A power factor meter is a type of electrodynamometer movement when it is made with two movable coils set at right angles to each other. The method of connection of this type of power factor meter, in a 3phase circuit, is shown in Figure 14. The two stationary coils, S and S1, are connected in series in Phase B. Coils M and M1 are mounted on a common shaft, which is free to move without restraint or control springs. These coils are connected with their series resistors from Phase B to Phase A and from Phase B to Phase C. At a power factor of unity, one potential coil current leads and one lags the current in Phase B by 30°Í¾ thus, the coils are balanced in the position shown in Figure 14. A change in power factor will cause the current of one potential coil to become more in phase and the other potential coil to be more out of phase with the current in Phase B so that the moving element and pointer take a new position of balance to show the new power factor.Power factor measurement provides us the knowledge of the type of loads that we are using, helps in calculation of losses happening during the power transmission system and distribution.
On the basis of supply voltage, Electrodynamometer type is further classified as:
The general circuit diagram of the single phase electrodynamometer power factor meter is given below.
Now the pressure coil is spitted into two parts one is purely inductive another is purely resistive as shown in the diagram by resistor and inductor. At present, the reference plane is making an angle A with coil 1. And the angle between both the coils 1 and 2 is 90°. Thus, the coil 2 is making an angle (90° + A) with the reference plane. Scale of the meter is properly calibrated shown the value values of cosine of angle A. Let us mark the electrical resistance connected to coil 1 be R and inductor connected to coil 2 be L. Now during measurement of power factor the values of R and L are adjusted such that R=wL so that both coils carry equal magnitude of current. Therefore, the current passing through the coil 2 is lagging by 90° with reference to the current in coil 1 as coil 2 path is highly inductively in nature.
Let us derive an expression for deflecting torque for this power factor meter. Now there are two deflecting torques one is acting on the coil 1 and another is acting on the coil 2. The coil winding is arranged such that the two torques produced, are opposite to each other and therefore pointer will take a position where the two torques are equal. Let us write a mathematical expression for the deflecting torque for coil 1.
Where M is the maximum value of mutual inductance between the two coils, B is the angular deflection of the plane of reference. Now the mathematical expression for the deflecting torque for coil 2 is-
At equilibrium, we have both the torque are equal thus on equating T1=T2 we have A=B. From here we can see that the deflection angle is the measure of the phase angle of the given circuit. The phasor diagram is also shown for the circuit such that the current in the coil 1 is approximately at an angle of 90° to current in the coil 2.
References
1.http://nuclearpowertraining.tpub.com/h1011v4/css/Power-Factor-Meter-96.htm
2.http://www.electrical4u.com/power-factor-meters-electrodynamometer-type-power-factor-meter/
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