Why is this? Because the current digital power factor meter has no sign at all.
So why is this happening? This is a long story!
1. What is the power factor?
We know that in an electrician, all voltages and currents are sinusoidal, and because they are sinusoidal, they can be conveniently represented by vectors. In the actual power system, the load is not necessarily pure resistance, that is, the current is not necessarily in phase with the current. If it is a pure inductor, the current will lag behind the voltage by 90 degrees. If it is a pure capacitor, then the current will be Will lead the voltage by 90 degrees. If it is not pure inductance or pure capacitance, then there will be a phase angle φ, which may be positive or negative, depending on the nature of the load. The inductive load φ is positive and the capacitive load φ is negative.
Figure 1. Voltage and current of a sine wave represented by a vector
For inductive or capacitive loads, because the current and voltage are not in phase, there are two different powers, active power and reactive power. The so-called active power refers to the product of the current component and the voltage in phase with the voltage. The so-called reactive power refers to the product of the current component perpendicular to the voltage and the voltage. The active power is actually the projection of the current vector on the voltage vector. This projection is to multiply the current vector by the cosine of its angle with the voltage, that is, Cosφ, and this Cosφ is called the power factor. When φ is 0 degrees, Cosφ is equal to 1, which is pure resistance. For inductive loads, 0 < Cosφ < 1; and for capacitive loads, 0 < Cosφ < -1. Therefore, the power factor is definitely positive and negative! And this is an important indicator to judge whether the load is capacitive or inductive! The power factor is also Cosφ, and Cosφ is the power factor. This is a matter of course.
In the power system, it is hoped that all loads are pure resistance, but it is practically impossible. Once Cosφ is not equal to 1, the reactive component is not really reactive, even if it is completely at right angle to the voltage. There is still I2R loss when it flows through the wire. So in the power system, we must try to correct the power factor as close as possible to 1. Because a large part of the load of the actual power system is an electric motor, it is an inductive load. In order to compensate for such inductive load, it is usually effective to use a large capacitor in parallel with the secondary of the power transformer. Make compensation. So for the power system they don't care much about the power factor problem because it is easy to compensate.
2. Power factor of the electronic system
There are many electronic devices in an electronic system, and almost all active electronic devices require DC power supply, and AC power is used as a power source in a general urban area or factory. So almost all electronic systems need a rectifier to convert the AC to DC, usually with an electrolytic capacitor to filter it later (Figure 2.).
Figure 2. Rectifier and electrolytic capacitor
This circuit, because it contains a rectifier diode, is actually a nonlinear circuit. This can be seen from the waveform of its power supply and current. The voltage and current waveforms at this time are shown in Figure 3.
Figure 3. Waveform of voltage and current of the rectifier
Obviously, although the voltage is still a sine wave, the current becomes a pulse wave. For such a nonlinear system, it is difficult to define its power factor because the power factor is originally derived from a linear system. Fortunately, most electronic systems are small household appliances that have little impact on large power systems. Therefore, the state specified that electronic systems below 75 watts did not require power factor. This is also very reasonable. Even now, the US Energy Star has specified a power factor of less than 100W for general lighting systems.
In the presence of fluorescent lamps, no power factor requirements have ever been proposed, so the power factor of an ordinary fluorescent lamp using inductive ballasting is about 0.5, and there is no regulation in the past.
However, in the country of energy-saving lamps, the power factor is not required below 15W, but since most energy-saving lamps are below 15W, this regulation has no effect on the large-scale promotion of energy-saving lamps. When the LED is specified, the power factor is not required to be 5W or less. It seems that the better the energy-saving effect, the more demanding the PF requirements! This is really difficult to understand!
Since the power factor of a low-power LED is now specified, there should always be a definition of the power factor for such a nonlinear system. How else to test? However, it is too difficult to define the power factor of a nonlinear system. To this end, many people have put forward various suggestions:
1. Use the cosine of the phase difference between the fundamental and the current of the current as the power factor. However, because the fundamental of the current is derived from the Fourier transform, the Fourier transform can only wait for its amplitude, but can not get its phase, the phase is relative, there must be a comparison to get the phase. However, when the Fourier transform is performed, the phase of the voltage is not used as a reference, so that the obtained result has no phase. So this kind of advice is impossible to achieve.
2. Take the zero crossing of the current as the starting phase of the current, and use the cosine of the phase difference of the phase and voltage as the power factor. But because the waveform of the current is not a sine wave, it is a pulse wave. Therefore, it is unreasonable to take the zero crossing of a pulse wave as its starting phase.
3. Finally, someone finally came up with a reluctant definition of power factor as the ratio of active power to reactive power.
PF = active power / reactive power
Although in the linear system of sine waves, if this definition is used, it is at least the same as the absolute value of Cosφ in absolute value. Active power is the projection of current onto the voltage axis and multiplication by the voltage value. The reactive power is simply multiplying the current vector and the voltage vector without considering the phase difference. Obviously, in a linear system, the absolute value is consistent with the absolute value of Cosφ, but the ratio is not signed.
Moreover, how to define active power and reactive power in a nonlinear system is also a big problem.
Most of them now use the following formula:
among them
Phase factor As for what is the phase factor, no one answered. Because this phase is not available.
And if this definition is used in a nonlinear system, the sign of the power factor Cosφ is lost because the power has no negative power. Since all digital power factor meters use this definition, the result is lost. The power factor without a sign also loses the fundamental meaning of the power factor!
3. The measured power factor
Let's take a look at the power factor situation in the real world. It is because the apparent power is difficult to define and difficult to measure, so the results measured by different instruments are different. For example, there is an 11-watt bulb that uses a bridge rectifier plus a 10uF electrolytic capacitor and is measured using three instruments. The results are as follows:
The test error of the three instruments is up to 8% or more, which is not as accurate as the measured voltage and current measurement power!
Such a large error, if the PF is required to be greater than 0.5, then only use the power factor meter of Tonghui, but if the tester must insist on using the remote test data, then it is not qualified. This result is simply making people feel at a loss.
If you want to think about it, you should use the most classic Cosφ meter that is officially certified in the electrician system. The result is the most authoritative. And the Cosφ tester is not like a digital tester, it can also get a sign.
Four-point power factor meter
A careful search reveals that most of the power factor meters used in the official power system are pointers. This power factor meter is also called the Cosφ meter. There are single-phase and three-phase. The rotary coil of this electric moving coil type power factor meter is changed to two vertical moving coils. The magnetic field of the meter is generated by the current in the load circuit. The vertical moving coils are A and B respectively. The A coil is connected in series with the load line and the B coil is connected in series with the load line. Therefore, the current of the B coil is lower than that of the A coil. When the power factor is 1, the current of the A coil will be in phase with the load current, so the A coil will produce the maximum torque, so that the pointer of the power factor meter points to the scale of 1.0. If the power factor is 0, the current of the B coil will be in phase with the load current, so the B coil will generate a torque, so that the pointer of the power factor meter points to the position of 0. If the power factor is between 0 and 1, the position of the last pointer is determined by the magnitude of the torque generated by the two coils. Its appearance is as follows.
Pointer power factor meter
Connection diagram during testing
We use a digital and analog power factor meter for a 102W LED light engine. The test results are as follows:
Light engine rectifier (using 124uF electrolytic capacitor)
102W light engine with constant current source
The results obtained with digital power and power factor meters are quite different from those measured with a pointer power factor meter.
Digital measurement PF = 0.6590
Pointer type measured PF = +0.9
This result is very surprising, because you don't need any power factor compensation to get a power factor of +0.9! And this is the most authoritative result and should be officially recognized.
V. Conclusion
In fact, there are many controversies in the international definition of the power factor definition of nonlinear systems. It is said that there are a total of seven definitions. It is no wonder that this issue is also the subject of a master's thesis in the United States, and is also being studied as a doctoral thesis in Sweden. In China's Baidu academics, 3135 papers can also be found to study this issue. The strange thing is that for such an internationally undetermined indicator, it is officially determined as an indicator that must be strictly enforced, and the more energy-efficient lamps are demanding! I really don't know what these officials think! If true This is the case, then I can only see that there are countermeasures under the policy! It is recommended that everyone use the pointer power factor meter to get the power factor value that the official can't deny!
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