Measurement / Measuring Instruments / Wattmeters / Electrostatic Wattmeter

Electrostatic Wattmeter Construction And Working

We use electrostatic wattmeter for measuring the small quantity of power. Particularly it is useful when the voltage is high and the power factor is low. We use electrostatic wattmeter mainly for measuring of dielectric loss of a power cable. Also, we use this wattmeter for calibration purpose of other wattmeters as well as energy meters.

Construction of Electrostatic Wattmeter

This is basically an electrostatic instrument. There is a noninductive resistance between quadrants of the meter. The quadrants and the noninductive resistance across them together connect to one terminal of the instrument. The suspension vane is connected to the other terminal of the instrument. We connect these two terminals of an electrostatic wattmeter across the load circuit of which we measure the power. The connection diagram of this instrument is given below.

electrostatic wattmeter

Here, i is the load current, R is the resistance, v is the voltage of the vane, v1 is the voltage of one pair of quadrants and v2 is the voltage of the other pair of quadrants.

As per the theory of electrostatic instrument, we can write the expression of instantaneous torque of an electrostatic wattmeter as,

electrostatic watt meter

From the above diagram, we can find,

electrostatic watt meter

Again, the supply voltage is the voltage across load + voltage drop in the noninductive resistance.

We can get this relation very easily by applying Kirchhoff”s voltage law in the above circuit diagram.

Therefore,

electrostatic watt meter

Since i is the load current and e is the load voltage ‘ei’ is the load power. So torque is proportional to the power in the load + half of the power loss in noninductive resistance. Again the deflection of the pointer of the instrument is proportional to the torque. Therefore, the deflection of the instrument gives a direct reading of power.

Advantages and Disadvantages of Electrostatic Wattmeter

  1. The electrostatic watt meter is a precision instrument.
  2. It does not have any error clue to waveform and frequency of the electrical signal.
  3. It is also free from eddy current error because no magnetic induction takes place in the instrument.
  4. The deflecting torque of the instrument is also very small.

 

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