• 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.
• The arrangement is shown in Fig.
• There are two current coils and two pressure coils.
• A current coil together with its pressure coil is known as an element. Therefore, a three phase wattmeter has two elements.
• The connections of two elements of a 3 phase wattmeter are the same as that for two wattmeter method using two single phase wattmeter.
• The torque on each element is proportional to the power being measured by it.
• The total torque deflecting the moving system is the sum of the deflecting torque of’ the two elements.
• Hence the total deflecting torque on the moving system is proportional to the total Power.
• In order that a 3 phase wattmeter read correctly, there should not be any mutual interference between the two elements.
• A laminated iron shield may be placed between the two elements to eliminate the mutual effects.
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| (fig) three phase wattmeter |
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