Taking Measuring Into Another Dimension

One of the nice things about working in the security field is we typically work within the two-dimensional world of direct current (DC). To get power to our DC equipment, however, the power distribution system of choice is alternating current (AC).

Most of the time, AC technology does not directly influence us. We simply plug in the “wall wart” (AC transformer) that comes with the alarm panel. If we need to estimate the power of a system, we pull out our handy Ohm’s law formula for power (watts) where power equals current times voltage (P = I x E) or variations such as P = I2 x R, which multiplies current with resistance.

We will shortly see that this basic power rule, which always works in the DC world, may not work the same in the AC world. Working with AC power and understanding how electronic components like capacitors and inductors respond takes us into another dimension – a 3-D world where the additional dimension is time.

Because of the temporal difference that can occur between AC voltage and AC current, we need to look at what power supplies – including uninterruptible power supplies (UPS) – actually do.

Power is expressed in watts or “active/real power.” Its relationship with specified, or apparent, power is shown in volt-amps. With that, we have entered a new dimension between what is real and what is apparent.

Switch Mode and UPS Basics
Most modern AC-powered security equipment – such as computers, monitors and DVRs (read more about networked DVRs on page 64) – are powered by a module called a switch mode power supply (SMPS). The SMPS has been around for many years. In fact, it is older than many might think.

One of the first SMPSs could be found in early automobiles as a mechanical vibrating circuit. The vibrator “chopped” the 6-V battery voltage into AC. In turn, that AC could be stepped up or down for plate and bias voltages in early tube radios.
UPSs also use SMPS technology and are often responsible for providing backup power in the kilowatt (1,000W) range. It is especially important for those working with these larger, head-end power systems to understand the power factoring concepts we will be discussing. Miscalculations could cause extra expense or failure of backup support when operating large UPS systems. I still read stories of system designers missing this important concept of power factoring.

The old, conventional “linear”  power supplies were typically only 40- to 50-percent efficient, while today’s SMPS power supplies can be 60- to 90-percent efficient. These “nonlinear” designs allow for less heat, increased battery life and less cost.

As with anything in nature, something good can come with a price. While the high-frequency switching design is a big plus for modern technology, it does some tricks with AC electrical power of which we should be aware.

Back to School on Sine Waves

Whether it was in a tech training class or in science class, we can all remember seeing an illustration of a sinusoidal (sine) wave. We also learned that any point on the wave was a representation of a value (in this case, voltage and current) with reference to a point in time on that wave, which is often expressed as frequency (1/time). In AC power, that frequency is 50Hz to 60Hz … or cycles per second, depending on in what part of the world you live.

However, you will notice in the diagram on page 28 of the July print issue of SSI  that the current sine wave temporally leads the voltage sine wave by 90°. This diagram is showing a purely capacitive circuit. Without getting too involved in formulas, it is important to note that the circuitry of a SMPS has some capacitive features. The same circuitry affects AC voltage and current in a similar manner with a phase difference in the 40° region. PF = cos q = cos (40°) = 0.78 (more on this in a moment).

Equations Spell Out ‘Power Factor’

An important term called “Power Factor” (PF) is defined by the Institute of Electrical and Electronics Engineers (IEEE) as the ratio of true, or real, power (W or watts) to apparent power (VA or volt-amps). PF = W / VA, therefore W = VA x PF. Since PF is also cos q ( the angle between real and apparent power due to the capacitive nature of the SMPS circuitry), volt-amps (VA) is V x I. Many of today’s SMPS have a power factor of between 0.60 and 0.95. A purely resistive circuit would have a PF of 1.

Some power supplies have power factor correction (PFC) circuits producing a higher PF. I once heard a story from a UPS user in which they had to modify a capacitive PFC circuit with a disconnect switch. The reason for this is they had to manually engage the circuit only after a full load was on the UPS in order to make the PFC work properly.
Taking a power factor measurement can be tricky. You will need a true root-mean-square (RMS) voltmeter, which all digital multimeters (DMM) are not.

If you are interested in testing how much real power your equipment is drawing, there is one interesting testing device that you might want to check out. It is called the “Kill-A-Watt” (www.p3international.com) and will test real power (watts), volts, current and even PF. The nice thing is the device only costs around $40.

An example of power factor calculations would be a system that was drawing 9.6kVA (120V x 80A). The UPS must be rated at 10kVA. But if the power factor of the load is 0.6, then only 6kVA of real power is being used (10kVA x 0.6 = 6kVa).

I hope everyone has a better understanding now of what to look for in switch mode power supplies. As with any system, it is good to work closely with the manufacturers and take a closer look at their specifications. Take some true power readings yourself to help confirm the data.

For those that want to learn more about AC circuits and other electrical circuits, I have found a great free online source called “Lessons In Electric Circuits” by Tony R. Kuphaldt. It’s located at www.ibiblio.org/obp/electriccircuits (Editor’s note: As of July 6, this site appears to be down).

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