Exercise

EXERCISE

1. A balanced star connected load of 20+j 15 ohms per phase is connected in star across a 400V three phase supply. Calculate (i) the line current, (ii) the power factor, (iii) the total power in KW.

[Ans: 9.24 A, (ii) 0.8 (lag), (iii) 5.12 KW]

2. Three resistance of 252 each are connected in Delta across a 500V, 3-phase AC supply. Calculate (i) line and phase currents, (ii) Phase voltage, (iii) Power consumed.

[Ans: 34.64A, 20A, (ii) 500V, (iii) 50KW]

3. A balanced 3-phase star connected load of 150KW takes a leading current of 100A with line voltage of 1100 volts, 50 Hertz. Find the circuit constant of the load per phase.

[Ans: 50Ω, 812 Micro farads]

4. 3-phase, 400V, 50 Hertz supply has a delta connected load with

Z_AB = 15 ∠ 0° Ω, Z_BC = 25 ∠ 60° Ω, Z_CA = 10 ∠ 30° Ω. Determine phase and line currents and draw the phasor diagram. Take the phase sequence as ABC.

5. A balanced 3-phase system supplies an unbalanced 3-phase delta-connected load made up of two resistors 100 Ω and 200 Ω and a reactor having an inductance of 0.3H with negligible resistance. V_L = 100V at 50 HZ. Calculate (a) total power in the systems and (b) the total volt amperes reactive.

[Ans: 150 watts; 106 VAR (lag)]

6. A three phase, four wire symmetrical 440V; RYB system supplies a star-connected load in which ZR = 10∠0° Ω, ZY = 10∠26.8° Ω and ZB = 10∠-26.8° Ω. Find the line currents, the neutral current and the load power.

[Ans: Taking E_RN as reference I_R = 25.2∠0°; I_Y = 25.4∠-146.8°; I_B = 25.4 ∠ 146.8°, I_N = 17.1∠0°; Power = 17967.7W]

7. A star-connected unbalanced load consists of the following loads; ZA = 10 Ω, ZB = 10 +  j 10 Ω, Zc = ∠ j 20 Ω. Take the voltage as EAB = 212 ∠ 90°V, EBC = 212 ∠-30°V, ECA = 212 ∠- 150°V. Find the line currents and magnitudes.

[Ans: 13.65A; 6.2A; 7.54 A]

[Note: The neutral of the load is not connected to neutral of supply.]

8. If the readings on two wattmeters connected to measure power in a 3-phase circuits are 5KW and 0.5 KW, the latter being obtained after the reversal of current coil connections, calculate the power and power factor of the load.

[Ans: 4.5 KW; 0.427 (lag)]

9. The power input to a 400V, 30 induction motor is measured by means of two wattmeters with their current coils connected in lines R and Y and their corresponding pressure coils connected across R and B and Y and B respectively. The phase sequence is RYB. If the total power input to the motor is 40 KW at 0.76 power factor, find the readings of the two meters.

If one of the wattmeters has its current coil connected in line R and its pressure coil across lines Y and B, what does its reading indicate?

[Ans: 28.875 KW; 10.125 KW; 19.75 KVAR]