The ratio between these two types of loads becomes important as you add more inductive equipment. Working power and reactive power make up Apparent Power, which is called kVA, kilovolt-amperes.
A high PF benefits both the customer and utility, while a low PF indicates poor utilization of electrical power. Here is an example. Because Edisto Electric must supply both the kW and kVA needs of all customers, the higher the PF is, the more efficient our distribution system becomes. Improving the PF can maximize current-carrying capacity, improve voltage to equipment, reduce power losses, and lower electric bills. Post a Comment.
Knowledge is Power. Power is TDK-Lambda. Switchmode power supplies without Power Factor Correction PFC tend to draw the AC input current in short bursts or spikes relative to the line voltage, as shown in Fig.
The Power Factor of a power supply is technically the ratio of the real power consumed to the apparent power Voltsrms x Ampsrms and is a decimal between 0 and 1. If left uncorrected the Power Factor PF of switchmode supplies will generally be around 0. Figure 1. Input of switchmode power supplies without PFC. The voltage waveform is a sinewave and the current waveform is a pulse or spike. Figure 2. Voltage and Current waveforms are sinusoidal and in-phase. Newer Post Older Post Home.
Subscribe to: Post Comments Atom. The most frequent way of describing the amount of distortion present in a signal due to the presence of harmonics is through the magnitude of total harmonic distortion THD , which represents the proportion of harmonic current relative to the fundamental current. THD can be calculated using Equation 2 :. The effect of distortion on the total power factor, however, is using the distortion factor, which is related to the total harmonic distortion with Equation 3 :.
The product of the displacement factor and the distortion factor makes up the power factor, calculated with Equation 4 :. At a large scale, this causes significant heating loss in the grid, and could even cause a power outage. Understandably, power suppliers have put forward limitations into the amount of power interference a device can apply to the grid.
The first attempt to do this was in , with the start of electric lighting, when they realized that interference from other devices was making incandescent lights flicker. Then, in , a regulation from the International Electrotechnical Commission IEC was put forward to force the introduction of power factor correction in consumer products.
Since then, different countries have created their own guidelines and regulations on power factor limitations. In the United States, the voluntary Energy Star guideline states that any computing equipment must have a PF of at least 0. In the EU, the legislation IEC is more stringent, dividing electrical devices into four categories: appliances A , power tools B , lighting C , and electronic devices D.
Each category has specific limitations on the relative weight that each harmonic up to the 39th can have relative to the fundamental frequency. Figure 8 shows the waveform restrictions established by the IEC for class-C devices, both in the frequency domain and the time domain. As you can see, the maximum harmonic amplitude values in the frequency domain follow the shape of a square wave, which is confirmed by observing the resulting waves in the time domain.
Therefore, a good power factor correction circuit is necessary in any commercialized device in order to increase operation efficiency and to be able to sell the device as a consumer product. Power factor correction PFC is a series of methods that manufacturers of electronic devices use to improve their power factor. As previously mentioned, a low power factor is caused by the presence of displacement or distortion in the signal. The negative effect of displacement on the power factor is relatively simple to solve, because capacitors drag the phase forward, while inductors drive it back.
To do so, there are two options:. The boost converter has two stages of operation. In the first stage, when the switch is closed, the inductor is charged by the voltage source in this case, the voltage coming out of the rectifier.
When the switch opens, the inductor injects the current it had stored in the previous stage into the circuit, increasing the voltage at the output. This current also charges a capacitor, which is responsible for maintaining the voltage level at the output while the inductor recharges.
If the switching frequency is high enough, neither the inductor nor the capacitor ever fully discharge, and the load at the output always has a greater voltage than the input voltage source.
This is called continuous conduction mode CCM. The longer the switch is closed i. If the duty cycle the time the switch is on relative to the total switching cycle is properly controlled, the input current wave can be shaped into a sinusoid. There is another method that offers fewer switching losses and cheaper circuits, although it sacrifices final PF quality.
This method, called boundary conduction mode BCM or critical conduction mode, switches the transistor just as the inductor is completely discharged see Figure This is called zero-current switching ZCS , which allows the diode in the boost converter to change polarity faster and more easily, reducing the need for high-quality, expensive components.
The converter tracks the input voltage, so the output current looks like a sine wave with a frequency of 50Hz. However, this current waveform still looks very different from a pure sinusoid, so it will logically have a large number of harmonic components. Because these harmonic components are multiples of the switching frequency, which is much higher 50kHz to kHz than the 50Hz fundamental, they will be very effectively filtered out. This significantly increases the power factor, which is why some switching power supplies reach PF values of up to 0.
This IC also compares the current and the voltage, shaping the current peaks to follow the shape of the input voltage. However, the selection of the right PFC circuit means analyzing the different tradeoffs available. First, detect whether the origin of your low power factor is due to displacement or distortion. Then, depending on the amount of power in the circuit, select active or passive PFC.
0コメント