Babu Banarasi Das National Institute of Technology and Management,Lucknow
Tutorial Sheet 3
Electrical Measuring Instruments
1. What do you mean by electrical measuring instruments? Describe its various types.
Explain i)indicating instruments ii) integrating instruments iii) Recording instruments with examples
2. Explain absolute and secondary instruments in detail.
3. What is PMMC (Permanent Magnet Moving Coil Instruments).explain its principle of operation, working, construction with necessary figure and expression in detail? Write advantages, disadvantages and applications of PMMC instruments.
4. What are moving iron ammeters and voltmeters instruments? What are its types?
Explain in detail attraction and repulsion type moving iron instruments with diagram.
5. What are the sources of errors in moving iron instruments? Explain characteristics of moving iron type instruments.
6. Explain single phase dynamometer wattmeter in detail with diagram. Also illustrate its principle of operation and working.
7. Write advantages, disadvantages, sources of errors and applications in case of single phase dynamometer wattmeter.

Three phase a.c. Circuits
1. For star connection prove that i) V
L
=1.73V
Ph
and ii) I
L
=I
Ph
with phasor diagram.
2. For delta connection prove thati) V
L
=V
Ph
& ii) I
L
=1.73I
Ph
with phasor diagram.
3. Explain i)3-Φ balanced supply ii) 3-Φ balanced load iii) 3-Φ unbalanced load
4. Derive expressions for power in 3-phase star and delta connections. Prove that i) neutral wire has a zero resultant current ii) sum of the three emfs at every instant is zero in 3-Φ system
5. Explain all the methods to measure power in 3-phase circuits.
6. Explain two wattmeter method to measure the 3-phase power in star connected load.
Derive the expression for phase angle in terms of wattmeter reading. Also give expression for power factor.
7. Three similar coils, connected in star, take a total power of 1.5KW at p.f. of 0.2 lagging from a 3Φ,400V,50Hz supply. Calculate the resistance and inductance of each coil.[ans:
Resistance and inductance of coil per phase are 4.27Ω and 0.0665Henry respectively.]
8. Three coils, each having a resistance of 20Ω and an inductive reactance of 15Ω, are connected in star to a 400, 3-Φ,5oHz supply. Calculate i) the line current ii) power factor iii) power supplied.[ans:I
L
=9.24A,P.f.=0.8 lag & Power=5121W]
9. A balanced 3-Φ,delta connected load has per phase impedance of (25+40j)Ω.if
400V, 3-
Φ supply is connected to this load, find i) phase current ii) line current iii) power supplied to the load.[ans:I
Ph
=8.48A,I
L
=14.7A & P=5397.76 watt]
10. In a 2 wattmeter method power measured was 30KW at 0.7 p.f. lagging. Find the reading of each wattmeter.[ans: 6165W & 23835W]
11. Three equal impedances, each consisting of R & L in series are connected in star and are supplied from a 400V, 50Hz, 3-Φ, 3-wire balanced supply mains. The input power to the load is measured by two wattmeters method and the two wattmeters read 3KW &
1KW.determine the values of R and L connected in each phase [ans:22.85Ω and 0.063
Henry]