When someone first mentioned “power factor” in a meeting I was attending, I did what any self-respecting person too embarrassed to admit she didn’t know what it was would do: I Googled it. If you’ve ever done this, chances are that unless you’re already an expert on power engineering, you were disappointed with the results.
The components of motor current are load current and magnetizing current (adding those instantaneous values yields the total motor current). Also, because load current is in phase with voltage and magnetizing current lags voltage by 90 degrees, their sum will be a sine wave that peaks somewhere between 0 and 90 degrees lagging which is the motor current's offset from voltage. There are negative effects associated with increased offset and that’s part of the power factor explanation. Anyhow, power factor represents the offset in time, or the delay, between voltage and the current being delivered and is defined as the cosine of that offset.
Ultimately, it was one of my colleagues that explained it in terms I could understand: beer.
When you go to a bar and order a pint, the bartender tips a glass and pours the beer up to that magic little line. A good bartender will have minimal head on the pour, but get a bad one, and you’re getting an inch of foam. You’re paying the same amount for either beer. More beer = good. More foam = bad.
So, combining the uber-geek definition with the over-simplified beer analogy, power factor is a way to measure the difference between “apparent power,” power that is lost through the process of induction (the beer AND the foam), and “real power,” which is usable to do work (the beer). Something like an incandescent light bulb has a perfect power factor, converting essentially every bit of electricity that’s flowing into it without magnetic loss (loss of conversion to heat is another matter all together!). Reactive loads, like motors, are a bit more complicated. They use a coil, or windings, connected to an AC power supply, and the coil needs to be magnetized before it can begin doing useful work of turning the motor through induction, which creates an inefficiency in the use of electricity.
Why does all of this matter?
A less-than-perfect power factor can be problematic for several reasons that engineers, particularly at industrial or manufacturing sites where lots of motors run heavy equipment, should note:
1. Poor power factor could cost you dearly. Because reactive power still requires capacity on the system even though it doesn’t do any useful work, some utilities charge more for it (particularly if the utility serves a region with a lot of industrial load). As we always remind folks, understanding how you’re billed for energy is the very first step to effectively managing it. A presentation by Michel Stuck at Fluke India brought the financial impact to life well:
Assume a utility adds 1% of demand charge for every 0.01 below a power factor of 0.97. Assume your power factor averages 0.86 each month and your demand charge is $7,000. (0.97 – 0.86)*100%=11%.
(11% x $7,000) x 12 months = $9,240 in avoidable costs.
2. Poor power factor is a sign of inefficiency that can result in additional costs associated with equipment maintenance. When machines break, there is costly down time and the products you’re manufacturing or the systems you’re running are also at risk.
3. In extreme cases, if you notice a really low power factor, you could have what’s known as three phase power imbalance, which can be due to improper wiring. If you do have an imbalance, your very expensive equipment is working against itself, creating a lot of wear and tear on the motors, shortening its life span, and generating a lot of heat, which could be a safety hazard.
The good news: it’s fixable.
First, you have to understand what equipment is causing the poor power factor. If your utility meter's based on KVAR, you can use your bill to determine you have a problem, but you can’t necessarily determine what load is causing the issue.
Real-time metering can help you identify where the problem is originating. Once you’ve identified where the problem is originating, capacitors or conditioners can be added on individual pieces of equipment, which essentially acts as a booster shot to make up for less-than-perfect power factor. Or you may find that a motor or even an entire motor control center has been improperly wired.
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