Que 21. According to Fleming’s left-hand rule if the forefinger points in the direction of the field then the middle finger will point in the direction of
Current in the conductor
Resultant force on the conductor
Movement of the conductor
None of the above
Answer 1. Current in the conductor
Explanation:
When a current-carrying conductor such as a wire attached to a circuit moves placed in a magnetic field, an electric current is induced in the wire due to Faraday’s law of induction.
The right hand is held with the thumb, first finger and second finger mutually perpendicular to each other than
The thumb is pointed in the direction of the motion of the conductor relative to the magnetic field i.e direction of the force.
The forefinger is pointed in the direction of the magnetic field.
The middle finger represents the direction of the induced or generated current within the conductor.
Que 22. Which DC motor is preferred for Elevator?
Differentially compound motor
Series motor
Shunt Motor
Cumulative compound motor
Answer 4. Cumulative compound motor
Explanation:
Compound motor finds applications where high starting torque or pulsating load are required.
In the elevator, the load is always pulsating and high starting torque is required to run the elevator.
This high starting torque is provided by the series winding of the compound motor and maximum speed at no load is limited by shunt winding of the compound motor.
Que 23. Which of the following application requires high starting torque?
Air blower
Elevator
Locomotive
Centrifugal Pump
Answer 3. Locomotive
Explanation:
To start up the locomotive we required very high starting torque.
DC series motor is used as the starting torque is about 500%
Que 24. Which DC motor is preferred for constant speed?
Compound motor
Series motor
Shunt motor
Differential motor
Answer 3. Shunt motor
Explanation:
In the case of DC motors Speed ∝ Back emf(Eb)/flux(φ)
The field winding in the shunt motor is connected in parallel to the armature winding and the supply.
The field winding is connected across the armature. The resistance offered by the field is constant so the current is constant.
As current is constant flux also constant because the flux is directly proportional to current.since flux is constant so the speed is also constant because speed depends on flux.
Hence the speed of the shunt motor can be assumed ALMOST constant under normal running conditions.
Que 25. The direction of rotation of D.C series motor can be changed by
Interchanging field terminal
Interchanging supply terminal
Both a & b
None of the above
Answer 1. Interchanging Field terminal
Explanation:
If field winding is reversed then the current flowing through the field winding reverses so the effective flux will be reversed. we know that Torque is proportional to flux.
As there is a reversal of flux i.e. from positive it becomes negative so torque generated also becomes negative resulting motor rotating in reverse direction.
Note: To reverse the motor, you need to change the polarity of the supply voltage to either the field winding or the armature winding, but not both.
Que 26. Which DC motor is used for the conveyor?
Series motor
Cumulative compound motor
Differentially compound motor
Shunt motor
Answer 1. Series motor
Explanation:
Conveyors require high starting torque because the material is placed before the starting of the Conveyors belt.
Therefore conveyor required high starting torque and constant speed.
Series motor is suitable for conveyor belts operating in a particular region of its torque-speed characteristics.
Que 27. What will happenwhen the D.C motor is connected to A.C supply then?
D.C motor will run at rated speed
D.C motor will burn
D.C motor will run at slow speed
Both 2 & 3.
Answer 4. Either 2 & 3
Explanation:
Series Connection:
In DC machine, T = kφi
where
k = number of poles and the number of turns in the winding. Φ = flux per pole i = current.
If saturation of core is neglected, then Flux is proportional to field current.
So Torque= k *field current*armature current.
For DC series machine, Armature current = Field Current,
Therefore, Torque = k × Ia2
If we apply AC then both, during positive as well as negative half cycle, torque is positive from above relation. So, torque is unidirectional, but due to AC, it will be pulsating.
Parallel connection:
But, In the case of parallel 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. D.C. motor will be heated up.
Que 28. Which D.C motor is preferred for machine tools?
Series motor
Cumulative compound motor
Differentially compound motor
Shunt motor
Answer 4 . Shunt Machine
Explanation:
DC shunt motor is a constant speed motor tool machine such as lathe machine requires being run at a constant speed.
As lathe machines roll down iron pieces reducing Diameters at a constant speed. and also for other purposes, it requires a constant speed motor.
Que 29. When the speed of D.C 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
Answer 1. Back E.M.F increases and current drawn decreases
Explanation:
The Emf equation of DC motor is given as
As, A, P, Z are constant therefore N is directly proportional to Eb & inversely proportional to flux.
As we know that in DC motor armature equation is given as
So As the back emf increase armature current reduced.
Que 30. The starting resistance of D.C motor is generally
High
Infinite
Low
None of the above
Answer 3. Low
Explanation:
The armature resistance of a motor is very small generally less than one ohm.
At the time of starting of DC motor, starting resistance is only the resistance of armature.
Therefore we use a starter to start a DC motor otherwise the starting current will be too high which may burn the motor.
