MCQs on Electric Circuits

01․ A 3 – φ delta connected symmetrical load consumes P Watts of power from a balanced supply. If the same load is connected in star to the same supply then what is the power consumption?

P_Δ = 3P_Y

02․ Condition for a balanced 3 - phase load is that, each phase has the same value of __________________.

The condition of a 3 - phase balanced are that each phase has the same value of impedance, resistance and power factor.

03․ A 3 wire, 3 - φ supply feeds a load consisting of three equal resistors. By how much is the load reduced if one of the resistors be removed when the load is in star?

If one resistor is removed from a 3 - φ circuit, then the circuit is no longer 3 - φ but converted into single phase circuit, having two resistor, each of R Ω connected in series across the supply of V_L Volts.

04․ Phase voltages of a star connected alternator are E_R = 240∠0^o V, E_Y = 240∠-120^o V and E_B = 240∠ + 120^o V. What is the phase sequence of the system?

Here E_R is reference voltage E_Y lags behind it by 120^o, E_B lags behind E_R by 240^o and E_Y by 120^o. So phase sequence is RYB.

05․ A balanced three Phase Y-connected load has one phase voltage V_B = 277∠45^o V. The phase sequence is RYB. The line to line voltage V_RY is

06․ In delta connected circuit when one resistor is open, then power will be

Power consumed in case of balanced delta connection P = V_ph2/R_eq. R_eq = 2R/3. So, P = 3V_ph2/2R. If one resistor is removed from a 3 - phase delta connected circuit, then the circuit will be converted into single phase circuit having two resistors in series. P’ = V_ph2/R_eq. R_eq = 2R. So, P’ = V_ph2/2R. P’ = P/3

07․ For balanced star load, if unbalanced line current are I_R, I_Y, I_B equal to - j10, - j10 and - j20 respectively, the neutral current will be equal to

All three current will be separated by 120_o in space.

08․ V_RN, V_YN and V_BN are three phase voltage of a balanced three phase system. If V_RN = 200∠30^o V and V_BN = 200∠- 90^o V, then phase sequence of the system is

Phase Y lags phase R by (30 + 90)^o = 120^o so the phase sequence RBY.

09․ Three identical inductors are connected in delta with a 220 V, 50 Hz, 3 - φ, supply. If the phase current is 24 A, then what is the value of each of the inductance?

10․ A balanced 3 - φ star connected load is fed from a 208 V, 3 - φ supply. Each load has resistance of 35 Ω. The total power is