- 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.
For a single-phase circuit, power factor meters provide a direct indication of power factor. Power factor can also be calculated as real power divided by apparent power, using measurements of real power, RMS current and RMS voltage. A wattmeter, ammeter and voltmeter can supply those values, but uncertainty from each instrument affects the calculated result.
In power systems, operators measure power factor at generating stations, large loads and each relevant electrical substation. Direct power factor meters help them assess current demand and the losses associated with a power transmission system and its distribution network.
A dedicated meter is useful when power factor must be read directly.
A traditional electrodynamometer instrument has a fixed current coil that is normally split into two sections. It also has two moving pressure coils on one spindle. The fixed coil carries the circuit current or a proportional current. The two pressure-coil branches are connected across the circuit and are designed to carry currents with different phase angles. Their interactions with the fixed-coil field create opposing torques. No control spring sets the pointer position; the pointer settles where the two torques balance.
Two traditional types of power factor meter are:
- Electrodynamometer type
- Moving iron type.
The electrodynamometer type is described below.
Electrodynamometer Type Power Factor Meter
Electrodynamometer type power factor meters are built for either of these supply arrangements:
- Single phase
- Three phase.
The general circuit diagram of a single-phase electrodynamometer power factor meter is shown below.
The fixed current coil carries the load current. Two identical moving pressure coils are mounted on the same spindle. Coil 1 is connected through a non-inductive resistor, and Coil 2 is connected through a highly inductive branch. The plane of Coil 1 makes angle A with the reference plane. The two moving coils are separated by 90o, so Coil 2 makes an angle of (90o + A) with the reference plane. The scale is calibrated in the cosine of the load phase angle. At the rated frequency, the external resistance R and inductor L are selected so the two moving-coil currents have approximately equal magnitudes. In the ideal model, R = ωL. The current in Coil 2 then lags the current in Coil 1 by approximately 90o.
Two opposing torques act in this power factor meter, one from each moving coil. Their balance sets the pointer position. Under the ideal assumptions used for the derivation, the torque expression for Coil 1 is:
Here, M is the maximum mutual inductance between the fixed and moving coils.
B is the angular deflection from the reference plane.
The corresponding torque expression for Coil 2 is:
At equilibrium, T1=T2. If the two pressure-coil currents are equal and exactly in quadrature, the derivation gives A = B. The pointer angle therefore represents the phase angle between sinusoidal load voltage and current. The phasor diagram shows the Coil 1 and Coil 2 currents approximately 90o apart.
The scale displays the cosine of that angle, with separate leading and lagging sides. This analogue design indicates displacement power factor for sinusoidal waveforms. It does not measure true power factor accurately when voltage or current contains harmonic distortion.
Advantages of Electrodynamic Type Power Factor Meters
- The air-cored measuring system avoids hysteresis and eddy-current errors. Its frequency error can be small near the calibration frequency compared with moving iron type instruments.
- The calibrated scale gives a direct indication of leading or lagging displacement power factor.
Disadvantages of Electrodynamic Type Power Factor Meters
- The operating torque is small, so friction and stray magnetic fields can affect the reading.
- The calibrated scale covers the intended leading and lagging power-factor range rather than a full 360o.
- A change in supply frequency changes the inductive pressure-coil current and can introduce calibration error.
- The construction is relatively costly compared with simpler indicating instruments.
