## What is UPF and LPF?

LPF(lagging powerfactor) meter are used to measure power in inductive circuits. As inductive loads have lagging current these LPF wattmeters can be used. UPF (unity power factor) meters are used to measure power in resistive circuits.

**What is a watt Metre?**

The wattmeter is an instrument for measuring the electric active power (or the average of the rate of flow of electrical energy) in watts of any given circuit. Electromagnetic wattmeters are used for measurement of utility frequency and audio frequency power; other types are required for radio frequency measurements.

**What are the types of watt meter?**

Types of Wattmeter

- Electrodynamometer wattmeter – for both DC and AC power measurement.
- Induction wattmeter – for AC power measurement only.

### What is the range of low power factor meter?

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 0.5 to 1.

**Why do we need UPF and LPF wattmeters?**

Answer: LPF wattmeter is used to measure power in inductive circuits. Inductive circuits have the property of lagging power factor and hence these wattmeters are used. Resistive circuits have unity power factor hence UPF wattmeters are used to measure power in these circuits.

**What is the significance of a unity power factor UPF and low power factor LPF wattmeter why they are different?**

Difference Between LPF and UPF Wattmeters :

LPF Wattmeter | UPF Wattmeter |
---|---|

The pressure coil current is very high. | The pressure coil current is small. |

The error caused by pressure coil inductance is large. | The error caused by pressure coil inductance is zero. |

The control torque is small. | The control torque is large. |

## How is reactive power measured?

Definition of reactive power

- Apparent power: S = V x I (kVA)
- Active power: P = V x Ia (kW)
- Reactive power: Q = V x Ir (kvar)

**Why do we use LPF wattmeter in OC test?**

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.

**How can you justify that two wattmeters are sufficient to measure 3 phase power?**

Two Wattmeter Method can be employed to measure the power in a 3 phase, three-wire star or delta connected the balanced or unbalanced load. The total instantaneous power absorbed by the three loads Z1, Z2 and Z3, is equal to the sum of the powers measured by the two wattmeters, W1 and W2.