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BE8251 Notes r2017 notes

Electrical Measuring Instruments and Classification of instruments

Electrical Measuring Instruments

Electrical Measuring Instruments

Classification of instruments

(i). Depending on the quality measured

(ii). Depending on the different principles used for their working

(iii). Depending on how the quantity is measured

Electrical Measuring Instruments classification Depending on the quality measured

  • Voltmeter
  • Ammeter
  • Energy meter
  • Ohm meter

Electrical Measuring Instruments classification Depending on the different principles used for their working

  • Moving Iron type
  • Moving coil type
  • Dynamometer type
  • Induction type

Electrical Measuring Instruments classification Depending on how the quantity is measured

  • Deflecting type
  • Integrating type
  • Recording type

Deflecting Torque

The deflecting torque moves the moving system and the pointer from the zero position.

The deflecting torque can be obtained through magnetic, thermal, electromagnetic or electro dynamic effects

Controlling torque

The controlling torque acts in a direction opposite to that of deflecting torque.

When the controlling torque (TC) and the deflecting torque (TD) are numerically equal the pointer takes a definite position.

In the absence of TC the pointer would deflect to maximum position irrespective of the quantity to be measured.

Moreover TC also helps in bringing the moving system to zero position when the instrument is disconnected from the circuit.

The controlling torque is obtained through spring control and gravity control

Spring Control:

 The arrangement for spring control consists of two phosphor bronze spiral hair springs attached to a moving system.

The springs are made of materials which

(i). are not affected by fatigue.

(ii). Have low temp-coefficient of resistance

(iii). Have low specific resistance

(iv). Are non-magnetic

As the pointer deflects the springs get twisted in the opposite direction.

The combined twist produces the necessary controlling torque which is proportional to angle of deflection of moving system θ.

If we consider a permanent magnet moving coil meter with spring control system the deflecting torque will be proportional to the current passing through it and the controlling torque will be proportional to the angle of deflection

Thus TD α I

TC α θ

Since TD = TC

We have θ α I

Thus the spring controlled instruments having uniform scale

Eddy current damping

Eddy current damping is the most efficient form of damping.

The essential components in this type of damping are a permanent magnet; and a light conducting disc usually of alumninum.

When a sheet of conducting material moves in a magnetic field so as to cut through lines of force, eddy currents are set up in it and a force exists between these currents and the magnetic field, which is always in the direction opposing the motion.

This force is proportional to the magnitude of the current, and to the strength of field.

The former is proportional to the velocity of movement of the conductor, and thus, if the magnetic field is constant, the damping force is proportional to the velocity of the moving system and is zero when there is no movement of the system.

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Other links 

DC Circuits and Ohm’s Law

AC Circuits and Kirchhoff’s law

Steady State Solution of DC Circuits and Problems based on ohm’s law

AC Instantaneous Value and RMS Value

RL Series Circuit And LR Series Circuit

Star Delta transformation

Gravity control

Damping Torque

Permanent Magnet Moving Coil Instruments (PMMC)

Operating Principles of Moving Iron Instruments Ammeters and Voltmeters

Electrodynamometer Type Wattmeter

Single phase Energy meter

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