Download - L15 Electrical Measuring Instruments

Transcript
Page 1: L15 Electrical Measuring Instruments

@ McGraw-Hill Education

1

Electrical Measuring

Instruments Amit Mishra

Next

Page 2: L15 Electrical Measuring Instruments

@ McGraw-Hill Education

2

Saturday, April 8, 2023 Electrical Measuring Instruments 2

Topics to be DiscussedTopics to be Discussed Essentials of an Instrument. Controlling Torque. Damping Torque. Dynamometer Type Instruments. Digital Multimeter. Analog Multimeter. Voltmeters.

Voltmeter Sensitivity. Voltmeter loading.

Next

Page 3: L15 Electrical Measuring Instruments

@ McGraw-Hill Education

3

Saturday, April 8, 2023 Electrical Measuring Instruments 3

Ammeters. Ammeter Sensitivity. Ammeter Loading.

Ohm-meters. Wattmeter. Watthour Meter.

Next

Page 4: L15 Electrical Measuring Instruments

@ McGraw-Hill Education

4

Saturday, April 8, 2023 Electrical Measuring Instruments 4

Measuring InstrumentsTwo Types :

1. Absolute instruments :-These give the magnitude of the quantity in terms of the constants of the instruments. Example :-

a tangent galvanometer (used for measuring electric current), it measure current in terms of the tangent of the angle of deflection produced by the current, radius and number of turns of the galvanometer and the horizontal component of the earth’s magnetic field

2. Secondary instruments :-These have to be calibrated by comparison with an absolute instrument.

1. Indicating instruments (e.g., voltmeters, ammeters, etc)

2. Recording instruments (e.g., ECG).3. Integrating instruments (e.g., energy-meter(watt-hour),

odometer in car (total distance covered)).

Next

Click

Click

Page 5: L15 Electrical Measuring Instruments

@ McGraw-Hill Education

5

Saturday, April 8, 2023 Electrical Measuring Instruments 5

Principle of operation1. Magnetic effect.

2. Thermal effect.

3. Chemical effect.

4. Electrostatic effect.

5. Electromagnetic induction effect.

Next

Page 6: L15 Electrical Measuring Instruments

@ McGraw-Hill Education

6

Saturday, April 8, 2023 Electrical Measuring Instruments 6

Essentials of an Instrument1. Deflecting Torque :

1. This can be produced by any of the effects of current (or of voltage).

2. It is proportional to the magnitude of the quantity being measured.

2. Controlling Torque : (i) Spring control, (2) Gravity control.

1. It opposes the deflecting torque. 2. It is proportional to the deflection of the pointer.

3. Damping Torque : (i) Air friction, (ii) Fluid friction, (iii) Eddy current.

1. It makes the moving system to come to its final steady position quickly.

Next

Click

Page 7: L15 Electrical Measuring Instruments

@ McGraw-Hill Education

7

Saturday, April 8, 2023 Electrical Measuring Instruments 7

Controlling Torque It opposes the deflecting torque.

At pointer’s zero position, the controlling torque is zero.

At some position , the controlling torque becomes equal to the deflecting torque.

The pointer then stops moving further. It serves two functions :

1. The pointer stops moving beyond the final deflection,

2. The pointer comes back to its zero position when the instrument is disconnected.

c

Next

Page 8: L15 Electrical Measuring Instruments

@ McGraw-Hill Education

8

Saturday, April 8, 2023 Electrical Measuring Instruments 8

(i) Spring Control Most commonly used. One or two hairsprings made of phosphor bronze are

used. The outer end of this spring is fixed and the inner end is

attached with the spindle. When the pointer is at zero of the scale, the spring is

normal. As the pointer moves, the spring winds and produces an

opposing torque. The balance-weight balances the moving system so that

its centre of gravity coincides with the axis of rotation, thereby reducing the friction between the pivot and bearings.

Next

Page 9: L15 Electrical Measuring Instruments

@ McGraw-Hill Education

9

Saturday, April 8, 2023 Electrical Measuring Instruments 9 Next

Page 10: L15 Electrical Measuring Instruments

@ McGraw-Hill Education

10

Saturday, April 8, 2023 Electrical Measuring Instruments 10

Double Springs

Next

Page 11: L15 Electrical Measuring Instruments

@ McGraw-Hill Education

11

Saturday, April 8, 2023 Electrical Measuring Instruments 11

Two springs A and B are wound in opposite directions.

On deflection, one spring winds while the other unwinds.

To make the controlling torque directly proportional to the angle of deflection, the springs should have fairly large number of turns.

Next

Page 12: L15 Electrical Measuring Instruments

@ McGraw-Hill Education

12

Saturday, April 8, 2023 Electrical Measuring Instruments 12

Advantages : Since

These instruments have uniform scale.

Disadvantages : The stiffness of the spring is a function of temperature. Hence, the readings given by the instruments are

temperature dependent. Furthermore, with the usage the spring develops an

inelastic yield which affects the zero position of the moving system.

c d c dand ; at final position, I Hence, I

Next

Page 13: L15 Electrical Measuring Instruments

@ McGraw-Hill Education

13

Saturday, April 8, 2023 Electrical Measuring Instruments 13

(ii) Gravity Control

Next

Page 14: L15 Electrical Measuring Instruments

@ McGraw-Hill Education

14

Saturday, April 8, 2023 Electrical Measuring Instruments 14

A small control weight is attached to the moving system.

In addition, an adjustable balance weight is also attached to make the centre of gravity pass through the spindle.

In zero position of the pointer, this control weight is vertical.

Next

Page 15: L15 Electrical Measuring Instruments

@ McGraw-Hill Education

15

Saturday, April 8, 2023 Electrical Measuring Instruments 15

When deflected by an angle θ, the weight exerts a force,

The restraining or controlling torque is thus developed is given as

sinW

c sin sinW L WL

d c dSince , and

or sin

I

WL kI

sin

or sin

WLI

k

I

Next

Page 16: L15 Electrical Measuring Instruments

@ McGraw-Hill Education

16

Saturday, April 8, 2023 Electrical Measuring Instruments 16

Disadvantage :

1. These do not have uniform scale.

2. These must be used in vertical position so that the control may operate properly.

Advantages :

1. Less expensive.

2. Unaffected by changes in temperature.

3. Free from fatigue or deterioration with time.

Next