Power Factor What Is It and Why Correct It?

Power Factor What Is It and Why Correct It?


Several MCL TWT amplifiers feature something known as Active Power Factor Correction, a term which is probably unfamiliar to some customers. This technical brief is intended to explain what it is and what advantages it provides to our customers and to MCL.

What is Power Factor?

In alternating current circuits, voltage, current and power all vary as functions of time, yet measured values are expressed as single numbers. For power, the simple average over time is referenced while voltages and currents are expressed as RMS values. The RMS value, which can be directly measured along with average power, is simply the equivalent direct current which would produce the same heating effect in a resistive load. It will be found, in general, that the product of voltage and current can be greater than the power; thus resulting in the concept of power factor, which is defined as the power in watts divided by the volt-ampere product.

Two principle reasons why the power factor can be less than unity are that the voltage and current may be displaced in time, which is a phase difference, or that the current may contain harmonics not present in the voltage. Harmonics are multiples of the power line frequency, which when present, cause the current wave shape to differ greatly from a sine wave. In some cases, the instantaneous power will be negative over part of the cycle. This represents stored energy being returned to the source, rather than as a loss, as commonly believed. We can also look at it in a different way in that the RMS current is in excess of that needed to account for the power. It would appear that there is little cause for concern over power factor, yet there are advantages to improving it.

Practical Advantages of Improving Power Factor

First, the excess current is using up additional current capacity of wiring, breakers and fuses. Lowering current could avoid the cost of installation of wiring of higher current capacity. The second reason is that third harmonic currents in a three-phase utility system all add in phase to produce high neutral currents. This is a situation that makes most utility companies unhappy to the point where power factor correction is required by law in Europe, and may soon be required worldwide.

Active Power Factor Correction

The main concern with some previous amplifier designs was that many used rectifiers with capacitor input filters, which are notorious for drawing large line current harmonics. So when addressing power factor concerns during the engineering of MCL's products, a unique electronic active power factor correction method was developed. This eliminated the use of the bulky passive filters which operated at power line frequencies and resulted in a significantly smaller and lighter method to address power factor correction.

The basic principle of active power factor correction is to derive an analog estimate of the input current as a function of time. This estimate is based on average input voltage, the instantaneous input voltage and the output voltage of the Power Factor Correction (PFC) circuit. The analog signal, so derived, is used as the reference for circuits which tightly control the input current. The analog reference in constantly revised--resulting in the correction of the input current as often as 200,000 times per second. In each increment of time, a small amount of energy is stored in inductors then discharged into a large capacitor bank in a manner in which the input current is shaped while the output voltage is fairly constant. Due to the high speed, there is little difficulty in making the current wave form follow that of the input voltage.

The Prime Power Converter (PPC) Module, as it is called, has several outputs. These are 48 volt blower power, auxiliary low voltage to other modules, and drive for a high frequency filament supply. It also contains its own control and protective circuits and its own control power circuit, which is power factor corrected once in operation.

Advantages of MCL's Active Power Factor Correction

With active power factor correction, it is possible to accommodate a wide range of input voltages and frequencies at the input, while the output voltage is a well-regulated constant value. For MCL's customers this means that the amplifier is less sensitive to input voltage variation. For MCL it means there is only one universal input power option which simplifies both the operations and sales process. Inventory is reduced, while the possibility of assembly and testing errors is minimized. Since the PPC produces constant internal interfaces regardless of input voltage, there is no need to change blowers, auxiliary power supplies, breakers or transformer taps to accommodate different input voltages.

Active power factor correction is just one of the innovative features that MCL has incorporated into our amplifiers. Like active power factor correction, the benefits of some of these features are often unknown to the customer. However, together they make MCL's products the most advanced and innovative power amplifiers available to the satellite industry.

For more information, please contact us.