Specifying an AC Power Source A Tutorial

To properly specify an AC power source, you need to fully understand the nature of the load that is connected to it. Certain characteristics can greatly impact the sizing of the source, including the load’s power factor (PF) and whether it is nonlinear or linear.

Of course, it also is important to know the voltage range and worst-case current as well as inrush current. Six basic steps can help simplify your selection of an AC source. But first, here is some basic information on AC power sources and loads.

Understanding Power Factor

In an AC circuit containing reactive and resistive elements, not all of the apparent power (VA) is true power. Some of the power occurs in reactive form. PF is the ratio of true power to apparent power. Its value can range from 0 to 1 and is determined by the phase angle in the circuit—the difference in degrees between the current and the voltage in an AC circuit as shown in Figure 1.

The amount of true power will vary depending on whether the load is predominantly resistive, capacitive, or inductive, even if apparent current drain is the same. Most linear-type AC sources derate output power as PF decreases (Figure 2). This is partly because the source must dissipate more reactive power internally, and power devices tend to run hotter. Newer switching sources run cooler and tend not to derate VA as much, even with poor PF loads.

Simply stated, PF is the ratio of watts (W) or useful power output to the VA output. VA is the product of the output volts multiplied by the amps and measures the output capacity of a power source. For an AC power source to have a meaningful specification, it must state how much current it can deliver to a range of PFs.

Now that you have some basic information about your power source, here are the steps to walk you through the process of sizing an AC power source.

1. Determine the Type of Load You Have

It is important to understand the characteristics of your load. There are many different types of loads, but basically they can be classified as either linear or nonlinear. Table 1 gives examples of various load types.

Linear loads can be composed of any combination of inductance, capacitance, or resistance. In reality, most loads are a combination of all three. Linear loads draw sinusoidal current. The current waveform will be shifted either positively or negatively in relation to the voltage sine wave, depending on the reactive element of the load.

Nonlinear loads draw current in a nonsinusoidal fashion. Switching power supplies are notorious for drawing high pulse currents while motors and capacitive input loads draw high inrush currents.

Although a switching load and a resistive load may both draw the same current, such as 10 Arms, the switching load has a higher peak current, such as 30 A peak. In this example, this means that the switching load has a crest factor (CF) of 3:1 or three times the rms value.

2. Determine How Much Current the Load Draws

Typically, you can find the current rating on the product itself measured in amps. In some cases, the power may be displayed in watts or VA. You also can use an external DMM or power analyzer to acquire the needed data.

3. Determine the Output Voltage Range Required From the AC Power Source

Typically, you need to perform margin testing of the load over a certain range of voltages. Many products going to Europe must be tested from 187 to 264 VAC. Find out the worst-case current at the highest output voltage. This allows you to determine the maximum VA required by the AC power source to cover worst-case conditions.

4. Calculate the Power From the Voltage, Current, and PF

VA = voltage × current

W = VA × PF

PF = W/VA

VA = W/PF

PF is unity (1.0) with a pure resistive load and less than unity with a reactive load (either leading or lagging).

5. Consider the Capability of the Power Source to Drive Reactive Loads

Some AC sources derate their overall VA due to the reactive load. See Figure 2.

6. Size the Power Source

Here is a sample scenario. The load—a triple-redundant DC power supply used in a server with eight microprocessors—is being tested to ship overseas. Each power supply is 800 W and paralleled with isolation diodes providing triple-redundant protection for the server. The normal power delivered from the supplies is less than 800 W; but with three units in parallel, the inrush current can exceed 170 A peak.

Figure 3 shows that when the inrush current reaches a maximum of approximately 130 A, the voltage waveform folds back. In this case, the AC source could not provide the maximum peak inrush current demanded by the load.

If you were to continue testing the product with this source, the components in the power supplies would not be properly stressed to weed out possible defects during the testing cycle. These problems can reappear after the inadequately tested units have been shipped to customers.

AC sources are designed with a few different current-limiting schemes. Some sources have a current-limit shutdown. This type of current limiting is not good for high inrush current loads because the AC source will shut down when its current limit is exceeded.

A better limiting scheme for high inrush demands is the current-limit foldback. Rather than shutting down, the AC source will continue to provide current to the load.

In this case, we need the current foldback mode and the capability to deliver 170 A peak at 264 VAC at 50 Hz since the servers are intended for international use. Figure 3 shows that the inrush current only lasts for the first quarter cycle and then goes down to about 20 A after a few cycles. The impedance of the AC source also should be low, typically 0.05 W on good AC sources, to minimize voltage distortion.

Other considerations include the length and gauge of the wire between the AC source and the load. Any added impedance could greatly impact the peak current delivered to the load. By keeping the wire length as short as possible and the gauge sized appropriately for maximum rms current, you will ensure maximum current transfer. Also, be aware that solid-state AC sources have only a certain amount of stored energy in internal capacitors that is available for the first few cycles to drive high inrush.

In this example, we will size the AC source to handle 170-A inrush for one quarter cycle at 270 VAC, 50 Hz. The current required for normal operation is not a problem.

In addition to the inrush, you also want to introduce sags, surges, and drops with durations of 20 ms, 50 ms, 100 ms, and 200 ms to emulate real-world problems. An AC source with a CF of 4:1 can handle four times its rated rms current at the peak of the voltage waveform. Working backwards, we can divide the 170 A by the CF of 4 to give a nominal current of 42.5 A.

Next, we determine the nominal VA: 270 VAC × 42.5 A = 11.5 kVA nominal power. AC sources typically are sized in rounded increments, so an AC source between 12- to 15-kVA single-phase with a CF of 4:1 should start up the load without any foldback on the voltage waveform.

Different manufacturers have ratings ranging from 1.4:1 CF to 6:1 CF. Even though a unit may be rated at 6:1 CF, it could only be rated for a fraction of the cycle and may not handle that current for one quarter cycle. If the source had a 3:1 CF, we would need a nominal rating of 15.3 kVA.

One AC source that provides the inrush current and performs complex sags, surges, and timed dropouts is the Model SW15750 from Elgar. This unit is rated at 15.75 kVA and has a CF of 4:1 (233 A peak) and 58 Arms in the single-phase mode. It has on-board measurements to determine true rms volts, amps, watts, repetitive peak current, frequency, and PF. The SW15750 also is reconfigurable for three-phase output and operates in both AC and DC modes.

About the Author

Martin Sanders is the senior applications engineer at Elgar. He holds an A.S. in electronics and a B.A. in organizational management and has nearly 20 years experience in the power electronics field. Elgar, 9250 Brown Deer Rd., San Diego, CA 92121, (619) 450-0085, e-mail: [email protected].

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Copyright 1999 Nelson Publishing Inc.

June 1999