What is power factor for LED driver?

When AC is overloaded, the AC voltage applied to the load and the AC passing through the load has a phase difference, from which people derive the concept of power factor. People's production and living electricity comes from the power grid, which provides AC with a frequency of 50Hz or 60Hz.

There are three types of AC loads: resistance, inductance, and capacitance.

When AC passes through a pure resistance load, the AC voltage applied to the resistor and the AC passing through the resistor are in phase, that is, the phase angle between them is ф= 0°. At the same time, active power is consumed on the resistance load, and the power grid needs to supply energy.

When AC passes through a pure inductive load, the phase of the AC voltage on it leads the phase of the AC by 90°, and the angle between them is ф= 90°, generating reactive power on the inductive load. The electric energy supplied by the power grid is converted into magnetic field energy in the inductor for short-term storage and then fed back to the grid to become electric energy. This cycle repeats endlessly. As a result, the power grid does not supply energy, so it is called "reactive power", but the "reactive current" that generates "reactive power" still exists.

When AC passes through a pure capacitive load, it is similar to this, except that the phase of the AC voltage on it lags the phase of the AC by 90°, and the angle between them is ф= - 90°. Here, the phase angle lead is positive and the phase angle lag is negative. The actual load is a composite of three types: resistance, inductive reactance of inductance, and capacitive reactance of capacitance. The composite is collectively called "impedance", which can be written as a mathematical formula: impedance Z=R+j(XL–XC). R is resistance, XL is inductive reactance, and XC is capacitive reactance. If (XL–XC) > 0, it is called "inductive load"; conversely, if (XL–XC) < 0, it is called "capacitive load".

When AC passes through an inductive load, the phase of the AC voltage leads the phase of the AC (0°<ф<90°);

when AC passes through a capacitive load, the phase of the AC voltage lags the phase of the AC (-90°<ф< 0°);

electrical engineering defines this angle ф as the power factor angle, and the cosine value of the power factor angle ф, i.e. Cosф, is called the power factor.

For a resistive load, the phase difference between the voltage and current is 0°, so the power factor of the circuit is 1 at its maximum (Cos 0°=1);

while for a pure inductive circuit, the phase difference between the voltage and current is 90°, and the voltage leads the current;

in a pure capacitive circuit, the phase difference between the voltage and current is -90°, i.e. the current leads the voltage. In the latter two circuits, the power factor is zero (Cos 90°= 0). For general load circuits, the power factor is between 0 and 1.

According to the mathematical formula impedance Z=R+j(XL–XC), if XL=XC, then Z=R, that is, impedance Z becomes a pure resistor, and the power factor is equal to 1. That is to say, inductive loads and capacitive loads can compensate for each other. If the inductive reactance value of the inductive element in a circuit is exactly equal to the capacitive reactance value of the capacitive element, it can be completely compensated. The method of power factor compensation originates from this.

 When AC passes through an impedance load, the total power S generated is called "apparent power". Apparent power S includes two components: active power P and reactive power Q. Among them, active power P = S*Cosф, reactive power Q = S*Sinф. Only when the power factor Cosф value is equal to the maximum value 1, that is, ф= 0°, the reactive component Q is equal to zero, and the active power P is equal to the value of apparent power S.

 However, the actual working capacity of the load is only related to the active power. For example, the cooling capacity of the air conditioner and the illumination of the lamp are only proportional to the active power. Therefore, people certainly hope that the power factor is higher. II. The harm of low power factor

1) The load-carrying capacity of the power supply equipment is discounted, that is, the load-carrying capacity is reduced.

 For example, if a certain device can supply 100KVA of apparent power, if the power factor is 0.7, it can only supply 70KW of active power; if the power factor is 0.9, it can supply 90KW of active power. It can be seen that improving the power factor is very meaningful.

2) The transmission line increases the loss of the transmission line due to the existence of reactive current.

For example, if the power factor is 0.7, to supply 70KW of active power, it is necessary to supply 100KVA of apparent power. The current of the transmission line increases, and the line loss will inevitably increase.

3. Power factor compensation method The power supply department calculates the power supply energy based on "apparent power", but the electricity fee is calculated based on "active power". The user's "watt-hour meter" is an "active power meter". There is a "power factor" discount between the two, so the power factor is data that the power supply department cares about very much. If the user does not achieve the ideal power factor, it is relatively consuming the resources of the power supply department. At present, the power factor regulation in China must be between 0.9 and 1 inductance.

The following methods can be used to compensate for the power factor:

1) Semi-centralized and centralized compensation methods require that each power distribution room of the power-consuming enterprise must be equipped with a power factor automatic control device to detect the power factor in real-time, automatically put in or cut off the number of compensation power capacitors for motor operation compensation (because the main power load of the enterprise is the motor), so that the power factor of the local power network meets the standard.

2) This method has been enforced since the late 1970s and early 1980s, and it has been at least 20 years. In addition, each power supply station also installs a power factor automatic control device to further compensate for the power supply area under its jurisdiction. Distributed compensation method requires that each electrical appliance adopts advanced technology when designing to meet the power factor standard so that the power factor can be guaranteed to meet the standard no matter when and where electricity is used. However, doing so will increase the cost and the size of the appliance, and some appliances have strict restrictions on the size, which increases the difficulty of design. Fourth, electric light source lighting fixtures and power factor compensation The electric light source started with incandescent bulbs, which are pure resistive loads and have no power factor compensation problem.

3) After the 1950s, fluorescent lamps quickly became popular and became the main lighting fixture. The ballast uses silicon steel sheet inductors, which have high reliability and long life. There are still a small number of them used today, and most of them have no power factor compensation measures. It may be affected by cost factors, or people do not understand power factor compensation very well and have a weak awareness of energy saving. There are also capacitors with appropriate capacity for power factor compensation, which are mostly used in 30W and 40W high-wattage fluorescent lamps and are rarely used below 20W.

4) Since the 1990s, people's awareness of environmental protection and energy saving has increased, and three-primary-color fluorescent powder energy-saving lamps have been developed, which have higher light efficacy. Electronic ballasts were also introduced later, and the energy-saving effect is more significant when equipped with three-primary-color fluorescent powder lamps. Some integrated circuit manufacturers at home and abroad have launched lamp chips with active power factor compensation for electronic ballasts. They have excellent performance, but they increase the cost and volume of electronic ballasts. Ordinary people cannot accept their prices, and they are only used in high-end lighting products. A large number of popular electronic ballasts, including those for energy-saving lamps, do not add any power factor compensation measures, which can be seen everywhere in popular energy-saving lamps and fluorescent lamps on the market. In other words, there were no power factor compensation measures for lamps in the past, but everyone was using them. V. Power Factor and LED Lighting LED consumes less power, and the power of lamps is even smaller than that of energy-saving lamps. LED lighting is of course more advanced and more suitable for environmental protection, energy saving and emission reduction.

The author's opinion on whether LED lamps should add power factor compensation is:

1) According to expert analysis, LED is a capacitive load. There are many inductive loads in the power grid, such as motors, transformers, etc. It is often necessary to connect capacitive loads for compensation, and the power factor automatic control device is used for this purpose. LED is a capacitive load, which just compensates for the problem of low power factor caused by many inductive loads in the power grid, which is just the right use. Based on this understanding, the author believes that LED lighting fixtures do not need to add power factor compensation measures in principle.

2) Single LED lamps used for indoor lighting are all low-power, and the power will not exceed 30W. The low power of the lamp has little impact on the power grid. The author believes that this type of lamp can completely avoid power factor compensation measures. Adding it is not good, but it will lose the function of LED lamps as capacitive loads that can compensate for the low power factor caused by many inductive loads in the power grid. These low-power lamps are mostly small and compact, and the internal space is very limited. For example, MR16, PAR30, and PAR38 lamp cups cannot be placed after the power PCB board is enlarged, so even if you want to add power factor compensation measures, you can't add them. There is also the side effect of reduced efficiency after adding power factor compensation, which may not be worth the loss. In addition, the increased cost affects sales. Moreover, the power supply department has taken countermeasures to compensate for the power factor of the power grid, so the lighting manufacturers do not need to add unnecessary measures.

3) For power above 100W, you can consider adding power factor compensation measures. The impact of large power loads on the power grid is also large, such as LED street lights of 100 watts to hundreds of watts. Streetlights are public welfare undertakings, and a slight increase in cost is not a big deal. There is also room for the power PCB board to be enlarged. Adding power factor compensation measures can help the power supply department reduce some of the regulation burden and prevent excessive compensation caused by excessive capacitive loads.