q22 | How are the poles and pitches in the concentric windings?
  • single pole, different pitches
  • different pole, different pitches
  • different pole, single pitch
  • single pole, single pitch
Answer: single pole, different pitches
Explaination:
the concentric windings have a single pole for a common centre. they have different pitches for each individual coil.
Q23 | What is the form of the progressive windings?
  • double layer diamond coil windings
  • single layer diamond coil windings
  • multi layer diamond coil windings
  • three layer diamond coil windings
Answer: single layer diamond coil windings
Explaination:
the progressive windings is one kind of the stator windings. they are in the form of the single layer diamond coil windings.
Q24 | When is the skein winding made use of?
  • when small amount of relatively small size wire is used
  • when large amount of relatively small size wire is used
  • when large amount of relatively large size wire is used
  • when small amount of relatively large size wire is used
Answer: when small amount of relatively small size wire is used
Explaination:
skein winding is one of the 3 kinds of single phase windings used. it is used when small amount of relatively small size wire is used.
Q25 | winding made use of?
  • maximum horse power single phase induction motor
  • fractional horse power single phase induction motor
  • minimum horse power single phase induction motor
  • zero horse power single phase induction motor
Answer: fractional horse power single phase induction motor
Explaination:
the skein winding is one of the 3 kinds of single phase induction motor. the skein winding is used when fractional horse power single phase induction motor is used.
Previous Next