Understanding of power factor

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Understanding of power factor [Copy link]

People often ask under what load conditions should the power factor of the power inverter be measured, resistive, capacitive, or inductive? In fact, there is a big misunderstanding here. Ignoring this misunderstanding may lead to serious disagreements between inverter manufacturers and users.

The voltage and current of the alternating current used in daily life on a purely resistive load are in phase, that is, the phase difference q = 0°, as shown in the left figure of Figure 1; the relationship between the voltage and current of the alternating current on a purely capacitive load is that the current leads the voltage by 90° (q = 90°), as shown in the middle figure of Figure 1; the relationship between the voltage and current of the alternating current on a purely inductive load is that the current lags the voltage by 90° (q = -90°), as shown in the right figure of Figure 1.

Figure 1 Voltage and current relationship

The active power on the resistive load is the apparent power, that is, the two are equal, so the power factor λ=1. The current and voltage phase difference on the pure capacitive and pure inductive loads is 90°, so the power factor λ = cos90°=0, that is, the active power on the pure capacitive and pure inductive loads is zero.

From here we can see a problem, the same power supply, for different nature of the load its output power size and nature are also different, so it can be said that the nature of the load determines the output of the power supply. In other words, the output of the power supply does not depend on the power supply itself, just like the water supply flow of a water tower depends on the degree of opening of the faucet, as shown in Figure 2.

Figure 2 Water tower and faucet

From the above discussion, it can be seen that the power factor is a parameter that characterizes the nature and size of the load. And generally speaking, a load has only one nature, just like a person has only one ID number. This nature is determined from the input end of the load, which is called the input power factor of the load. Once a load circuit is completed, its input power factor is determined. For example, if UPS is used as the load of the mains or generator, such as a UPS with six-pulse rectifier input, its input power factor is 0.8. Regardless of whether it is a mains grid or a generator, if it requires an apparent power input of 100kVA, it needs to obtain 80kW of active power and 60kvar of reactive power from the power supply in front. If the input power factor of UPS is 0.6, it needs to ask the power supply in front for 60kW of active power and 80kvar of reactive power. For such output distribution, the power supply in front has no right to decide.

How is the load power factor characterized?

As we all know, a long time ago, the clothes people wore were made one-to-one by tailors according to each person's size. In modern times, due to the development of society and division of labor, many industries have emerged. For example, the clothing industry needs to make a variety of clothes in advance to meet social needs. The question is how big the clothes should be? What color clothes? This requires a plan in advance, and this plan comes from the social population. What size do most people wear? What style do they like? So various sizes are formulated, that is, so many codes. The same is true for UPS power supplies in the power supply industry. This is also a type of inverter. It cannot be manufactured one-to-one. It is also necessary to pre-manufacture one or several batches of machines with different power factors and power specifications according to the form and scale of current electrical appliances in advance, so as to prepare for spot sales in the market. The basis for pre-manufacturing one or several batches of UPS is the load power factor. When the load power factor of the UPS is equal to the input power factor of the load, it is called a perfect match, and the UPS can output all the power. Figure 3 shows the relationship between the UPS load power factor and the load input power factor.

Figure 3 UPS load power factor and output power factor

It is because some users mistake the load power factor for the "output power factor" of the equipment that they not only make a mistake in the relationship, but also derive a concept that does not exist at all. Since it is the "output power factor of the equipment", the output power of the equipment must obey this power factor value, that is, "a 100kVA device with a power factor of 0.8 should also give an output power of 80kW when carrying a linear load." If this misunderstanding is limited to individual users, it will at most lead to conflicts between users and suppliers. However, if the standard setters fall into this misunderstanding, the harm will be to equipment manufacturers across the country.