LED lamp power

Active power refers to the actual output power, and apparent power refers to the product of the input voltage rms value and the input current rms value. This is perfectly equivalent to Cosφ in a sine wave system, so there is no problem. However, in a nonlinear system, what is the active power and the apparent power are worth exploring. Because in the non-linear system, its current waveform has a lot of higher harmonics, so in the end what to take as its apparent power is a big problem. There are various practices now. LED will go out a strict route, that is to say, five-watt light is like a child less than one-meter-two does not need to buy a ticket on the train, as a requirement, more than 5W must require a power factor of > 0.7. The LED lamps except for the small MR16 spotlights are 3 watts, the vast majority are more than 5 watts. So this rule just stuck in the neck of the LED.

The rectified voltage and current waveforms are not sine waves, and although the voltage waveform before rectification is a sine wave, the current waveform is not a sine wave. So the entire system is a nonlinear system. The original power factor is defined for a linear system, and the input and output voltage and current are all sinusoidal at the same frequency, otherwise Cosφ cannot be used. However, in non-sinusoidal systems, because the voltage and current waveforms are not sine waves, there is no phase angle to say. So the power factor in the nonlinear system must be redefined.

1. The effective value of the voltage is multiplied by the effective value of the current as the apparent power. Many digital power factor meters now use the product of voltage rms and current rms as the apparent power. But the definition of power must be the product of the voltage rms value of the same frequency sine wave and the current rms value. The product of the current higher harmonics rms value and the fundamental voltage rms value cannot be considered as power because the frequencies are not the same and therefore are meaningless numbers. So using this method to define the apparent power is problematic. Unfortunately, many digital meters are measured this way.

2. Take the cosine of the fundamental current phase as the power factor, or take the cosine of the phase of the zero crossing of the current waveform as the power factor, or multiply the effective value of the fundamental wave of the current and the rms value of the sine voltage as its apparent power. . Some instruments measure it this way. From the waveform of this current, it can be seen that the higher harmonics of this waveform are very rich, and the fundamental wave is very small. If the fundamental wave current is used to multiply the fundamental voltage, the obtained power is compared with the active power. Very small, so its power factor will be high and may even be greater than 1. This is the case, for example, in some pointer-type power factor meters.

3. In nonlinear loads, the biggest problem is the harmonic current, because although the harmonic current cannot form the apparent power with the fundamental voltage, the square of the harmonic current multiplied by the line resistance causes heat loss. Moreover, this harmonic current cannot be compensated with a simple capacitor or inductor. So what really needs to be limited is the harmonic current value. Not the so-called "power factor."

In fact, this issue has always been controversial in the academic community, so both the master's thesis in the United States and the doctoral thesis in Sweden are still studying this issue. For example, Stefan Svensson of Sweden pointed out in his doctoral dissertation that in the case of non-linearity, seven different definitions have now been proposed for power factor, and that a nonlinear system may be obtained under different definitions. Different power factor values. And no matter what kind of definition it is, it does not meet the original intention of presenting the power factor in the linear system. E.g. In a linear system, a pure or pure inductor can be used to compensate for inductive or capacitive loads. This is obviously ineffective in nonlinear systems. So these defined power factors completely lose the meaning of the original power factor.