The importance of power factor to LED lamps

1. Power factor

The power factor characterizes the ability of a lamp to output active power. Power is a measure of the rate of energy transfer. In a DC circuit, it is the product of voltage V and current A. In an AC system, it is more complicated: that is, part of the AC current circulates in the load without transferring electrical energy. It is called reactive current or harmonic current, which makes the apparent power (voltage Volt multiplied by current Amps) greater than the actual power. The difference between apparent power and actual power leads to the power factor, which is equal to the ratio of actual power to apparent power. Therefore, in an AC system, actual power is equal to apparent power multiplied by power factor. That is: power factor = actual power/apparent power. Only linear loads such as electric heaters and light bulbs have a power factor of 1. The difference between actual power and apparent power of many devices is very small and can be ignored, while the difference is large and important for capacitive devices such as lamps. A recent study by PC Magazine in the United States showed that the typical power factor of lamps is 0.65, that is, the apparent power (VA) is 50% greater than the actual power (Watts)!

2. Apparent power

Apparent power: the product of AC voltage and AC current. It is expressed by the formula: S=UI. In the formula, S is the rated output power, the unit is VA (volt-ampere); U is the rated output voltage, the unit is V, such as 220V, 380V, etc.; I is the rated output current, the unit is A. Apparent power consists of two parts: active power (P) and reactive power (Q). Active power refers to the part that does work directly. For example, it makes the light light up, the motor rotate, and the electronic circuit work. Because this power turns into heat after doing work and can be directly felt by people, some people have an illusion that active power is the apparent power, but they don’t know that active power is only a part of the apparent power, which can be expressed by the formula: P=Scosθ=UIcosθ=UI?F. In the formula, P is the active power, the unit is W (watt); F=cosθ is called the power factor, and θ is the phase difference when the voltage and current are out of phase when the nonlinear load is in nonlinear load. Reactive power is the power stored in the circuit but not directly doing work, expressed by the formula: Q = Ssinθ = UIsinθ. In the formula, Q is reactive power, the unit is var (var).

3. Reactive power

For lamps and other electronic circuits that rely on DC voltage, they cannot work without reactive power. Most users think that lamps and other equipment only need active power, not reactive power. Since reactive power does not do work, what is the use of it? So of course they think that lamps with a power factor of 1 are the best, because they can give the maximum output power. However, this is not the case in reality.

假如有一灯具，当交流市电输入后进行整流，就得到脉动直流电压，若不将脉动电压进行任何加工，就直接提供给灯具，毫无疑问，电路根本无法正常工作。虽然这时灯具的功率因子接近于1，可这又有何用呢。为了让灯具电路能正常工作，必须向其提供平滑了的直流电压。这个“平滑”工作必须由接在灯具整流器后面的滤波电容器来完成。这个滤波器就像一个水库，电容器里面必须储存足够数量的电荷，在整流半波之间的空白时，使电路上的工作电压仍不间断，能保持正常电平。换句话说，即使在两个脉动半波之间无输入电能时，Uc的电压电平也无显著的变化，这个功能是靠电容器内的储能来实现的，储存在电容器内的这部分能量就是无功功率。所以说，灯具是靠无功功率的支持，才能保证电路正确运用有功功率实现正常使用的。因此可以说，灯具不但需要有功功率，也需要无功功率，两者缺一不可。