L15 Electrical Measuring Instruments

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@ McGraw-Hill Education

Electrical Measuring

Instruments Amit Mishra

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.

Ammeters. Ammeter Sensitivity. Ammeter Loading.

Ohm-meters. Wattmeter. Watthour Meter.

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)).

Principle of operation1. Magnetic effect.

2. Thermal effect.

3. Chemical effect.

4. Electrostatic effect.

5. Electromagnetic induction effect.

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.

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.

(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.

Saturday, April 8, 2023 Electrical Measuring Instruments 9 Next

Double Springs

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.

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

(ii) Gravity Control

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.

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

The restraining or controlling torque is thus developed is given as

c sin sinW L WL

d c dSince , and

or sin

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.

L15 Electrical Measuring Instruments

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