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Basic AC Electric

Basics of Alternating Current Electricity

Part Two—Phase and Power
Richard Perez

© 1996 Richard Perez This series of basic electric articles

continues with a discussion of

power in alternating current circuits. Learn why powering loads such as motors and electronics is difficult for inverters and generators.

In the first article in this series (Home Power #52, page 74), we discussed the repetitive and sinusoidal nature alternating current electricity. We got a hold of the concepts of amplitude, frequency, period, and phase in sinusoidal waveforms. If you missed this article, or if the concepts have grown fuzzy during the last two months, then give Part One of this series a read. You will need the concepts presented there to understand what you will read here.


The concept of phase is essential to understanding alternating current electricity. Phase means time, or more specifically a time interval between when one repetitive thing happens and another repetitive thing happens. In the case of alternating current electricity, we are talking about the time when maxima and minima happen on sinusoidal voltage and current waveforms. An event which happens after a related event is said to lag. An event which precedes a second and related event is said to lead. The preferred unit for phase is degrees as in 360° in a circle or a single cycle. This unit of measurement works well because of the repetitive nature of sinusoidal alternating current electricity.

Voltage and Current in Resistive Loads

In a resistive load, such as an electric heater or an incandescent lightbulb, the voltage and current waveforms are always exactly in phase. Figure 1 illustrates the voltage and current waveforms powering a 1000 watt electric heater. Note that the voltage and current waveforms reach maxima, minima, and zero at exactly the same instant. This is the definition of “in phase”. Note that the current waveform varies from zero to a positive peak of 12 Amperes and a negative peak of -12 Amperes. These peaks in current are called exactly that—peak current.

AC Volts & Amperes in a 1000 Watt Resistive Load

200 150 100

20 15 10

50 5 00 -50 -5

-100 -10 -150 -15 -200 -20

Figure 1

Voltage and Current in Inductive Loads

Not all electric loads are resistive like the heater and the lightbulb. Some appliances convert the alternating current electricity into magnetic fields rather than heat. These appliances include electric motors, and any device that uses a transformer to convert power from one voltage to another. The list of appliances which convert the electric power into magnetism for whatever reason is very long: well pumps, table saws, coffee grinders, microwave ovens, TV sets, VCRs, and in fact, most of the appliances we wish to power in our systems. All appliances that primarily convert electric power into magnetism are known as “inductive” loads. Alternating current electricity is converted into magnetism by a process called electromagnetic induction, hence all the appliances that use this effect are called inductive loads.

In the conversion from electric power into magnetism, a strange thing happens to the electric power—voltage and current become out of phase. Instead of being in phase with voltage, the current lags behind the voltage—the current waveform is delayed in relation to the voltage waveform.

Figure 2 shows the operation of a 1000 watt electric motor. Note that the voltage and current waveforms are no longer in phase. The current waveform lags some 45 degrees behind the voltage waveform. Another thing to notice is that, in order to deliver 1000 watts of power to the inductive load, the current peaks must increase to 17 amperes. More on why this happens when we shortly discuss power in inductive circuits.

Voltage and Current in Capacitive Loads

Some appliances convert alternating current electricity into electrostatic fields. These appliances include anything with the new “switching” power supplies which

44 Home Power #53 • June / July 1996

Volts (Black Curve)

Amperes (Gray Curve)

Image | Basics of Alternating Current Electricity Part Two—Phase and Power

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