April 1, 2010
Understanding energy costs and calculations can lead to big savings for some operators.
by Daniel J. Meyer, P.E., Ph.D.
Does your operation pay bills for diesel fuel on portable engine/generator sets to power electrical motors or pay for electrical bills on motors at stationary sites that are connected to the electrical grid? If so, placing capacitors, which correct power factors, on your electrical system can offer returns such as 200 percent per year. To achieve these savings, however, an operator must first understand how power works.
When there is an electrical load involving inductance (magnetic elements), there is a new term beyond amps and volts called power factor (PF) that enters into the basic unit of measurement called the kilowatt-hour. There are two components of electricity flow. One portion is absorbed to do useful work (at the shaft end), which is called real power (KW). The other portion is literally borrowed from the power company or the engine/gen set and used to magnetize the magnetic portions of the circuit (KVAR), which are known as the motor windings. Due to the reversing nature of AC power, this borrowed power is subsequently returned to the power system when the AC cycle reverses. This borrowing and returning occurs on a continuous basis (60 cycles per second). PF then becomes a measurement of the amount of real power that is used, divided by the total amount of power, both borrowed and used (KVA). The PF is variable and changes with the amount of load that is applied to the motor. It increases steadily with the horsepower loading that is applied to the motor. Energy savings increase even more when there is no load or a light load.
The concept is best explained by looking at the “power triangle.” It involves KVA (total or apparent power), KW (true power), and KVAR (reactive power). See Figure 1. The square root of the sum of the KW squared and the KVAR squared gives KVA. When the PF is 100 percent, the KVAR is 0, and the KVA is equal to the KW. Since this seldom happens, capacitors can offer significant savings.
A capacitor is a passive electrical component that can store an electrical charge or energy on its plates. The amount of energy stored equals voltage squared times the capacitance (KVAR) divided by 2.
The motor has no way of intelligently adjusting the amount of electricity it consumes in relation to the job or work it does. Most AC motors operate in the 65 to 80 percent PF range under full load and drop further under partial loading, especially anything with low horsepower motors. For example, a large saw in a sawmill can run as low as 8 percent when idling and as high as 85 percent when cutting through logs. Capacitors improve system capacity, improve efficiency, minimize line losses, prevent voltage drops, and decrease infrastructure costs, as well as help avoid PF penalties.
Running the numbers
How does your motor’s PF fit into the bigger picture of energy savings? Let’s go through two examples. The first is a portable rock crusher with several conveyors under no load. This recorded setup had a measured PF of 33 percent, the KW was 11, and the KVA was 32. The KVAR or capacitor rating at 100 percent PF was measured to be about 30. Let’s use 24 KVAR of capacitance to see what happens to the power demand KVA. This drops the KVA to 12.5. The result, shown in Figure 2, is a 61-percent drop in power needed (KVA) which is (32-12.5)/32 x 100 percent. Even when the crusher is loaded, which means the KW will go higher, the PF will also be higher. The KVAR usually increases about 10 percent. Ideally, capacitors are designed for average operating load and not for the maximum peak power because the capacitor would need to be bigger, which costs more, and the current could lead the voltage, which is not good. A better option is to get the two (amps and current) “in sync,” which means about 95 percent power factor.
The second example is an asphalt plant with a 2,200 horsepower engine powering a 1,500 KW generator. The measured KW was 470 under normal load, and the KVA was 618. This worked out to a 62-percent average PF on the plant’s 21 motors. The 100-percent KVAR rating needed is 401, but let’s use 300 KVAR (see Figure 3). This drops the KVA to 481, which is a decrease of 22 percent in power demand. Expect the final result to be better than this because full load may only occur 80 percent of the time. Does this mean your engine will drop 22 percent exactly in fuel cost? It is probably not a perfect line between dropping KVA and corresponding decreases in your diesel fuel usage, but it would be a very significant drop.
Now, let’s check out a crushing operation on an electrical grid. When your operation is connected to the electrical grid, it may be getting a power factor penalty because the power company decides it’s big enough that it doesn’t want to subsidize all your power cost. In this case, it will add a PF penalty. This is not the same as peak demand, which is charged per KW used. The penalty might be an additional 10 percent on your bill and could be explained on your statement with a notation that your site doesn’t meet a minimum PF percentage, so you will be assessed a PF penalty.
To remove this penalty, you can add capacitance (capacitors) to the motor system. They might offer to do this for your operation by adding a capacitor bank outside your operation and billing a monthly rental fee. Alternatively, your operation can add its own capacitors, which is often less expensive. This can be done at the larger motors so it takes fewer capacitors or, if everything is operating at one time, you can add one or two large capacitors to the main electrical panels. Normally, capacitors are added after the motor starter so the capacitor is off when the motor is off. If you have motors running intermittently, that is how they should be installed. Capacitors pull amps all the time if they are put on an electrical panel, so if your operation is not operating the motors, the power is still adding up on your bill.
If your operation is not being charged a PF penalty and it is connected to the electrical grid, it would not be a wise investment to add capacitors. Although, it would drop the KVA, the power company will not reward your operation. Instead, your operation will be subsidizing them. Using capacitors when connected to the grid does not offer nearly as good of a return as compared to placing the capacitors on your own power system (engine/generator set).
The bottom line
Let’s look at the economics of capacitors on the big asphalt system outlined earlier. If capacitors were to be added, two large ones (about 150 KVAR) could be purchased for approximately $5,000 if these motors operate all the time the plant is running. The operation needs to protect the capacitors with fuse disconnect switches, which might cost another $3,000 for a total of $8,000. What is the payback? At this asphalt plant, the total power (KVA) will drop 22 percent, but we’ll use 17 percent as a conservative estimate. Savings are calculated based on these assumptions:
Hours run per season: 1,200 hours
Diesel fuel costs: $2.20/gallon
Engine fuel usage: 40 gallons/hour
17 percent/100 percent x 40 gal/hr x 1,200 hrs x $2.20/gal = $17,950 per year (ROI = 17,950/8,000 = 224 percent/year)
Now, let’s look at the capacitor brands on the market. Some have two-year warranties, but warranty periods vary. The prices increase along with the longer warranty life units. The anticipated life is usually based on how many hours the motors are run. If you calculate the earlier example for five years, it would be 224 percent x 5 or a 1,120-percent return of $89,600 on an $8,000 investment. Most capacitors have a 17-year or longer life expectancy. Some have internal fuses to protect against indirect lightning coming in the power line if it is connected, but still suggest additional protection.
Another consideration related to the warranty is the voltage rating of the capacitor. Do not use lower-cost, 480-volt capacitors on 480-volt motors, particularly if connected to the electrical grid. Their life will be short due to surges. Typically, power companies are within their rights if the voltage surges up to 10 percent. Most capacitor companies have 600-volt rating models that, typically, last three times as long as 480-volt rated capacitors. The best models are 660-volt rated capacitors, which tend to have the 10-year warranties.
Calculating capacitor size
Standard power analyzer equipment works to size motors or electrical panels for capacitors. Some companies have a secret patented system to measure for capacitor sizing so the person measuring has no clue what KVAR setting is needed. The main two items an operation needs are a power factor and a KW reading, but more data improves accuracy (visit http://usenergyadvisor77.vpweb.com to download a data form).
It is ideal to use a power analyzer and leave it on for 5 to 10 minutes or longer to record the low, mid, and high data. Many power analyzer units do up to 3,000 amps. Remember, it is dangerous to take measurements when a lot of amps are involved. Use an electrician or a qualified maintenance person or engineer to collect the electrical data. The motors need to be in operation to get the data. Once it has been collected, gather quotes from multiple dealers and make sure you get the right size of capacitor.
If your operation has soft starts or variable frequency drives or variable speed motors there are some critical wiring details needed to connect them properly. Not all capacitor companies understand this. Don’t let them tell you that your operation can’t wire up these types of motors to capacitors.
If the operation is connected to the electric grid, it may be worthwhile to add surge protection to minimize external voltage surges. Also, consider adding surge protection if the operation has intermittent operation of large motors because this can set up internal voltage surges. Two items needed on the data sheet to be sized for surge protection are the total monthly kilowatt-hours and total amps. Another location for surge protectors or clean power systems is in the control room for your motors’ operation, which can be affected by voltage surges, harmonics, and electrical noise. Any expensive monitoring equipment should be protected. These single-phase units are much cheaper than three-phase clean power systems. They work well on either engine/gen sets or on operations connected to the electrical grid.
The ROI of capacitors in many motor usage systems can be very worthwhile. By investing in some capacitor expertise to collect data on your operation, you may be able to drop your operational costs and improve profits. AM
Dan Meyer, P.E., Ph.D., is president of Modern Ag Systems Design, Inc., in Waukon, Iowa. He has spent 25 years serving on the faculty of three major universities and an additional eight years as a private consultant. He can be contacted at 563-517-7679 or via e-mail at firstname.lastname@example.org. His Web site, including an electrical survey form, is http://www.usenergyadvisor77.vpweb.com.