9032 Experiment 01

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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION o 1

Electrical Measurements (9032) Experiment No. 1

EXPERIMENT NO. 1

1.0 TITLE :

a) Preparation for laboratory work .

b) To study the constructional details of Permanent Magnet Moving Coil and Moving iron

instruments.

Part a) Preparation for laboratory work.

Before coming in the laboratory for performing the practicals, please do the following

1. Study the symbols and units.

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2 o MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION


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2. Answer the questions on basic knowledge

Questions :

Write answers to QQQQ (Question numbers to be allotted by the teacher)

1) Why symbols are necessary in electrical engineering?

2) What information is obtained from the meter marked with following symbols? Write it in

two lines.

3) Draw a circuit to indicate use of Auto - transformer, A. C. Voltmeter, A. C. Ammeter and

load.

4) Draw a circuit to indicate use of potential divider, D.C. Voltmeter, D. C. Ammeter and

Rheostat.

5) Draw symbols for P.M.M.C. and M. I. Type instruments.

6) List the ranges of voltmeters, ammeters and wattmeters in your laboratory.

(Space for answers)

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Signature of Teacher

(Space for answers)

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Part B :To study the constructional details of Permanent Magnet Moving Coil and Moving Iron

Instruments.

2.0 PRIOR CONCEPTS:

Deflecting torque, Controlling torque, Damping torque.Force on a current carrying conductor placed in a magnetic field

Magnetic repulsion.

3.0 NEW CONCEPTS

Proposition 1 : P.M.M.C. instrument (construction).

The instrument has Deflecting system, Controlling system and Damping system. Deflecting system

consists of a permanent magnet made of Alnico, a core of soft iron and moving coil of enameled

copper wire. The moving coil has a thin cross-section and about 50 - 100 turns. It is wound on a

rectangular aluminium former. The former is pivoted on a spindle. The moving coil is free to rotate

in the gap of permanent magnet and fixed core.

Controlling system consists of springs, called control springs, made of phosphor bronze. (springs

also act as the leads for the current).

Damping system is based on principle of eddy current. Eddy current damping system generates

eddy currents in the permanent magnet and damps the oscillations of indicating pointer by providing

force in the opposite direction.

Concept Structure 1 :

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Proposition 2 : P.M.M.C. instrument (operation).

When the moving coil in the field of permanent magnet caries current, a deflecting torque is produced

which is proportional to current through the moving coil.

Control springs produce the controlling torque which is proportional to angle of deflection. Eddy

currents circulating in the aluminium former produce damping torque. Which is proportional to eddy

emf induced in it.


Proposition 3 : P.M.M.C. instrument (scale and use)

In PMMC instrument, the scale is uniform as the deflecting torque is linearly proportional to current

flowing through the coil. Direction of deflection depends on the direction of current through the

moving coil, so the instrument is used for DC measurement only and polarities must be observedwhile connecting the instrument.

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Proposition 3 : P.M.M.C. instrument (used for AC measurements).

With rectifier, a P.M.M.C. instrument can be connected to AC supply, and the instrument is called as

Rectifier type instrument.

The rectifier type instrument measures average or r. m. s. AC value depending upon calibration of

its scale.


Part II : Moving Iron Instrument

Proposition 5 : Repulsion type MI instrument (construction).

The instrument has Deflecting system, Controlling system and Damping system.

Deflecting system consists of a circular coil. A soft iron piece called fixed iron is attached to the coil

from inside. Spindle is along the axis of coil. Another soft iron piece called as moving iron is attached

to the spindle.

Controlling system consists of springs, called control springs made of phosphor bronze.

Damping system consists of air friction damping.

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Proposition 6 : Repulsion M. I. Instrument (operation).

While coil carries current, the soft iron pieces gets magnetized. Similar poles are formed on the

nearer faces of soft iron pieces and they repeal each other. Thus the deflecting torque is due to the

repulsion and it is proportional to square of the current through the coil.

Control spring provide the controlling torque which is proportional to angle of deflection.

Air friction damping is provided to the instrument. So damping torque is proportional to movement

of piston in air chamber.

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Proposition 7 : M. I. Instrument (Scale and Use).

In M. I. Instruments, the deflecting torque i.e. the magnitude of deflection is proportional to square

of the current (I) so the scale of the instrument is non - uniform and cramped at the beginning.

The direction of deflection does not depend on the direction of the current through the coil so the

instrument can be used for DC as well as AC measurements and polarities need not be observed

while connecting the instrument.

(P.M.M.C. instruments being more accurate, they are preferred for DC measurements).

4.0 LEARNING OBJECTIVES:Intellectual skills:

To understand the concept of P.M.M.C. instrument.

To understand the concept of M. I. instrument.

Motor skills:

Ability to use P.M.M.C. instrument.

Ability to use M. I. instrument

5.0 APPARATUS :

Part A :

P.M.M.C. instruments (Ammeter, Voltmeter, - any range).

Part B :

M. I. Instruments (Ammeter, Voltmeter, - any range).

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6.0 STEPWISE PROCEDURE :

Part A :

Open a given P.M.M.C. instrument.

Observe Deflecting, Controlling and Damping systems.

Draw sketch showing all parts of the instrument and label them.

(Space for sketch)

Part B :

Open a given M. I. instrument.

Observe Deflecting, Controlling and Damping systems.

Draw sketch showing all parts of the instrument and label them.

(Space for sketch)

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7.0 QUESTIONS :

Write answers to Q. 1, 2 and 15. Write four answers from remaining questions to QQQQ

(Question numbers to be allotted by the teacher).

1. Identify the instrument shown below and label the marked components of it -

Instrument _____________________

Components :

1. __________________

2. __________________

3. __________________

4. __________________

5. __________________

Instrument _____________________

Components :

1. __________________

2. __________________

3. __________________

4. __________________

5. __________________

Instrument _____________________

2. Identify the instrument and label the components marked with arrow -

Instrument _____________________

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Complete the following table :

3. Draw sectional view of PMMC and MI instruments to scale.

4. What type of damping is provided in PMMC and MI instruments?

5. Give applications of PMMC and MI instruments.

6. What are the functions of deflecting, controlling and damping torques?

7. Why PMMC instrument is used for d.c. only?

8. State the advantages and limitations of PMMC instrument.9. State the advantages and limitations of MI instrument.

10. Which meter will you use for measuring 7.5 amp DC?

11. Which type of meter will you use for measuring 6 A a.c.?

12. Is it possible to measure 6A a.c. with a PMMC instrument? Give reason.

13. Identify the type of meter. Draw its IS symbol.

14. State the material used for PMMC and MI instruments in their constructional parts.

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(Space for answer)

Signature of Teacher

Documents

9032 Experiment 01