Dynamometer Type Instrument
ElectroDynamometer Instrument
The electrodynamometer type instrument is a transfer instrument. A transfer instrument is one which is calibrated with a D.C source and used without any modifications for A.C measurements. Such a transfer instruments has same accuracy for A.C and D.C measurements. The electro-dynamometer type instruments are often used in accurate A.C voltmeters and ammeters, not only at the power line frequency but also in the lower audio frequency range with some little modifications, it can be used as a wattmeter for the power measurements.
Principle: Electro-dynamometer type instruments are very similar to PMMC type instrument in which the operating field is produced, not by a permanent magnet but by another fixed coil (usually two fixed air cored coils are used).
The PMMC instrument cannot be used on A.C currents or voltages. If A.C supply is given to these instruments, an alternating torque will be developed. Due to moment of inertia of the moving system, the pointer will not follow the rapidly changing alternating torque and will fail to show any reading. In order that the instrument should be able to read A.C quantities, the magnetic field in the air gap must change along with the change in current. This principle is used in the electro-dynamometer type instrument.
Construction: The below fig shows the construction of the electro-dynamometer type instrument.
The various parts of the dynamometer type instrument are:Fixed coil: The necessary field required for the operation is produced by the fixed coil. This coil is divided into two halves to give a more uniform field near the centre and to allow passage of the instrument shaft. The fixed coils re usually air cored to avoid hysteresis, eddy currents and other errors when the instrument is used on A.C fixed coils are wound with fine wire for using as voltmeter, while for ammeters and wattmeter’s it is wound with heavy wire ( i.e. stranded heavy wire). The coils are usually varnished and baked to form a solid assembly. They are then clamped in place against the coil supports. This makes the construction rigid.
The mounting supports are preferably made of ceramic. If the metal parts would have been used when it would weaken the field of the fixed coil.
Moving coil: the moving coil is wound either as a self-sustaining coil or else on a non-metallic former. Metallic former cannot be used because eddy currents would be induced in it by the alternating field. Light but rigid construction is used for the moving coil. It is also air cored.
Controlling: the controlling torque is provided by tow control springs. These springs acts as leads to the moving coil.
Moving system: The moving coil is mounted on an aluminium spindle. It consist of counter weights and pointer sometimes a suspension may be used, in case a high sensitivity is desire.
Damping: The damping torque is provided by air friction, by a pair of aluminium vanes which are attached to the spindle at the bottom. These vanes move in sector shaped chambers. Eddy current damping cannot be used in these instruments as the operating field is very weak (or would be destroyed).
Shielding: The field produced by these instruments is very weak. Even earth’s magnetic field considerably affects the readings in D.C measurements. So shielding is done to protect it from stay magnetic fields. It is done by enclosing in a casing of high permeability alloy.
Cases and Scales: laboratory standard instruments are usually contained in highly polished wooden cases which usually contained in highly polished wooden cases which are rigid. The cases are son constructed as to remain stable over long periods of time. The case is supported by adjustable levelling screws. A pririt level also provided to ensure proper levelling.
The scales are hand drawn, using machine sub-dividing equipment. Diagonal lines for fine sub-divisor are usually drawn for marking on the scale. Most of high-precision instruments have a 300 mm scale with 100, 120 or 150 divisions.
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