## Dynamometer Type Three-Phase Wattmeter

β’ A dynamometer-type three-phase wattmeter consists of two separate wattmeter movements mounted together in one case with the two moving coils mounted on the same spindle.

β’ The arrangement is shown in Fig.

β’ There are two current coils and two pressure coils.

β’ A current coil together with its pressure coil is known as an element. Therefore, a three-phase wattmeter has two elements.

β’ The connections of two elements of a 3-phase wattmeter are the same as that for two wattmeter method using two single-phase wattmeters.

β’ The torque on each element is proportional to the power being measured by it.

β’ The total torque deflecting the moving system is the sum of the deflecting torque of the two elements.

β’ Hence the total deflecting torque on the moving system is proportional to the total Power.

β’ In order that a 3-phase wattmeter read correctly, there should not be any mutual interference between the two elements.

β’ A laminated iron shield may be placed between the two elements to eliminate the mutual effects.

## 3-Phase Electrodynamometer Wattmeter – Construction, Working & Shape of Scale

### Working of 3-Phase Electrodynamometer Wattmeter :

A 3-phase electrodynamometer wattmeter is a type of electrical meter used to measure the power in a 3-phase AC electrical circuit. It is based on the principle of electromagnetic induction. The construction of a 3-phase electrodynamometer wattmeter typically consists of a moving coil, a stationary coil, and a pointer.

The moving coil is connected to the current-carrying conductor and rotates in the magnetic field of the stationary coil, which is connected to the voltage-carrying conductor. The pointer is attached to the moving coil and indicates the power measurement on a scale.

The working of a 3-phase electrodynamometer wattmeter involves the interaction of the current-carrying conductor and the voltage-carrying conductor, with the resulting electromagnetic forces causing the moving coil to rotate. The angle of rotation is directly proportional to the power in the circuit. The scale of the wattmeter is typically a circular or rectangular shape, with markings indicating the power measurement in watts or kilowatts.

In the case of a 3-phase system, the wattmeter must have 3 separate moving coils, one for each phase and 3 separate current coils, and 3 separate voltage coils. The three moving coils are positioned 120 degrees apart.

It should be noted that the electrodynamometer wattmeter has a limited range of measurement and is typically used for measuring power in medium to high-voltage circuits.

## The shape of the Scale of the Electrodynamometer Wattmeter :

In electrodynamometer wattmeters, the final steady deflection ΞΈ is given by,

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