SSC JE 2010 Electrical question paper with Solution

Nr = 750 RPM

s = 0.04

Since Nr = Ns(1 – s)

= 750 = Ns(1 – 0.04)

Ns = 781.25 RPM

Now synchronous speed of motor

Ns = 120f/P

781.25 = 120f/4

f = 26.04 Hz

Positive feedback is used to have more gain. For oscillator, the gain should be at maximum to have sustained oscillations 

In a magnetic circuit, permeance is a measure of the quantity of magnetic flux for a number of current-turns. A magnetic circuit almost acts as though the flux is ‘conducted’, therefore permeance is larger for large cross-sections of a material and smaller for longer lengths. This concept is analogous to electrical conductance in the electric circuit.

Resistance of a wire

R = ρl/A …………..1

where ρ is resistivity of the material

L is length in meters

A is cross-sectional area in m²

R’ = ρl’/A’……………..2

Now length of wire is stretched to 1/10 of its original length

l’ = l + l/10 = 11l/10

l’/l = 11/10

Since  On stretching, Volume will be constant

lA = l’A’

A’/A = l/l’

from equation 1 & 2

R/R’ = lA’/l’A

R’ = R x 121/100

R’  = 1.21 x R = 1.21 x 10 = 12.1Ω

Voltage transformers (VT), also called potential transformers (PT), are a parallel connected type of instrument transformer. They are used in the electrical power system for stepping down the system voltage to a safe value which can be fed to low ratings meters and relays.

The currents on the two sides of a 3-phase transformer in general differ in both magnitude and phase. The CT connection must compensate for these differences, as a percentage relay must compare currents which, under normal conditions, are identical (in magnitude and phase). In star/star or delta/delta transformers, the currents on the two sides have the same phase angle, the three CTs on both sides are, therefore, identically connected (star or delta) and an appropriate choice of CT ratios would be sufficient for magnitude equalization.

In case of a star/delta transformer, the currents on the two sides also differ in phase by 30°. This is compensated by connecting CTs in Delta on the star side, and in star on the delta side of the transformer

The voltage between the Base and Emitter ( V_BE ), is positive at the Base and negative at the Emitter because for an NPN transistor, the Base terminal is always positive with respect to the Emitter.

In electrical braking method, the stored energy of rotating part is converted into electrical energy and dissipated by the resistance in the form of heat or returned to the supply.

Advantages of Electrical Braking

• Electric braking is fast and cheap.
• Mechanical Braking due to excessive wear on the braking blocks or braking lining needs frequent and costly replacement and also due to wear and tear increase the maintenance cost. In electric braking, there is no maintenance cost like replaced brake shoes periodically.
• In electric braking negligible amount of heat is generated.
• In some cases of electrical braking, we can return about 60 to 80% of the kinetic and potential energy of the apparatus back to the line which is not possible with mechanical braking

In order for an induction motor to be rotated, there should be rotating magnetic. In case of single phase motor during the positive half cycle, the rotor will rotate in one direction and in the negative half cycle, it rotates opposite to that. So the net torque developed is zero and hence single phase induction motor is not self-starting.

In order to self-start a single phase induction motor one must create a rotating magnetic field on the stator. Hence, the single phase motor needs an auxiliary winding which produces two magnetic fields displaced in time which provides a magnetic field and used to create the torque needed to start a single phase induction motor

To increase the range of a voltmeter, the high-value resistance should be connected in series

The logic is that, if we increase the resistance, less current will flow as the voltmeter is connected in parallel to the supply voltage. This results in the decrease in the signal current for the voltmeter to process. [For analog voltmeter, the decrease in current leads to less deflection of the needle.] Thus, by re-calibrating the scale, the range of voltmeter is increased.