Power Factor Meter

Power Factor Meter

Power factor of a single phase circuit is given by
cos  = P/VI.
By measuring power, current and voltage power factor can be calculated using the above equation.
This method is not accurate.
It is desirable to have instantaneous indication of power factor.
Power factor meter indicate directly the power factor of the circuit.
Power factor meters have : 1. Current Coil
2. Pressure Coil.
1. The current circuit carries the current whose PF is to be measured.
2. The pressure circuit is connected across the circuit whose PF is to be measured and is usually split into two
3. The deflection of the instrument depends upon the phase difference between the main current and currents in two
     paths of pressure coil i.e., the power factor of the circuit.
4. The deflection is indicated by a pointer.

Types of power factor meters
There are two types:1. Electrodynamometer Type      2. Moving Iron Type
Construction of electrodynamometer type power factor meter.

Construction is shown in Fig.1
It consists of two coils 1.Fixed coil which acts as current Coil.
                                     2. Moving coil or pressure coil.

Single phase Electrodynamometer Power Factor Meter

Current coil:
1. Split into two parts and carries the current of the circuit under test.
2. The magnetic field produced by this coil is proportional to the main current.
Pressure coil:

1. Two identical coils A & B pivoted on a spindle.
2. Coil a has a non inductive resistance R connected in series with it.
3. Coil B has a highly inductive choke coil L connected in series with it.
4. The two coils are connected across the voltage of the circuit.
5. The value of R & L adjusted to carry the same current at normal frequency.

Working Principle:
1. Current in coil is in phase with the circuit voltage.
2. Current through coil B lags the voltage by an angle 90°(Δ).
3. The angle between the planes of the coils is made equal to Δ.
4. There is no controlling torque.
5. Minimum control effect using silver or gold ligaments for connecting moving coils.

Assumption made:
Current through coil B lags voltage by exactly 90°.
Angles between the planes of the coils is exactly 90°.

Now, there will be two deflecting torques:
1. Torque acting on coil A.
2. Torque acting on coil B.

The coil windings are arranged in such a way that the torques due to two coils are opposite in direction. Therefore the pointer will take up a position where these two torques are equal.
Consider the case of a lagging power factor of cos .

Deflecting torque acting on coil a is:
TA = KVI Mmax cos sinθ
Where θ = angular deflection from the plane of reference. & Mmax = maximum value of mutual inductance between the two coils.
This torque acts in clockwise direction.
Deflection torque acting on coil B is:
TB = KVI Mmax cos (90° -) sin(90°+ θ) = KVI Mmax sin  cos θ.
This torque acts in the anticlockwise direction. The value of Mmax is the same in both
the expressions, due to similar construction of coils.

The coils will take up a position where the two torques are equal.
or KVI Mmax cos sinθ =KVI Mmax sin cos θ or θ = 
Therefore the deflection of the instrument is a measure of phase angle of the circuit. The
scale of the instrument can be calibrated directly in terms of power factor.

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