Recently, you may have noticed advertisements for a device that claims to reduce your monthly home electricity bills. The advertising literature states that you are paying for the added electricity that must flow when power factor (PF) is less than “unity” within your house. But to what extent does PF influence energy consumption? And do the energy savings accrued through power factor correction (PFC) justify the purchase of standalone PFC devices? Moreover, do the energy savings justify the addition of PFC components within home appliances where PF is less than one?
Numerous pieces of equipment in the home are candidates for PFC. Some of these equipment types have capacitive inputs (for example, switching power supplies). However, some of the larger loads are the motor-driven appliances such as refrigerators and washing machines, which have inductive inputs. An analysis of these types of equipment examines their typical energy requirements and the impact of PFC on their energy usage within the home.
The electricity provided to your house varies in amplitude and direction of flow in the wires leading to and within the house. It is varying in a sinusoidal manner. You pay for the electricity by paying for the electric power provided multiplied by the time for which this power is delivered. Electric power is simply computed by multiplying the voltage times the current times a “fudge factor” (i.e., the PF).
The work done is computed by determining this product; therefore, the maximum work is done when the voltage and current reach their maxima at the same time (Fig. 1). For a fixed amount of power, if the PF is less than unity, additional current must flow to compensate for the current and voltage that do not have simultaneous maxima.
When the current and voltage simultaneously achieve maxima, the PF is said to be unity. The presence of some types of electrical equipment causes the electrical current to continually play catch-up with the electrical voltage. If the current is always playing catch-up, the circuit is said to have a lagging PF. Electrical motors — in fact, any device that has a coil of wire — will contribute to a less than one lagging PF.
In the instance where a circuit has a PF less than unity, more current must flow to produce the desired electrical work. This additional current flow causes more power losses in the conductors located in the walls of your house, for which you derive no advantage except for a small amount of additional heat generated (this might be considered a benefit in the winter but a detractor in the summer).
Fig. 2 depicts a single-house wiring circuit that is subject to a lagging PF due to the load being an electric motor. Although every house is different, certain assumptions can be made with regard to the elements in this diagram that will allow an analysis of energy consumption with and without PFC. Once the analysis is complete, these assumptions can quickly be evaluated to determine whether variations in home environments, appliances or their usage may alter the analysis.
For the purpose of this analysis, the following conditions are assumed:
Most appliances using an electric motor will be fed by a #12 gauge cable and protected at the load center (main panel) by a 20-A circuit breaker.
The average two-conductor cable length from the load center to an appliance containing an electric motor is 25 ft. This yields a total conductor length of 50 ft.
Motor-driven appliances are the devices in a house most likely to contribute to a lagging PF.
Most motor-driven appliances have a 1-hp motor with an efficiency of 85% and a lagging PF of 0.75.
The average cost of electricity in the United States is $0.10 per kWh.