Que 31. Which of the following will decreasewhen the supply voltage of a D.C motor is increased?
Operating speed
Starting Torque
Full-Load Current
No load current
Answer 3.Full load current
Explanation:
As we know that T ∝ Ia
Therefore increase in load on the motor demands more torque from the machine, consecutively the motor requires more current for driving the load it is subjected to.
When there is no load on the motor, the motor takes less current for its operation since load torque is less armature torque is less.
The power output of the motor remains the same and thus the full load current is reduced when the voltage is increased to maintain constant power.
Que 32. The compensating winding in the conductively compensated DC series motor is provided as
Parallel with field winding
In series with armature winding
Parallel with armature winding
Series with field winding
Answer 3. In series with armature.
Explanation:
The cross-magnetizing effect of armature reaction may cause trouble in d.c. machines subjected to large fluctuations in load.
In order to neutralize the cross magnetizing effect of armature reaction, a compensating winding is used.
The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature. The series-connected compensating windings produce a magnetic field, which varies directly with armature current.
Therefore this induced emf developed in compensating winding acts opposite to armature emf and as a result commutation increases.
Que 33. Sparking of commutator at D.C motor result in
Increase in power consumption
Damage to commutator insulation
Damage of commutator segments
All of the above
Answer 4. All of the above
Explanation:
Sparking in DC motors is discouraged because of wear and tear on the commutator and brushes and increase power consumption.
Que 34. For highly explosive atmosphere which of the following motor is generally preferred?
Battery operated motor
Series motor
Air motor
Shunt motor
Answer 3. Air motor
Explanation:
A pneumatic motor (Air motor) or compressed air engine is a type of motor which does mechanical work by expanding compressed air.
Pneumatic motors generally convert compressed air energy to mechanical work through either linear or rotary motion.
Air motor generates less heat and can be used in more volatile atmospheres as they do not require electric power and do not create sparks.
Que 35. Which DC motor is preferred for crane and hoist?
Series motor
Cumulative compound motor
Shunt motor
Differentially compound motor
Answer 1. Series motor
Explanation:
Crane and hoist require high starting torque during starting because they carry heavy load.
Therefore series motor is suitable for such kind of machine as it provides high starting torque.
Que 36. What will happen if the 220 volts DC series motor is connected to 220 Volt Ac supply?
The motor will not run
The armature winding of the motor will burn
The motor will run with less efficiency and more sparking
Either 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 cycles will cancel out each other. D.C. motor will be heated up.
Que 37. If Ta be the armature torque and Ia be the armature current then which of the following relation is valid for DC series motor before saturation?
Ta ∝ Ia
Ta ∝ Ia2
Ta ∝ 1/Ia
Ta ∝ 1/Ia2
Answer 2. Ta ∝ Ia2
Explanation:
We know that torque is directly proportional to the product of armature current and field flux, Ta ∝ ɸ.Ia.
In DC series motors, the field winding is connected in series with the armature, i.e. Ia= If. Therefore, before the magnetic saturation of the field, flux ɸ is directly proportional to Ia.
Hence, before magnetic saturation Ta ∝ Ia2.
After magnetic saturation of the field poles, flux ɸ is independent of armature current Ia. Therefore, the torque varies proportionally to Ia only, T ∝ Ia.Therefore, after magnetic saturation, Ta-Ia curve becomes a straight line.
Que 38. The reason for providing field winding of DC motor with thick wire is
To provide large flux
To carry large load current
To reduce eddy current
To increase the flux
Answer 2. To carries large load current
Explanation:
The use of thick and thin wires in DC machines is to have appropriate resistance.
In DC shunt machine thin wire of larger number of turns is used
In the series machine, thick wires of smaller turns are used.
In the series machine, if you use thick wires the resistance of the wire will be lower(inversely proportional to the area), and hence larger current will flow as it is directly connected across the supply.
Que 39. In D.C motor the percentage of residual magnetism is
2 – 3%
10 -20%
50%
80 – 90%
Answer 1. 2 -3 %
Explanation:
Residual magnetism is the amount of magnetization left behind after an external magnetic field is switched off.
The amount of residual magnetization is about 2-3% in DC motor.
This residual magnetism helps in the starting of the motor and building up of speed.
Que 40. Why is the DC motor preferred over AC motor?
High-speed operation
Low-speed operation
Rated speed operation
Variable speed operation
Answer 4. Variable speed operation
Explanation:
DC motor provides Speed control over a wide range both above and below the rated speed.
The speed control can be achieved by the armature control method and field control method.
