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:

- Pressure coil resistance is kept low to increase the torque.

- Compensating coil is used to compensate the Pressure coil current.

- Capacitor is connected across the pressure coil to reduce the error due to pressure coil inductance.

- Control torque is made very less for high deflection.

**Why a LPF Wattmeter is needed?**

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.

- The value of deflecting torque is very low even though we fully excite the current and pressure coils.

- Errors due pressure coil inductance.

**Modifications:**

**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.

**Compensation for Pressure coil current:**

In this category two cases
arise and their diagrams are shown below:

In the first category both
the ends of the pressure coil is connected to supply side i.e. current coil is
in series with the load. The supply voltage is equal to the voltage across the
pressure coil. Thus in this case we have power shown by the first wattmeter is
equal to the power loss in the load plus power loss in the current coil.

In the second category, the
current coil is not in series with the load and the voltage across the pressure
coil is not equal to the applied voltage. The voltage across pressure coil is
equal to the voltage across the load. In this power shown by the second watt
meter is equal to the power loss in the load plus the power loss in the
pressure coil.

Therefore, it is absolutely
necessary to compensate for the pressure coil current in a LPF wattmeter.

The compensating coil is
connected in series with the pressure coil circuit and is made nearly identical
to the current coil. The current coil and the compensatig coil carry currents
of (I + Ip) and Ip, respectively, and they produce fields corresponding to
these currents. The compensating coil is so connected that it opposes the field
of the current coil. Thus, the resultant field is due to current I only.

Hence, the error due to
pressure coil current flowing in current coil is neutralized.

**Compensation for Inductance of PressureCoil:**

The error caused by pressure
coil inductance is

Error=VIsin(Ф)tan(β)

With low power factor, the
value of Ф is large, and therefore the error is correspondingly large. The
compensation of the error caused by the inductance in the pressure coil is done
by connecting a capacitor across a part of series resistance in the pressure
coil circuit.

**Small Control Torque:**

LPF wattmeters are designed
to have a small control torque so that they give full scale deflections.

Incorporating all these
modifications into an electrodynamometer wattmeter gives the following circuit:

A modern low power factor
meter is designed such that it gives high accuracy while measuring power
factors even lower than 0.1.

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