Powerful savings

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 power triangle (shown in Figure 1) includes total or apparent power (KVA), true power (KW), and reactive power (KVAR). Figures 2 and 3 illustrate how these factors come into play at a portable crushing operation and an asphalt plant, respectively.

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.

Capacitors, shown in the images above, should have a higher voltage capacity than the motors on which they are installed. For example, a 480-volt capacitor should not be installed on a 480-volt motor because power surges can shorten their life.

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: