An aggregate quarry’s utility bill shows a power factor of 0.78 for the month, along with a penalty line item. The plant has not added any equipment. Its crushers, conveyor drives and pumps simply run at partial load most of the day. Induction motors draw reactive power to build their magnetic fields, and lightly loaded motors draw a larger share of it. Improving power factor reduces the current the system carries for the same amount of real work.
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Why Does Low Power Factor Matter?
Power factor is real power in kW divided by apparent power in kVA. At 0.78, cables and transformers carry about 28 percent more current than they would at 1.0 for the same load. That extra current heats the wires. It also uses up transformer room that could serve new equipment. On the long feeders common at mines, it also causes voltage drop at the far end, which makes motors run hotter. Many utilities add demand charges or penalties when power factor falls below a threshold such as 0.90 or 0.95.
How Do Facilities Improve It?
Capacitors supply reactive power locally, so less of it has to flow from the utility through the whole system. A fixed bank stays on all the time and suits a steady load. An automatic bank uses a controller and contactors to switch capacitor steps in and out as the load changes. Mounting banks near large motors, or at the motor control center, reduces current on the longest runs.
What Should Be Checked First?
A power quality study logs kW, kVAR and harmonics at the main service and at large loads for several days. Variable frequency drives on conveyors and pumps create harmonic currents. Capacitors can resonate with the system at one of those frequencies. That makes the distortion worse and can blow capacitor fuses. Where harmonics are present, detuned banks with series reactors, or filter banks tuned to a specific harmonic, avoid that problem. IEEE 519 sets the harmonic limits most utilities use.
What About Maintenance?
Capacitor banks need routine checks of fuses, contactors and the capacitor cans. A can that bulges or leaks has failed inside. Blown fuses on one phase leave the bank unbalanced. An infrared scan during operation shows hot connections and failing contactors.
Operations looking for metal-enclosed capacitor banks built for mining conditions can review guidance from Becker Mining on selecting the right bank for their system.
