Answer / maniveer

LOW POWER FACTOR(LPF) & HIGH POWER FACTOR(UPF)

 In open circuit test the secondary winding of transformer is kept open.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. So LPF(Low Power Factor) Wattmeter is used in OPEN CIRCUIT TEST of Transformer.
          
            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. So UPF(Unit Power Factor) Wattmeter is used in SHORT CIRCUIT TEST of Transformer.

Answer / venkata krishna

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 / potnur hemarao

Lpf is used in power measuring in high inductive circuit and upf is used in power measuring in resistance circuit which power factor ranges 0.5 to unity

Answer / nitish kennedy

Low Power Factor wattmeters are specially designed to measure power at low power factor .

Ordinary wattmeters (i.e. UPF) are not accurate in measuring power at low power factor beacuse of -

low deflecting torque.
error due to inductance of of pressure coil is large at low power factor.
So some modifications are done in LPF wattmeters-

low value of pressure coil resistance to increase the torque.
pressure coil current is compensated using compensating coil.
capacitor is connected in pressure coil to reduce the error due to pressure coil inductance.
control torque is made very small.

Answer / k.prakashchandra

LPF -low pf UPF- unity pf.In lpf meter pf compensation is
done and is accurate at the specified low pf.

Answer / suresh

HERE LPF USED TO measure the low current & UPF is used to measure the high current. Then low pf and unity power factor

Answer / subha

can i get the expansion of lpf and upf

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