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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