LPF wattmeter is used for measuring power in highly inductive circuits & UPF wattmeter is used for measure power in resistance circuit in which the power factor ranges from 0.5 to unity.
Answer is given by :Er. M KUMAR YADAV
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Alright, honey, listen up. The UPF wattmeter is used in short-circuit tests to measure the power factor of the equipment under test. It helps in determining the efficiency and performance of the system during a short-circuit condition. So, in simple terms, it's like having a nosy neighbor peeking in to see if everything's running smoothly when things go haywire.
If, by 'upf', you mean 'unity power factor', then allwattmeters measure the in-phase component of the load current, so the term is quiet unnecessary.A short-circuit test is used to determine the (true) power loss in the transformer, which is exactly what a wattmeter measures
Unity power factor has a value of 1.0. This means the current and voltage waveforms are in phase. This is only possible if the net load is non-reactive (resistive). If the load is either capacitively or inductively reactive, the power factor will be other than unity. If an inductively reactive load such as a motor is offset by a capacitively reactive load such as a PF correction capacitor, it is possible to acheive a net load that has unity power factor. Some loads, such as resistance heaters, are intrinsically non-reactive, and present a unity power factor.
A leading power factor means that the current is leading the voltage. That means that in the AC cycle (50 or 60 Hz) the current reaches its peak before the voltage. The amount of phase-lead can be up to 90 degrees which means one quarter cycle, or 5 milliseconds in a 50 Hz system.