Electrodynamometer wattmeter and Low Power Factor LPF wattmeters are mostly similar in construction and operation, except for the following modifications are carried out in the electrodynamometer wattmeter to convert it into a LPF wattmeter:. As the name suggests the low power factor meter are the instruments that measures lower values of power factor accurately. There is a requirement of low power factor meter because, the usage of ordinary electrodynamometer wattmeter to measure power factor of a low pf load gives inaccurate results. Now there are two main reasons that would suggests us that we should not use ordinary wattmeter in measuring the low value of power factor. Pressure coil current:. The pressure coil circuit is designed to have a low value of resistance, so that the current flowing through it is increased to give an increased torque.

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The low power factor meter is used for measuring the power of the highly inductive circuit. It is also used in the resistive circuit which has a power factor range from 0.

The ordinary Wattmeter used for measuring the low power factor gives the inaccurate result. This happens because of two reasons. Some additional features are added on the ordinary Wattmeter so that the meter can measure the power of the low power factor circuit.

The pressure coil is designed for having a low value of resistance so that the high value of current passes through it. This current produces the deflecting torque on the moving coil. There are two substitute methods of connecting the Wattmeter in the circuit.

The figure below shows these two methods. In figure a the pressure coil is parallel connected to the supply voltage. And the current coil is connected in series with the supply voltage.

The magnitude of the voltage across the pressure coil is equal to the supplied voltage. The total power measured by the pressure coil is equal to the sum of the power loss in the load and the power loss in the current coil. In figure b the pressure coil is not connected in parallel with the load. Thereby, the magnitude of the pressure coil voltage is not equal to the supplied voltage. The output power obtains from the circuit is equal to the sum of the load power loss and the power loss of the pressure coil.

In low power circuit, the value of current is high, and that of the power is low. The high-value current causes the error in the Wattmeter reading. For reducing the error, the compensation coil is used in the circuit. The compensating coil compensates the error in the circuit which induces because of low power factor. The compensating coil and the pressure coil are in series with each other.

The coil is coordinate with the current coil. The pressure coil current I p flows through it. The field induces in the compensating coil is because of the current I p. The field of the compensating coil and the current coil opposes each other.

Thus the error occurs in the pressure coil is neutralized. The small amount of inductances is present in the pressure coil of the Wattmeter. This inductance causes the error in the reading. The error occurs in the pressure coil is given by the expression. The compensating coil is used in the circuit for compensating the inductance error occurs in the Wattmeter. Along with the compensating coil, the capacitor is used in the circuit. The capacitor is placed in parallel with the pressure coil resistance.

The capacitor eliminates the error. Above 10kHz the compensation is accurately held in the Wattmeter. The control torque of the low power factor Wattmeter is kept small so that the full-scale deflection is obtained even for the small value of the power factor.

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UPF and LPF Wattmeters

As the secondary side is open, the entire coil will be purely inductive in nature. So the power will be lagging due to inductive property of the circuit. In short circuit test the secondary winding of transformer is short circuited. As secondary side is short circuited the entire coil will be purely resistive in nature. So the power factor will be unity. Lpf is used in power measuring in high inductive circuit and upf is used in power measuring in resistance circuit which power factor ranges 0.


Low Power Factor Wattmeter


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