Mechanical braking Friction braking or Magnetic Braking
In this type of braking, the motor is stopped by using a brake shoe or band on a brake drum. In friction braking, it is difficult to get the smooth stop of the motor and It requires frequent maintenance and replacement of brake shoe, and braking power is was as well as it dissipates more heat energy.
Ques 52. When the current through the coil of an electromagnet is reversed, the
Magnetic field collapse
The direction of the magnetic field reversed✔
The direction of the magnetic field remains unchanged
Magnetic field expands
For the DC current (Direct current), when you change the direction of the current in an electromagnet, the North and South poles of the elec-magnet will be reversed.
With the AC current, it constantly (continuously) changes its current direction, hence the North and the South poles will change continuously, too. That is an application for the motor.
Ques 53. An arc blow is a welding defect that is countered with the help of carrying?
The resistance welding
The arc welding using DC supply
The thermit Welding
The arc welding using AC supply✔
Arc Blow
Arc blow is the, usually unwanted, deflection of the arc during arc welding. The welding current flowing through a plate or any residual magnetic fields in the plate will result in unevenly spaced magnetic flux lines. These uneven flux lines can, in turn, cause the arc between the electrode and the work to move during welding.
Controlling the Arc Blow
The best method to eliminate arc blow is to use alternating current since alternating current changes directions, the flux lines don’t become strong enough to bend the arc before the current changes direction.
Ques 54. The per-phase D.C armature resistance of an alternator is 0.5 ohm. The effective AC armature resistance would be about?
0.25 ohms
0.5 ohms
1 ohms
0.75 ohms✔
The AC resistance per phase is obtained by multiplying the dc resistance by the factor that varies from 1.2 to about 1.8, depending on the frequency, quality insulation, etc. For purposes here, we shall use a factor of 1.5 in computing the effective AC armature resistance per phase.
Rac = 1.5 Rdc = 1.5 x 0.5
Rac = 0.75Ω
Ques 55. The superposition theorem is used when the circuits contain
A single voltage source
Passive elements only
Active elements only
A number of voltage sources✔
The superposition theorem for electrical circuits states that for a linear system the response (voltage or current) in any branch of a bilateral linear circuit having more than one independent source equals the algebraic sum of the responses caused by each independent source acting alone, where all the other independent sources are replaced by their internal impedances.
Ques 56. One sine wave has periods of 2 ms, another has a period of 10 ms. Which sine wave is changing at a faster rate?
All are at the same rate
Sine wave with period 2 ms✔
Sine wave with a period of 10 msec
sine wave with the period of 5 ms
Frequency is inversely proportional to time period, therefore, sin wave of 2ms will change at the faster rate as compared to the sine wave of 10 ms.
Ques 57. Humans are more vulnerable to Electric shock current at?
50 Hz
48 Hz
45 Hz
40 Hz✔
Lower frequency AC is more dangerous than higher frequency AC because less frequency means less inductive reactance so high current.
Ques 58. An active element in the circuit is one
Dissipates energy
Receives Energy
Both receives and supplies
Supplies Energy✔
The active elements generate energy. Batteries, generators, operational amplifiers etc are active elements.
Ques 59. The permissible voltage drop from the supply terminal to any point on the wiring system should not exceed?
2 % ± 1 volt
1% ± 1 volt
4% ± 1 volt✔
3% ± 1 volt
The permissible voltage drop from the supply terminal to any point on the wiring system should not exceed 4% ± 1 volt of the nominal supply voltage i.e for 230 V AC supply the permissible voltage drop is about 9.2 V.
Ques 60. The reactive power generated by a synchronous alternator can be controlled by?
Changing the alternator speed
Changing the field Excitation✔
Changing the terminal Voltage
Changing the prime mover input
If alternator is Overexcited, it will deliver reactive power with lagging current while in Underexcited, it absorb reactive power with leading current
