This deflecting torque deflects the moving coil to such a position where the controlling torque of the spring is equal to the deflecting torque. Operation with AC Let, i1 and i2 be the instantaneous values of the current carried by the coils. As the quantity i2 is always positive, the current varies and the instantaneous torque also varies. But the moving system due to its inertia cannot follow such rapid variations in the instantaneous torque and responds only to the average torque.
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This type of instrument can be used either as an ammeter or as a voltmeter, but is generally used as a wattmeter. Besides, their use as an ammeter, voltmeter, and wattmeter; they are also used to transfer calibration of working instruments. The fixed coil is usually air-cored to avoid hysteresis effects when used on AC circuits. Fixed coils are wound with fine wire for use as a voltmeter. But, if the instrument is to be used as an ammeter or wattmeter, then the fixed coils are wound with heavy wire carrying the main current.
The moving coil is mounted on an aluminium spindle. It is wound either as self sustaining coil or else on a non-metallic former so as to prevent eddy currents. Moving coils are also air-cored.
These springs act as leads to the moving coil. Since, there is no iron, field strength and hence flux density is proportional to I1. The deflection produced in the instrument is proportional to the product of currents flowing in the fixed and the moving coil. They can be used on both AC and DC. They are used as transfer instruments.
Costlier than PMMC and moving iron type. Non-uniform scale.
Electrodynamometer type Instruments - Construction and Operation
Circuit Globe Electrical Measurement Electrodynamometer Wattmeter Electrodynamometer Wattmeter Definition: The instrument whose working depends on the reaction between the magnetic field of moving and fixed coils is known as the Electrodynamo-meter Wattmeter. It uses for measuring the power of both the AC and DC circuits. The working principle of the Electrodynamometer Wattmeter is very simple and easy. Their working depends on the theory that the current carrying conductor placed in a magnetic field experiences a mechanical force. This mechanical force deflects the pointer which is mounted on the calibrated scale.
Except that the permanent magnetic field coil is replaced by a coil which carries the current to be measured. Such instruments may also serve as transfer instruments. A transfer instrument is that which may be calibrated with dc source and then used on ac without any modifications. Because they have precision grade accuracy both for ac and dc measurements.