My question is why do PFC circuits cost so much? How do they work.

With a purely inductive and resistive load, you can solve the problem with capacitance - and vice versa. My guess is that the problem gets more expensive when the load deviates from a sine wave. It may be more in design than in components. This business is customer driven. The more we ask for it, the harder they'll think about delivering it.

John’s guess is correct. It is when the voltage waveform induced by the load deviates from a sine wave that power factor correction gets expensive. For instance, in the strictest sense magnetic HMI ballasts are “Power Factor Corrected.” In order to understand what I mean, it would help to understand some basic electrical engineering principles. If you haven't already, I would suggest you read the article I wrote for our company newsletter on the use of portable generators in motion picture lighting. In it I cover some of the basic electrical engineering principles behind harmonic distortion and how it can adversely effect generators. The article is available on our website.

Here is a much simplified explanation of power factor and why it is necessary in HMI & Kino ballasts. With a purely resistive AC load (Incandescent Lamps, Heaters, etc.) voltage and current waveforms are in step (or in phase), changing polarity at the same instant in each cycle ( a high power factor or unity.) With “non-linear loads” (magnetic and electronic HMI & Fluorescent ballasts) energy storage in the loads, impedes the flow of current and results in a time difference between the current and voltage waveforms – they are out of phase (a low power factor.) In other words, during each cycle of the AC voltage, extra energy, in addition to any energy consumed in the load, is temporarily stored in the load in electric or magnetic fields, and then returned to the power distribution a fraction of a second later in the cycle. The "ebb and flow" of this nonproductive power increases the current in the line. Thus, a load with a low power factor will use higher currents to transfer a given quantity of real power than a load with a high power factor

As John correctly surmises, basic power factor correction brings the voltage and current waveforms back in phase (closer to unity power factor) by supplying reactive power of opposite sign, adding capacitors or inductors which act to cancel the inductive or capacitive effects of the load, respectively. For example, the inductive effect of motor loads may be offset by locally connected capacitors. If a load had a capacitive value, inductors are connected to correct the power factor. In the electricity industry, inductors are said to consume reactive power and capacitors are said to supply it, even though the reactive power is actually just moving back and forth on each AC cycle. The make up of a magnetic HMI ballast is very similar to an electric motor and hence, like an electric motor, has an inductive effect on the power supply. Between the power input and the HMI lamp is a transformer that acts as a choke coil. The transformer provides the start-up charge for the igniter circuit, rapidly increasing the potential between the electrodes of the head’s arc gap until an electrical arc jumps the gap and ignites an electrical arc between the lamp electrodes. The transformer then acts as a choke, regulating current to the lamp to maintain the pulsating arc once the light is burning.

Essentially a large coil of wire that is tapped at several places to provide for various input voltages and a high start-up voltage, the transformers of magnetic HMI ballasts exhibit high self-inductance. Self-inductance is a particular form of electromagnetic induction characteristic of coils (like those in magnetic HMI ballasts and electric motors) that inhibits the flow of current in the windings of the coil. This opposition to the flow of current is called inductive reactance. In the case of a magnetic HMI ballast, the multiple fine windings of the ballast transformer induces appreciable voltage and considerable current that is in opposition to the primary current, causing the primary current to lag behind voltage, a reduction of current flow, and an inefficiency in the use of power supplied by the generator. Put simply, the ballast draws more power than it uses to create light. As John Sprung' suggests, the addition of capacitors will compensate for the high inductance of the transformer and bring the current partially back in phase with the voltage. For this reason a bank of capacitor is typically included in the design of magnetic HMI ballasts as a power factor correction circuit. In this sense magnetic ballasts are power factor corrected.

If, in the case of a magnetic ballast, you were to measure the current (using a true RMS Amp Meter) and voltage (using a Volt Meter) traveling through the cable supplying the magnetic HMI ballast and multiply them according to Ohm’s Law (W=VA) you would get the “apparent power” of the ballast. But, if you were to instead, use a wattmeter to measure the actual amount of energy being converted into real work (light) by the ballast, after the applied voltage overcomes the induced voltage, you would get the “true power” of the ballast. The ratio of “true power” to “apparent power” is a measure of the “power factor” of the ballast and is expressed by a number somewhere between 0 and 1. Where a typical 1200W magnetic HMI ballast takes 13.5 Amps at 120 Volts to generate 1200 Watts of light the power factor is .74 (13.5A x 120V= 1620W, 1200W/1620W= .74). The favorite analogy electricians like to use to explain power factor is that if apparent power is a glass of beer, power factor is the foam that prevents you from filling it up all the way. When lights with a low power factor are used, a generator must be sized to supply the apparent power (beer plus foam), even though only the beer (true power) counts as far as how much actual drinking is possible.

By comparison to magnetic HMI ballasts, electronic HMI ballasts are quite a bit more complicated. In an electronic HMI ballast, AC power is first converted into DC. Then, a high-speed switching device (micro processor controlled IGBTs) turns the flat current into an alternating square wave. Hence, they are commonly referred to as square wave ballasts. Electronic square wave ballasts utilize solid state electronic components (rectifiers, capacitors, and IGBTS) which use only portions of the input power sine wave. Put simply, they place all their load on the peak values of the power waveform. These devices then return the unused portions to the power stream as harmonic currents. These harmonic currents stack on top of one another creating harmonic distortion that likewise creates an opposition to the flow of current, pulls the voltage and current out of phase, and when the power is supplied by a generator can lead to severe distortion of the voltage waveform in the power distribution system.

For example, the power waveform below left (from my article) is typical of what results from the operation of a 2500W non-Power Factor Corrected load (electronic HMI & Kino ballasts) on a conventional portable generator (a Honda EX5500 with a Barber Coleman Governor.) The severe harmonic noise exhibited here can cause overheating and failing equipment, efficiency losses, circuit breaker trips, excessive current on the neutral return, and instability of the generator's voltage and frequency

*Left: Conventional generator power w/ pkg. of non-PFC Elec. HMI Ballasts & Kino Flo Wall-o-Lite. Right: Inverter generator power w/ Pkg. of PFC Elec. Ballasts & Kino Flo Parabeam 400.*

The opposition to the flow of current caused by harmonic distortion is called capacitive reactance. Capacitive reactance acts on the waveform in a way opposite to inductive reactance. It causes current to lead voltage. Since an electronic ballast also puts current and voltage out of phase with one another, it also has a power factor. An electronic square wave HMI ballast typically has a power factor less than .6, meaning the ballast has to draw 40 percent or more power than it uses. Where a typical 1200W non-power factor corrected electronic HMI ballast takes 18.5 Amps at 120 Volts to generate 1200 Watts of light the power factor is .54 (18.5A x 120V= 2220W, 1200W/2220W= .54).

When using a lighting package with low power factor (like the pkg. of non-PFC electronic HMI & Kino ballasts depicted above), the conventional wisdom in the past has been to not load the generator beyond 75% for more than a short period. Where the maximum recommend continuous load on a 6500W generator is 5500W, the de-rated continuous load rating would be roughly 4000 watts. By de-rating the load capacity in this fashion, the Gaffer minimizes the adverse effects of high THD so that both the generator and the loads placed upon it operate more reliably. However, this conventional wisdom no longer holds true if the HMI & Kino ballasts are power factor corrected and powered by an inverter generator. For example, the power waveform above on the right, is the same 2500W load but with power factor correction operating on our modified Honda EU6500is Inverter Generator. As you can see, the difference between the resulting waveforms is startling. Even though the load is the same, the fact that it is power factor corrected, and the power is being generated by an inverter generator, results in virtually no power waveform distortion. What this means is that an inverter generator can be loaded to capacity with PFC HMI and Kino Flo ballasts. The substantial reduction in line noise that results from using PFC ballasts on the nearly pure power waveform of an inverter generator creates a new math when it comes to calculating the continuous load you can put on a portable gas generator (in the case of our modified Honda EU6500is generator a capacity of 7500 Watts.)

Since power factor correction can be of tremendous benefit when operating HMIs and Kinos on portable gas generators, it is worth understanding in more detail. Since, as exhibited in the waveform above, capacitive reactance distorts the shape of the voltage waveform from a sine wave to some other form, the addition of linear components such as inductors cannot counteract the harmonic currents as the addition of capacitors counteracted the inductive reactance of magnetic HMI ballasts. In the case of electronic ballasts, other more complicated (translate expensive) means of power factor correction is required to smooth out the power waveform they induce.

To understand how power factor correction circuits work in electronic HMI ballast it helps to understand the source of the harmonic currrents. The harmonic currents produced by electronic HMI ballasts are primarily generated by the diode-capacitor section of the ballast. As you may recall from our discussion above, the diode-capacitor section rectifies the AC input power into DC, which is then used by the power module to create the square wave. The diode-capacitor section accomplishes this by first feeding the AC input current through a full wave bridge rectifier, which inverts the negative half of the AC sine wave and makes it positive. The rectified current then passes into a bank of capacitors which removes the 60 Hz rise and fall and flattens out the voltage-making it essentially DC. The required DC is then fed from these capacitors to the power module where the IGBTs switch it into an alternating square wave.

(ILLUSTRATION COURTESY OF HARRY BOX)

*Step 1: Rectifier Bridge converts AC power to rectified sine wave. Step 2: capacitors flatten the rectified sine wave to DC. Step 3: micro processor switching alternates polarity of DC creating an AC square wave. *

The source of harmonics currents lies in the rectifying circuit of the diode-capacitor section of the ballast. The rectifying circuit only draws current from the AC line during the peaks of the supply voltage waveform, charging the capacitors to the peak of the line voltage. Since the capacitors only charge when input voltage is greater than its stored voltage, a non-PFC circuit only charges the capacitors for a brief period of the overall cycle time. After 90 degrees, the half cycle from the bridge drops below the capacitor voltage; which back biases the bridge, inhibiting further current flow into the capacitor. During this brief charging period, the capacitors must be fully charged, requiring large pulses of current to be drawn for a short duration. As can be seen in the illustration below, electronic ballasts draw current in high amplitude short pulses. The remaining unused current feeds back into the power stream as harmonic currents.

(ILLUSTRATION COURTESY OF FAIRCHILD SEMICONDUCTOR)

*Thin Black Trace: Rectifier Bridge converts AC power to rectified sine wave. Thick Black Trace: Stored Capacitor Voltage. Red Trace: Current drawn by capacitors once input voltage is greater than voltage stored in the capacitor (thick black trace.) *

Notice how big the input current spike (red trace) of the rectifier circuit is. All the circuitry in the ballast as well as the supply chain (the generator plant, distribution wiring, circuit breakers, etc) must be capable of carrying this high peak current. In order not to have these high amplitude current pulses, the capacitors in the diode-capacitor section of the ballast must charge over the entire cycle rather than just a small portion of it. The power factor correction circuitry of electronic HMI ballasts use a multi-stage boost converter topology to accumulate energy in the capacitors over the entire cycle, which averages out the peak load, and greatly reduces the huge peak current. In the “Active Power Factor Correction” circuits used in electronic square wave ballasts, a boost converter is inserted between the bridge rectifier and the main input capacitors. The boost converter maintains a constant DC bus voltage on its output while drawing a current that is always in phase with and at the same frequency as the line voltage. Another switch mode converter produces the desired output voltage from the DC bus. Where the input to the converter is the full-rectified AC line voltage, the output voltage of the boost converter is set higher than the peak value (hence the word boost) of the line voltage (a commonly used value is 385VDC to allow for a high line of 270VACrms.)

Now that the capacitors charge throughout the AC cycle rather than just a brief portion of it, harmonic currents are not generated. And, with the boost pre-converter voltage higher than the input voltage, the load is forced to draw current in phase with the AC main line voltage. In this fashion, the PFC circuit realigns voltage and current and induces a smoother power waveform at the distribution bus. PFC circuits successfully increase the power factor to as much as .9, making ballasts with it near linear loads. As a result, the ballast uses power more efficiently with minimized return current and line noise and also reduces heat, thereby increasing their reliability.

If you still don’t entirely understand how power factor correction works in electronic HMI ballasts, I would suggest you read the article I wrote for our company newsletter on the use of portable generators in motion picture lighting. In it, is a more detailed explanation of the basic electrical engineering principles behind harmonic distortion and how it can adversely effect generators. The article is available on our website.

Guy Holt, Gaffer, ScreenLight & Grip, Boston