Electrical Metering for Linemen and Techicians


Power Factor

Resistive Load
Watt-hour meters compare alternating current voltage and current signals at virtually every moment in time. When the load beyond the meter is made up of a purely resistive load, the voltage and current are exactly in phase. Meaning, the individual voltage and current sine waves rise and fall simultaneously. When this condition exists, the circuit is said to be at unity power factor. In other words, all of the current in the circuit is doing useful work. Some examples of purely resistive loads are incandescent lights, heating elements in electric ranges and dryers, etc… These devices are changing electrical energy into some other energy form such as light, or heat.

Inductive (Motor) Load
An electric motor changes electrical energy into mechanical energy. A portion of the total alternating current flowing through the motor is used to set up the magnetic field required for the motor to operate. The portion of current used to set up the magnetic field will not be in phase with the voltage in the circuit. The waveforms do not rise and fall together. The meter will only register the portion of alternating current that does rise and fall with the voltage. In other words, the meter only registers the current that is doing useful work in the circuit, not the magnetizing current. Generically speaking, if 70% of the current in the circuit is in phase with the voltage, the circuit is said to be operating at 70% power factor. Once again, the watt-hour meter only registers the portion of current that is in phase with the voltage. However, the transformer supplying energy to the circuit must be sized to deliver the total current required. Regardless of whether it is in phase with the voltage.

Capacitors (Power Factor Correction)
Capacitors are capable of temporarily storing and delivering the magnetizing current required by motors. In other words, an appropriately sized capacitor can provide the motors magnetizing current rather than delivering it through the transformer and secondary service. By utilizing capacitors, power factor can be corrected. You might say it makes the circuit more efficient and may even allow the transformer and service wires to be downsized.

One common misconception is that as power factor is corrected toward unity through the use of capacitors, the watt-hour meter will run faster. Remember that the capacitor simply cancels the need for the service to supply magnetizing current. The meter will simply continue to meter the useful current in the circuit as it did before. You might ask, “Why bother to correct power factor if the meter still registers the same amount of energy”? As stated earlier, the transformer and secondary service may be able to be downsized when power factor is corrected. In addition, the reduction of current flowing through electrical system also reduces line loss.