Ircon electrical questions with answers
1. Select the application of fluid coupling fitting from the following:
(A) acts as a voltage limiter
(B) enables no-load start-up of prime-mover
(C) works on the principle of eddy current
(D) none of the above
2. Application of occupancy sensors is well suited for ___.
(A) day light based controllers
(B) night based controllers
(C) motor controllers
(D) movement or noise detector in room space
3. When a synchronous motor is running at synchronous speed, the damper winding
(A) damping torque.
(B) eddy current torque.
(C) torque aiding the developed torque.
(D) no torque.
4. If a transformer primary is energised from a square wave voltage source, its output voltage will be
(A) A square wave.
(B) A sine wave.
(C) A triangular wave.
(D) A pulse wave.
5. The most suitable servomotor for low power applications is
(A) a dc series motor.
(B) a dc shunt motor.
(C) an ac two-phase induction motor.
(D) an ac series motor.
6. A balanced three-phase, 50 Hz voltage is applied to a 3 phase, 4 pole, induction motor. When the motor is delivering rated output, the slip is found to be 0.05. The speed of the rotor m.m.f. relative to the rotor structure is
(A) 1500 r.p.m.
(B) 1425 r.p.m.
(C) 25 r.p.m.
(D) 75 r.p.m.
NS = 120f /P = 120 x 50 /4 =1500rpm
N = NS ( 1-s) = 1500 (1-0.05) = 1425
\relative speed = 1500 – 1425 = 75 rpm
7. In a 3-phase synchronous motor
(A) the speed of stator MMF is always more than that of rotor MMF.
(B) the speed of stator MMF is always less than that of rotor MMF.
(C) the speed of stator MMF is synchronous speed while that of rotor MMF is zero.
(D) rotor and stator MMF are stationary with respect to each other.
8. The synchronous reactance of the synchronous machine is ______________.
(A) Ratio between open circuit voltage and short circuit current at constant field current
(B) Ratio between short circuit voltage and open circuit current at constant field current
(C) Ratio between open circuit voltage and short circuit current at different field current
(D) Ratio between short circuit voltage and open circuit current at different field current
9. In case of a universal motor, torque pulsation is minimized by _________.
(A) load inertia
(B) rotor inertia
(C) both rotor and load inertia
(D) none of the above
10. Inverse definite minimum time lag relay is also called ___________
(A) pilot relay.
(B) differential relay.
(C) over current relay.
(D) directional overcurrent relay.
11. The direction of rotation of hysteresis motor is reversed by
(A) Shift shaded pole with respect to main pole
(B) Reversing supply lead
(C) Either A or B
(D) Neither A nor B
12. The short circuit test on a transformer is conducted to obtain
(A). copper losses.
(B). core loses only.
(C). eddy current loss
(D). hysteresis loss
13. CRGO Steel is used in the core of transformer to reduce
(A). Eddy Current loss
(B). Ohmic loss
(C). both Eddy Current and Hysteresis loss
(D). Hysteresis loss
14. Two resistances R1 and R2 give combined resistances 4.5Ω and 1Ω when they are connected in series and parallel respectively. What would be the values of these resistances ?
(A). 3Ω and 6Ω
(B). 1.5Ω and 3Ω
(C). 3Ω and 9Ω
(D). 6Ω and 9Ω
Ex: Here, R1 + R2 = 4.5 ..................(1)
and R1.R2/(R1 + R2) = 1
=R1.R2 = 4.5..................(2)
Combining (1) & (2) we get R1 = 1.5 or 3 and R2 = 3 or 1.5
15. Two equal resistors R connected in series across a voltage source V dissipate power P. What would be the power dissipated in the same resistors when they are connected in parallel across the same voltage source ?
Ex: Say R is the resistance of the identical two resistors. When they are connected in series across a voltage source V, then total power dissipated by the resistors will be P = V2/2R. When they are connected in parallel across the same voltage source V, then total power dissipated by the resistors will be V2/R/2 = 4V2/2R = 4P.
16. The phase angle difference between current and voltage is 90o, the power will be
Ex: The expression of active power P = V.I.cosθ.
Where, V is voltage, I is current and θ is the angle between current and voltage.
here, this θ = 90o
Therefore Power=V.I.Cos 900 = 0[Since, cos 900=0]
17. In a synchronous generator in order to eliminate the fifth harmonic the chording angle should be
18. A hunting sound is produced in a synchronous motor when
(A) load fluctuates
(B) supply frequency varies
(C) both (A) and (B) above
(D) none of the above.
19. Stampings in transformers are provided to reduce
(A) hysteresis loss
(B) eddy current loss
(C) copper loss
(D) all of the above.
20. By adding silicon to ferromagnetic , materials
(A) electrical resistivity increases and also magnetic permeability increases
(B) electrical resistivity decreases and also magnetic permeability increases
(C) electrical resistivity decreases and magnetic permeability increases
(D) electrical resistivity increases and magnetic permeability decreases.