Explanation of Dynamometer Type Wattmeter

Dynamometer Type Wattmeter

This is also similar in design and principle to the dynamometer type ammeter and voltmeter.

Pic 1

Construction: In the dynamometer type instrument is used as a wattmeter, the fixed coil, which is divided into two equal portions, the fixed coils are connected in series with the load and carry the circuit (or load) current, therefore it is also called as current coil. The moving coils is connected across the load and carries the current proportional to the voltage across the circuit. Therefore, it is known as pressure coil or potential coil.

A high non-inductive resistance ‘R’ is connected in series with the moving coil to limit current through it.

The magnetic field of the fixed and moving coils react on one another causing the moving coil to turn about its axis. The movement of the coil is controlled by hair springs also lead the current into and out of the moving element. Damping torque is provided by the air friction.

Working: When the instrument is connected in the circuit as shown in the below fig. the main circuit current I₁ passes through the current coil and the current, I₂ is proportional to the voltage to be measured passes through the pressure coil. Due to the current in the two coils. Mechanical force exists between them. The result is that the moving coil moves causing that pointer to deflect on a graduated scale. The pointer move till deflecting torque is equal to controlling torque.

The reversal of current through the fixed and moving coils reverses the field due to both the coil hence, the deflecting torque remains unchanged. Therefore these instruments can be used on A.C power measurement.s

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