Ques 51. The synchronous speed of a three-phase induction motor having 20 poles and connected to a 50 Hz source is
1200 rpm
300 rpm✔
600 rpm
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
fo/4
2fo
fo
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
375 AT/m✔
225 AT/m
675 AT/m
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
70%
20%
40%✔
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:
Peak line to line voltage
RMS phase voltage
Peak Phase Voltage
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:
1.00
1.65
1.45
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
For half-wave rectifier
Irms = Im/2
Idc = Im/π
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 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
127. 4 V
Slightly less than 200 V
Zero✔
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
4
1
2
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.
Thank u soo much sir.
Sir,
If you have uee material Or notes, please sent Me.
95 answer pls sir…..
sent me
Chetan I will explain you the question tomorrow morning currently i am uploading the solution of SSC JE 2009. Sorry for the delay
plz sir ll u send this p paper t me
Which paper @raway
sir ll u plz send previous ppr to me
Ques no 95 have no answer.
Answer would be option (B)
And Questions no 96 has no figure.
Please check and update it.