Sometimes, you need to correct power factor (PF) at a single motor. This means sizing PF capacitors for that motor. The good news is if you size your PF capacitors correctly, you lower your electrical costs. The bad news is if you size your PF capacitors too small, you don't accomplish much. The worst news is if you size your PF capacitors too big, the vector swings past the perpendicular, leaving you in a very unfavorable situation. But don't worry; calculations to get the proper kVAR value and correct PF capacitor size are straightforward.
What's the first step? Collect the full-load PF and efficiency information on the motor. You can often get this information from product data sheets, product CD-ROMs, or your supplier. It also may not hurt to check your manufacturer's web site.
Most PF tables need your motor input rating in kilowatts. As such, you'll probably need to do simple conversions to use a standard table. This conversion starts with a very simple calculation, as shown in the sidebar "Equations to Know," Eq. 1. (Note: that equation cannot show in this medium. For more information, contact EC&M at (800)441-0294 and ask for the January 1998 article that begins on page 91.)
You'll need to determine the input kW, as shown in Eq. 2. Once you have that number, refer to the PF correction table (Table 1). Go in from the left with the existing PF and come down from the top with the desired PF. Where the two lines intersect, you'll find the multiplier you need.
Take that multiplier times the motor input kilowatts to get the required kVAR of correction. Then, using Table 2, select the capacitor, rounding down to the next lowest value. You now have the correct capacitor for PF correction.
Now how do you hook it up? A typical way to connect capacitors, when correcting a single motor, is between the starter and the motor. This connection, as shown in the diagram (on page 92), reduces the current flowing through the starter and overload relay.
Think about the overload heaters, for a moment. You install them in the starter, right? And you size them according to the current the starter will see, based on the motor load, right? So, if you reduce the current through the starter, the "correct" heater won't protect the motor, right? Right! So, you have to reduce the heater size to accommodate the reduced current through the starter, or you run an under-protected motor.
The motor itself will draw the same number of amps at a full load as it would without the PF correction. However, the PF capacitor will supply a portion of the current to the motor. Only the balance will come through the starter from the power line. You can determine the new value of current passing through the overloads by using Eq. 3, shown in "Equations to Know," on page 91. If you were going to do this operation for the motor in the previous example, the math would look like the example in the sidebar "Heater Current Calculation Example" below.
Don't get overly corrective with your PF correction. Target the PF correction for your motor's running current, not its starting or full load current. Always under correct; the cost is a slight loss of PF. Never over correct; the costs pile up because of high transient torques, overvoltages, and other problems you don't need. Consider 95% correction the upper limit, with 90% being the optimum target.
A final note of caution: Never use this type of PF correction where you're controlling a motor with a solid-state device (such as a soft start or VFD). If you have a PF problem and are using such a device, contact the device manufacturer for a solution. Their expertise and exposure to thousands of users can save you time, money, and downtime. The manufacturer will be more than happy to help you, and you'll be glad you brought them your PF correction problem.
Note: The tables and equations integral to this article do not appear here. This text outlines the idea of the calculations. For more information, contact EC&M at (800)441-0294 and ask for the January 1998 article that begins on page 91.
Suggested Reading EC&M's Practical Guide to Electrical Energy Efficiency & Reduced Costs, Order #5844. EC&M's Practical Guide to Motors and Motor Controllers, Order #4570. Electric Motor Control, Fifth Ed, Order #2506. Electronic Drives, Order #6113. Illustrated Electrical Calculations, Order #4622. Industrial Motor Control, Third Ed, Order #2522. For ordering information, call (800) 543-7771.
