ELE101/102 Dept of E&E,MIT Manipal 1

Tutorial

1. A 400V, 3phase supply feeds an unbalanced four-wire star

connected load. The branch impedances of the load are ZC =

600 Ω; ZB = 6300 Ω and ZA = 5450 Ω. Find the line

currents and the neutral current. Assume CBA system.

Ans: IC = 38.50°AIB = 38.5-150°A; IA =46.2-285°AIN = 30.656°A

ELE101/102 Dept of E&E,MIT Manipal 2

Tutorial

Ans:

104.8-120.35V

IR = 28.41-69°A

IY = 29.85136.54°A

IB =1327.60°A

2. A 400V, 3phase supply feeds an unbalanced three wire star

connected load. The branch impedances of the load are ZR =

(4+j8) Ω; ZY = (3+j4) Ω and ZB = (15+j20) Ω. Find the neutral

displacement voltage. Also find the line currents using (i)

Neutral displacement method. (ii) Mesh current analysis.

Assume RYB phase sequence.

ELE101/102 Dept of E&E,MIT Manipal 3

3. Three similar loads 20-450 Ω are connected in delta across a

balanced, 3 phase, 400 V ac supply. Determine (i) the line

currents (ii) active power (iii) apparent power (iv) power factor and

(v) the readings of the 2 wattmeters connected to measure the 3

phase power.

Tutorial

Ans:

34.6415°A, 34.64-105°A,34.64-225°A

16.97KW

24KW

0.707

3586 W and 13384 W

ELE101/102 Dept of E&E,MIT Manipal 4

4. Two wattmeters are connected to measure the power in a 3

phase circuit read 5 KW and 1 KW, the reading of second

wattmeter being obtained after reversing the potential coil of the

wattmeter. Calculate the power and power factor.

Ans:

4000 W

0.36

Tutorial

ELE101/102 Dept of E&E,MIT Manipal 5

5. Three loads ZR = 1000 Ω; ZY = 15-300 Ω and ZB = 20450 are

connected in star across a balanced, 3 phase, 400 V, RYB

supply. Determine the readings of the 2 wattmeters connected to

measure the 3 phase power, if the current coils of the wattmeters

are inserted in the lines R and Y.

Tutorial

Ans:

WR = 2907.2 W

WY = 7489.5 W