Ques 31. Two coils have an inductance of 8 mH and 18 mH and a coefficient of coupling of 0.5. If the two coils are connected in series aiding, the total inductance will be
32 mH
38 mH
40 mH
48 mH
Answer.2. 38 mH
Explanation:
Total inductance is given as
Leq = L1 + L2 + 2M……………..1
Where M = K√L1L2
= 0.5√8 x 18
= 6
Putting the value of M in equation number 1
= 8 + 18 + 2 x 6
= 8 + 18 + 12
= 38 mH
Ques 32. Which material is preferred in permanent magnets?
Cast Iron
Steel
Pure Iron
None of these
Answer.2. Steel
Explanation:
The material which we choose for making magnets should have high coercivity. The high coercivity ensures that magnetism in the material stays for a longer time. As steel and alnico have high coercivity, therefore, they are used for making permanent magnets.
Ques 33. As per Faraday’s Law of electromagnetic induction, an e.m.f is induced in a conductor whenever it
Lies perpendicular to the magnetic flux
Lies in a magnetic field
Cuts magnetic field
Move parallel to the direction of the magnetic field
Answer.3. Cuts magnetic field
Explanation:
Faraday’s Law of electromagnetic induction states that an emf s induced in the conductor whenever it cuts across the lines of force of a magnetic field.
Ques 34. Electrolyte capacitor has _________ tolerance Value.
Wide
Low
Zero
None of these
Answer.1. Wide
Explanation:
Electrolytic capacitors have a very wide tolerance. Often capacitors may be quoted as -20% and +80%. This is not normally a problem in applications such as decoupling or power supply smoothing, etc
Ques 35. In order to minimize loss due to hysteresis, the magnetic material should have
High resistivity
Low Hysteresis co-efficient
Smaller B-H loop area
High retentivity
Answer.3. Smaller B-H loop area
Explanation:
Hysteresis loss is associated with the cyclic magnetization and demagnetization of the material as the magnetic field changes with the sinusoidally varying currents.
The hysteresis loss is proportional to the area of the hysteresis loop and therefore in order to reduce the loss the material of the core should have a smaller hysteresis loop area.
Ques 36. Closed-looped T.F is K/ S2(1 + kTs), the system will be?
Stable
Unstable
Marginally stable
Conditionally stable
Answer.3. Unstable
Ques 37. Silicon steel is used in electric machines because it has
Low coercitivity
Low Retentivity
Low hysteresis loss
High coercitivity
Answer.3. Low Hysteresis loss
Explanation:
Electrical steel or silicon sheet steel is used in transformers because of its high permeability and low hysteresis losses. Conventional steels would overheat and make an inefficient transformer.
Ques 38. For 30 V, 5-ohm resistance if the current flowing is 6A, what will be the state of the fuse if resistance decreases.
Open
Short
Burts
No effect
Answer.1. Open
Explanation:
According to Ohm’s law if resistance decrease, while voltage is held at a constant value then the current flow in the circuit will increase. This increase in the current may cause the fuse to trip because the combined current in the parallel device has exceeded the fuse safety limit. Therefore the fuse gets open.
Ques 39. If the capacitance is short-circuited what will happen to single phase induction motor
It will start and stop
Won’t start
No effect
Better start
Answer.2. Won’t Start
Explanation:
When the capacitor is short-circuited high inrush current will flow in the winding which may burn the starting winding of a single-phase induction motor. Therefore the motor will not start at all.
Ques 40. Reciprocal of reluctance is
Reluctivity
Permeance
Permeability
Susceptibility
Answer.2. Permeance
Explanation:
Reluctance is the measure of the opposition offered by a magnetic circuit to the setting up the flux and the reciprocal of reluctance is known as permeance.
