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Home » Power
Everything You Need to Know About Power Supplies
Author: Gabriel Torres 671,771 views
Type: Tutorials Last Updated: May 29, 2008
Page: 10 of 14
Power Factor Correction (PFC)

All equipment with motors and transformers, such as the power supply itself, use two types of power: active (measured in kWh) and reactive (measured in kVArh). Active power produces the real work, for example, a motor axe rotation. Reactive power (also called magnetizing power) is the power required to produce the magnetic fields to enable the real work to be done on transformers, motors, etc. The vector sum of the reactive power and the real power components is called apparent power and is measured in kVAh. For the industrial customer, the electrical power utilities measure and charge based on the apparent power, but for residential and commercial customers, the measured and charged power is the active power.

The problem is that, although it's necessary for motors and transformers, the reactive power “occupies space” on the system which could be used by more active power.

Power factor is the ratio between the active power and the apparent power of a circuit (power factor = active power / apparent power). This ratio can vary from 0 (0%) to 1 (100%), and the nearest to 1 this factor is, the better, because it means that the circuit is absorbing less reactive energy.

In order to optimize reactive power consumption, many countries have established through legislation the maximum reactive power percentage to be consumed by users. If the customer has a power factor inferior to the value set up by the government (i.e., the reactive power is above the limit set up by law), the customer will pay a penalty.

The penalty concept exists to force industry to improve its power factors in order to prevent them from using more reactive power. As we have already mentioned, this type of power overloads the system with an energy type which is not effectively used, but it's necessary to make motors and transformers able to operate.

Generally, this improvement includes checking if there are no motors or transformers operating “in blank” or over dimensioned. The reactive power necessary for operating in “peak load” is almost the same necessary for operating in a lower load. That is, if a motor operates with a lower load, it consumes less active power, but its reactive power consumption is almost the same as if it were operating in peak load, resulting in a low power factor. Other matters usually discussed are: if the network current level is above specifications and if the fluorescent lamps (which need a reactor, a type of transformer) use power correction circuits and also the installation of capacitors to correct the power factor (power correction circuits, our next issue) of the electrical system.

Many countries are starting to adopt legislation which enforces end user-oriented electroelectronic equipment manufacturers to respect power factor, as well as it's demanded from industrial customers. As of January, 2001, the European Union started to require that all electroelectronic equipment sold in the country with power exceeding 70W have power factor correction circuits, so as to consume the least possible reactive power of the electrical system. It is expected that other countries start taking the same measures.

For this reason, the power supply manufacturers who wished to sell to Europe as of year 2001, had to start producing power supplies with power factor correction circuits, which are called power factor correction or simply PFC.

There are two types of power factor correction circuits: passive and active. Passive PFC uses components that don't need power to operate (such as ferrite core coils) and fits the power factor between 0.60 (60%) and 0.80 (80%). Active PFC uses electronic components such as integrated circuits, transistors, and diodes, and per manufacturers’ statement, it's able to generate a power factor of over 0.95 (95%). Power supplies with no power factor correction circuits have a power factor below 0.60 (60%).

The power correction is not related to efficiency. This is the most common mistake we see on the market; the PFC circuit doesn't make your computer consume less electricity. As we have already explained, the PFC's function is to prevent the power supply from consuming more reactive power from the electrical system, resulting in electrical network optimization (allowing the utility to provide more active power). The insertion of this type of circuit was created in order to fulfill legislation demands regarding electricity consumption, particularly European legislation. Because adopting the same legislation is the tendency in other countries, manufacturers are preparing themselves by producing power supplies with this type of circuit.

Honestly, there is no advantage for the end user having or not a power factor correction circuit (PFC). Saying that a power supply with this type of circuit is better is a marketing move of power supply manufacturers to persuade customer to by a more expensive power supply. In fact, this type of power supply is better for the electrical power utility, which will need to provide less reactive power, which overloads the system. But for end-user, there is no difference, because, at least for now we are not being overcharged in case of our reactive power consumption exceeds a fixed level, as it happens with industrial customers. Neither non-industrial user is charged by the electric power utilities for using this type of power.

In practical terms, a power supply with PFC basically means that the manufacturer can sell it in Europe.

As we mentioned earlier, a side effect of power supplies with active PFC is that they are “auto range,” not requiring you to select the input voltage through a 110 V/220 V switch.

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