- Power Factor Meters Definition: Power factor meters are devices used to accurately measure the power factor in AC circuits, crucial for industrial applications.
- Electrodynamometer Type: This type of meter measures power factor by using two coils—one resistive and one inductive—to determine the phase difference between voltage and current.
- Working Principle: The meter works by balancing the deflecting torques of the coils, with the deflection angle indicating the phase angle.
- Advantages: These meters have a high torque-to-weight ratio, low losses, and less error over small frequency ranges.
- Disadvantages: They have small working forces, a limited scale range, are sensitive to frequency changes, and are costly.
Before introducing various types of power factor meters, it is essential to understand their importance. We cannot directly calculate power factor in an AC circuit by dividing power by the product of current and voltage from a wattmeter, ammeter and voltmeter because this method is inaccurate and prone to errors. Therefore, industrial applications require accurate power factor meters.
In power transmission and distribution systems, power factor is measured at every station and electrical substation using power factor meters. This measurement helps us understand the type of loads in use and calculate the losses during power transmission system and distribution.
Hence we need a separate device for calculating the power factor accurately and more precisely.
A power factor meter circuit generally includes two coils: a pressure coil and a current coil. The pressure coil connects across the circuit, while the current coil carries the circuit current or a fraction of it. By measuring the phase difference between voltage and current, the power factor can be calculated. The pressure coil usually splits into an inductive and a resistive part. No controlling system is needed because the pointer balances at equilibrium due to opposing forces.
Now there are two types of power factor meters-
- Electrodynamometer type
- Moving iron type.
Let us study electrodynamometer type first.
Electrodynamometer Type Power Factor Meter
In electrodynamometer type power factor meter there are further two types on the basis of supply voltage
- Single phase
- Three phase.
The general circuit diagram of single phase electrodynamometer power factor meter is given below.
Now the pressure coil is split 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 90o. Thus the coil 2 is making an angle (90o + A) with the reference plane. Scale of the meter is properly calibrated as 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 lags by 90o with reference to current in coil 1 as coil 2 path is highly inductive in nature.
To understand deflecting torque in this power factor meter, we recognize that there are two deflecting torques: one on coil 1 and another on coil 2. The coil windings are arranged so that these torques are opposite, balancing the pointer when they are equal. The mathematical expression for the deflecting torque for coil 1 is given as:
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 as equal thus on equating T1=T2 we have A = B. From here we can see that the deflection angle is the measure of 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 90o to current in the coil 2.
Given below are some of the advantages and disadvantages of using electrodynamic type power factor meters.
Advantages of Electrodynamic Type Power Factor Meters
- Losses are less because of minimum use of iron parts and also give less error over a small range of frequency as compared to moving iron type instruments.
- They high torque is to weight ratio.
Disadvantages of Electrodynamic Type Power Factor Meters
- Working forces are small as compared to moving iron type instruments.
- The scale is not extended over 360o.
- Calibration of electrodynamometer type instruments are highly affected by the changing the supply voltage frequency.
- They are quite costly as compared to other instruments.
