SSC JE 2014 Electrical question paper with solution(Morning-Shift)

SSC previous year solved electrical papers / March 25, 2022

Ques 51. The synchronous speed of a three-phase induction motor having 20 poles and connected to a 50 Hz source is

  1. 1200 rpm
  2. 300 rpm✔
  3. 600 rpm
  4. 1000 rpm

Synchronous speed of synchronous motor is given by

Ns = 120f/P

=120 x 50/20 = 300 RPM

 

Ques 52. A circuit with a resistor, inductor and capacitor in series is resonant of fo Hz. If all the component values
are now doubled the new resonant frequency is

  1. fo/4
  2. 2fo
  3. fo
  4. fo/2✔

The resonant frequency of LC is:

fo = 1/2π√LC

New resonant frequency

fnew = 1/2π√2L2C

fnew = f0/2

 

Ques 53. A 2 cm long coil has 10 turns and carries a current of 750 mA. the magnetising force of the coil is

  1. 375 AT/m✔
  2. 225 AT/m
  3. 675 AT/m
  4. 450 AT/m

For a coil of wire Magnetizing force (H)

= NI/l = 10 x 750 x 10-3/2 x 10-2

= 375 AT/m

 

Ques 54. A consumer has an annual consumption of 700800 units. If his maximum demand is 200 kW. The load factor will be

  1. 70%
  2. 20%
  3. 40%✔
  4. 50%

Percent load factor = Average consumption/Maximum demand

Average consumption = Annual consumption/Hour used per day x days used per year kW

Average consumption = 700800/24 x 365

= Load factor = 70800 x 100/24 x 365 x 200 = 40%

 

Ques 55. The rated voltage of a 3-phase power system is given as:

  1. Peak line to line voltage
  2. RMS phase voltage
  3. Peak Phase Voltage
  4. RMS line to line voltage✔

In AC system, we can’t take average value because average value is equal to zero over a period in AC.

So now to calculate different parameters like voltage, current, etc we need RMS value. This is the reason why we use RMS quantities while dealing with AC.

Now In a 3-phase supply, there exists 3 lines and a common neutral line. The voltage between two lines (for example ‘L1‘ and ‘L2‘) is called the line to line (or phase to phase) voltage.The line to line voltage is the vector sum of the phase to phase voltage across each winding. This is not the same as the arithmetic sum and is given by the following equation VLL = √3 VLN

 

Ques 56. For a half-wave rectified sine wave the ripple factor is:

  1. 1.00
  2. 1.65
  3. 1.45
  4. 1.21✔

Ripple Factor (r): It is the ratio of root mean square (RMS) value of AC component to the DC component in the output and is given by

Ripple factor

For half-wave rectifier

Irms = Im/2

Idc = Im/Ï€

For the half-wave rectifier, RMS Current is given as Im/2 and DC current Im/π which results in the ripple factor of 1.21.

 

Ques 57. Which one of the following bridges are generally used for measurement of frequency and also capacitance?

  1. Wein Bridge✔
  2. Hay’s Bridge
  3. Owen’s Bridge
  4. Schering Bridge

The Wien bridge is one of many common bridges.Wien’s bridge is used for precision measurement of capacitance in terms of resistance and frequency.It is also used to measure audio frequencies.

 

Ques 58. Two voltmeters of (0-300 V) range are connected in parallel to an A.C. circuit. One voltmeter is moving iron type reads 200 V. If the other is PMMC instrument, its reading will be

  1. 127. 4 V
  2. Slightly less than 200 V
  3. Zero✔
  4. 222 V

PMMC instrument is used only for DC current and voltage. For AC supply, no deflection will be there due to instantaneous opposite torques.

 

Ques 59. The least number of 1Φ wattmeters required to measure total power consumed by an unbalanced load fed from a 3Φ, 4 wire system is

  1. 4
  2. 1
  3. 2
  4. 3✔

According to Blondel, a French engineer, when power is supplied by N-wire AC system, the number of wattmeters required to measure power is one less than the number of wires, i.e(N- 1) regardless the load is balanced or unbalanced.

Hence for 3 phase 4 wire system number of wattmeter required is 3.

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