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Dc machines for Mechanical engineering students, Exercises of Electrical Engineering

University of Cebu (UC)Electrical Engineering

EE hand notes for Mechanical engineering subject

Typology: Exercises

2019/2020

Uploaded on 01/22/2020

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CENTRAL COLLEGE OF ENGINEERING AND MANAGEMENT
SUBJECT: ELEMENTS OF ELECTRICAL ENGINEERING
QUESTION BANK
UNIT-1(DC NETWORKS)
1 .Define Thevenin’s Theorem. List the steps for obtaining Thevenin’s equivalent circuit.
2 .Define and explain Norton’s theorem.
3 .Define kirchoff’s Law.
4 .Use nodal analysis method to find current in the various resistors of the circuit shown in fig.
5 . Give the comparison between magnetic and electric circuit?
6 . Write down the statement of Thevenin's theorem Also write the various steps to find the Thevenin's
equivalent?
7. State Kirchhoff’s current law and Kirchhoff’s voltage law.
8. Define Hysteresis and Eddy current losses. How these losses are reduced, practically in electric
machines?
9. State Norton theorem?
10. State the superpositiontheorem?
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CENTRAL COLLEGE OF ENGINEERING AND MANAGEMENT

SUBJECT: ELEMENTS OF ELECTRICAL ENGINEERING

QUESTION BANK UNIT-1(DC NETWORKS)

1 .Define Thevenin’s Theorem. List the steps for obtaining Thevenin’s equivalent circuit.

2 .Define and explain Norton’s theorem.

3 .Define kirchoff’s Law.

4 .Use nodal analysis method to find current in the various resistors of the circuit shown in fig.

5. Give the comparison between magnetic and electric circuit?

6. Write down the statement of Thevenin's theorem Also write the various steps to find the Thevenin's

equivalent?

7. State Kirchhoff’s current law and Kirchhoff’s voltage law.

8. Define Hysteresis and Eddy current losses. How these losses are reduced, practically in electric

machines?

9. State Norton theorem?

10. State the superpositiontheorem?

  1. Use nodal method to find current in branch a b of circuit shown in Fig. All resistance in ohms.
  2. For the network shown below, draw a Norton’s equivalent circuit and Determine the current flowing through 18.75ohms resister.
  3. Find the current in different branches of the network shown in Fig.Using superposition theorem.
  4. The Circuit Shown in Fig. contains two voltage sources and two current sources. Calculate (i) V th (ii) R th Betweenthe open terminals A & B of the circuit.

And (ii) are if a material having magnetic characteristics giver by the Following table: H (AT/m) 100 210 340 500 800 1500 B (Tesla) 0.2 0.4 0.6 0.8 1.0 1. Determine the current necessary in a coil of 4000 turns wound on Selection (ii) to produce a flux density of 0.7 Tesla in the air-gap. Neglect magnetic leakage.

  1. A SERIES MGNETIC CIRCUIT HAS AN IRON PATH OF LENGTH 50 cm AND AN AIR GAP OF 1 mm. THE CROSS SECTIONAL AREA OF IRON IS 6 cm2 AND COIL WOUND ON IRON HAS 400 TURNS. DETERMINE THE CURRENT REQUIRED TO PRODUCE A FLUX OF 0.9 mWb IN CIRCUIT. THE POINTS ON MAGNETISATION CURVE OFIRON IS GIVEN.
  2. An iron ring has a cross sectional area of 3cm2 and mean dia of 25cm. An area gap of 0.4mm has been cut across the section of ring. The ring is wound with a coil of 200 turns carrying current of 2A. If the total magnetic flux is 0.24mWb find relative permeability ur of iron.
  3. An iron ring has a cross section area of 2 cm and a mean diameter of 20 cm^2. An air of 0.4 mm has been cut across the section of the ring. The ring is wound with a coil of 300 turn. The total magnetic flux is 0.2 mwb. The relative permeability of iron is 1000. Find the value of current passed in the turn.
  4. Determine the value of p.f. from wattmeter readings. A 400 V ,3-phase, induction motor has an output of 20 HP and operates at a p.f. of .85 lagging with an efficiency of 90%. Calculate the reading on each of the two wattmeter connected to measure the input. 8.. A three phase star connected balance load of value Z=40 angle 60 ⁰ ohms per phase is connected across 400V,ohms per phase is connected across 400V, 3-phase balance supply. Find power draw from supply and reading of two wattmeters connected to measure the input. Draw circuit diagram?
  5. A steel ring of 15cm in diameter and of circular section 3cm in diameter Has an air gap of 1.5mm length. It is would uniformly with 750 turns of wire carrying of 2.1A. Calculate: (i) Magneto motive force. (ii) Flux density in air gap. (iii) Magnetic flux. (iv) Relative permeability of steel ring. Flux density Wb/m 2 1.2 1.35 1.45 1. H(At/m) 500 1000 2000 4500

UNIT -4(SINGLE PHASE TRANSFORMER AND DC MACHINE)

2 marks questions->

  1. Derive the torque equation of D.C. machine.
  2. Derive the EMF equation of a D.C. machine.
  3. A series motor never be started without load on it. Give reason.
  4. Explain function of commutator. 8/7 marks questions->
  5. Enlist various parts of a D.C. machine and explain the function of each part.
  6. Describe the principle of three phase induction motor
  7. Suggest the appropriate motor for following applications:-  Ceiling fan  Electrical traction lathes  Crane &hoists  Blower and fan  Floor mill  Tape recorder  Tube well pump.
  8. Write the application of D.C. generator .The armature of a 12-pole D.C. shunt generator has 50 slots and is wave wound with 12 conductor per slots .The generator is running at a speed of 625 rpm and supplies a resistiv load of 15Ω at a terminal voltage of 300 v. The armature resistance is 0.5Ω and field resistance is 60Ω.Find the armature current, the generated EMF and flux per pole.
  9. Derive the expression for maximum torque and draw the torque –slip curve.
  10. Define slip and sate its importance.
  11. A 4 pole , 3 phase induction motor operates from a supply whose frequency is 50 Hz. Calculate:  The speed at which the magnetic field of the stator is rotating.  The speed of the rotor when the slip is .04.  The frequency of rotor current when slip is 0.03.  The frequency of the rotor current at stand still.
  12. Derive the expression for developed torque in a 3-phase induction motor.
  13. With usual notation, derive the torque equation of O.K. motor.
  1. Explain working principle of 3-phase induction motor. Draw torque/slip curve for two different values of rotor resistance.
  2. The no load armature current of 230V shunt motor is 2A at speed of 1200rpm. If the full load armature current is 40A, find full load torque and speed. Take armature resistance=o.25 ohm and field flux unchanged from no load to full load.
  3. Describe the classification of D.C. motor. Explain their torque /slip characteristic. Derive the expression for torque developed by D.C. motor. Give the torque speed characteristic for D.c. series and D.C. shunt motors. 25.. What is a transformer? Give classification of the transformer.
  4. Derive e.m.f. equation of a single phase transformer.
  5. Explain maximum efficiency of a single phase transformer. UNIT- 5(ELECTRICAL MEASURING INSTRUMENTS) 2 marks Questions ->
  6. Define damping. Name 3 different types of damping torque used in electro mechanical instruments.
  7. State the essential requirements of indicating type instrument. Explain in brief
  8. How do you extend the range of an armature?
  9. What is creeping error? 7/8 marks Question
  10. Describe the following torque and explain why they are required in electrical measure in system. a. Deflecting torque b. Controlling torque c. Damping torque
  11. Draw a neat diagram of PMMC instrument and explain its working.
  12. Explain essentials of indicating instruments
  13. A millimetre with a resistance of 4 ohm is connected with a shunt of 0.005 ohm. what will be the current flowing through the instrument if it is connected to a circuit in which a current of 10A is flowing?
  14. What is the principle of moving iron voltmeter?
  15. Explain the operation of amoving iron voltmeter.
  16. What do you understand by shunt and multiplier. How these are useful for extension of instrument? Explain.
  1. Why damping torque is provided in an electrical instrument ?what are the different methods of providing damping force? Describe them. What type odf damping is provided in following instruments? a. PMMC b. Moving coil c. Moving iron d. Wattmeter
  2. A moving coil instrument has a resistance of 10ohm and gives full scale deflection when carrying a current of 50mA. Show how it can be adopted to measure voltage and current upto 750V and current upto 100A.
  3. A moving coil instrument has a resistance of 10 ohm and gives full scale deflection when carrying a current of 50 mA. Show how it can be adopted to measeru voltage upto 750V and current upto 100A.
  4. a. What is meant by indicating, recording and integrating instruments? b. Explain difference b/w controlling and damping torque in an electrical instrument.
  5.  Show how a PMMC instrument can be employed as (A) AN AMMETER, (B) voltmeter.  How would you extend the range of ammeter and voltmeter?