Low Power Factor Wattmeter

Low Power Factor Wattmeter Definition

A low power factor wattmeter is an instrument used to measure low values of power factor accurately.

Why Standard Wattmeters Fail

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

Errors due to pressure coil inductance.

The above two reasons give very inaccurate results thus we should not use normal or ordinary watt meters in measuring the low value of the power factor.

Low Power Factor Wattmeter Design

The modified circuit is shown below:

We have used here a special coil called compensating coil, it carries a current equal to the sum of two currents i.e load current plus pressure coil current.

The pressure coil is placed such that the field produced by the compensating coil is opposed by the field produced by the pressure coil as shown in the above circuit diagram.

• Thus the net field is due to the current I only. Hence in this way errors caused by pressure coil can be neutralized.

• We require compensating coil in the circuit in order to make the low power factor meter. It is the second modification that we have discussed in detail above.

• Now the third point deals with the compensation of the inductance of the pressure coil, which can be achieved by doing modification in the above circuit.

Now let us derive an expression for the correction factor for pressure coil inductance. And from this correction factor, we are going to derive an expression for error due to the inductance of the pressure coil.

When considering the inductance of the pressure coil, the voltage across it is not in phase with the applied voltage.

Hence in that case it lags by an angle

Where, R is electrical resistance in series with pressure coil, rp is pressure coil resistance, here we also conclude that the current in the current coil is also lagging by some angle with the current in pressure coil. And this angle is given by C = A – b. At this time reading of the voltmeter is given by

Where, Rp is (rp+R) and x is angle. If we ignore the effect of the inductance of pressure i.e putting b = 0 we have expression for true power as

On taking the ratio of equations (2) and (1) we have an expression for correction factor as written below:

And from this correction factor error can be calculated as,

On substituting the value of correction factor and taking suitable approximation we have an expression for error as VIsin(A)*tan(b).

Now we know that the error caused by pressure coil inductance is given by the expression e = VIsin(A) tan(b), if the power factor is low (i.e in our case the value of φ is large hence we have a large error).

Thus in order to avoid this situation, we have connected the variable series resistance with a capacitor as shown in the above figure.This final modified circuit is known as the low power factor meter.A modern low power factor meter is designed such that it gives high accuracy while measuring power factors even lower than 0.1.