Most Missed Question

by: Mark Reeder

Franklin Electric’s Certified Contractor Program recognizes those water systems contractors that have made a commitment to understanding the proper application and installation of Franklin products. Attendance at an all-day Franklin Tech session, either at the factory, or “on the road” automatically certifies a contractor.

However, water systems contractors with 5 or more years experience with Franklin products can “test-out” of BASIC Certification with an exam.

Many of you have taken the Franklin Electric Certified Contractor exam on our Key Dealer website at www.keydealer.franklin-electric.com. Looking back at the results so far, there is one question that is most frequently missed on the test:

True or False? Amp draw is a reliable indication of electrical power consumption.

The correct answer is False, but it’s an easy one to get wrong. Here’s why:

In an AC system, power consumption, which is actually what we pay for, is a combination of three things: voltage, current (amp draw), and power factor. Of the three, voltage and current are the easiest to understand. Voltage is electrical pressure and current is electrical flow. Using our commonly used analogy to a water system, this makes sense. A 5 hp pump will produce a greater combination of pressure and flow than a ½ hp pump.

The big difference between a water and an electrical system is dealing with alternating current in an electrical system. That is, the current flows back and forth to do the work, kind of like the motion of a jigsaw. Because we are dealing with alternating current, power factor also has to be considered.

The reason so many of us answer true to the above is that somewhere along the way we learned that electrical power is simply voltage times current. That’s completely correct for a DC system. Say we’ve got a 12 Volt DC battery connected to a headlight and that headlight is drawing 1 amp.

Power consumed will be:

12 Volts x 1 amp = 12 Watts.

So, if amperage goes down at a fixed voltage, power has go to down as well. End of story with a DC system.

However, in an AC system, when calculating the power consumed this third component called power factor complicates things. Power in a single-phase AC system is:

Voltage x current  x power factor

Power factor is always a number between 0 and 1 and is sometimes expressed as a percentage.

Power factor results because the voltage and the current in an AC system aren’t necessarily in phase. Actually, the current wants to lag behind the voltage.

In the picture below, the current (in red) is completely in phase with the voltage (blue).

In reality, in an electric motor, the relationship between the voltage and current will probably look more like this:

This difference in phase between the voltage and the current is the power factor. As it turns out, the more “in-phase” the voltage and current, the higher the power factor. As a matter of fact, in the first picture, the power factor is 100%. In the lower picture, the power factor is around 75%.

So, looking at the two systems above, the power consumption of the bottom one is lower because the power factor is smaller, even though the values of the voltage and current are the same in each case

When comparing two motors, amp draw doesn’t tell the whole story on power consumption. You need to look at the manufacturer’s published power consumption.

Power factor can be changed with a run capacitor, or in the case of smaller 2-wire motors, with a permanent split capacitor motor (Franklin doesn’t manufacture this for the North American market). In these motors, a capacitor in the motor serves as both a start and run capacitor. Compared to other types of 2-wire motors, this results in much lower starting torque, but as a trade-off, it does lower amp draw and increase power factor. That is, the amp draw goes down, but the power factor goes up. The net result is that the power consumption between the two motors is nearly the same.

It takes some time to get there, but amp draw by itself doesn’t tell the whole story in terms of power consumption. So, now you know why amp draw is not a reliable indication of power consumed.

Get more industry and business insights from Mark Reeder by following his blog at www.franklininthefield.com.

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  1. Pingback: Column by Column: Power Factor | Franklin AID

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