AC and DC

The distinction between alternating and direct current decides which devices can be connected to what, and it is the reason the electrical grid looks the way it does.

The two waveforms

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DC is not necessarily low voltage and AC is not necessarily high. The difference is direction over time, not magnitude, and a question conflating the two is testing exactly that.

Frequency and period

Frequency is the number of complete cycles per second, measured in hertz, the unit named for the physicist Heinrich Hertz.

US household supply is 60 hertz, meaning the current reverses direction 120 times a second and completes 60 full cycles.

Most of Europe and much of the rest of the world uses 50 hertz.

Period is the time for one cycle, and it is the reciprocal of frequency. At 60 hertz the period is one sixtieth of a second, about 16.7 milliseconds.

Aircraft electrical systems often run at 400 hertz, because a higher frequency allows smaller and lighter transformers and motors for the same power. That is an aviation-relevant fact and it does appear.

Measuring AC

An AC voltage is changing constantly, so "the voltage" needs a definition.

Peak voltage is the maximum the waveform reaches.

Peak-to-peak is from the top of a crest to the bottom of a trough, which is twice the peak.

RMS voltage - root mean square - is the value that produces the same heating as an equal DC voltage. It is what a meter reads and what a supply is rated at.

US household supply is 120 volts RMS, and its peak is about 170 volts. RMS is about 0.707 times the peak, and the peak is about 1.414 times the RMS.

When a voltage is quoted without qualification, it is RMS.

An oscilloscope shows the waveform itself - voltage plotted against time - which a multimeter cannot. So a scope on a 120-volt outlet shows a peak of about 170 volts while the meter reads 120, and neither is wrong: one reads peak and the other RMS.

Phase, impedance and power factor

This is the part of AC that has no DC equivalent, and the subtest asks it more than you might expect.

Phase

Phase describes timing: where one waveform is in its cycle compared with another. Two waveforms that peak at the same moment are in phase; two that are 180 degrees apart, one peaking as the other bottoms out, are completely out of phase.

Voltage and current are not always in step.

A memory aid electricians use is ELI the ICE man: in an inductor (L), voltage (E) comes before current (I); in a capacitor (C), current comes before voltage.

Reactance and impedance

Reactance is the opposition a capacitor or inductor offers to AC. It is measured in ohms like resistance, but it depends on frequency; the Capacitors and Inductors lesson covers which way.

Impedance is the total opposition to AC: resistance and reactance combined, also in ohms. They do not simply add, because they are out of step with each other. They combine like the sides of a right triangle:

A circuit has 30 ohms of resistance and 40 ohms of reactance. What is its impedance?

Impedance is the hypotenuse: the square root of 30 squared plus 40 squared, which is 50 ohms - the 3-4-5 triangle scaled by ten. The bank's impedance calculations all use a 3-4-5 or 6-8-10 triangle, so recognizing those two is enough.

Power factor

In a circuit with reactance, voltage times current overstates the useful power, because some of the current simply flows back and forth without doing work.

• Apparent power is volts times amps, measured in volt-amperes.
• True power is the part that does useful work, in watts.
• Power factor is the ratio of the two, from 0 to 1.

A circuit draws 10 amperes at 120 volts with a power factor of 0.8.

Apparent power is 10 x 120 = 1,200 volt-amperes. True power is 1,200 x 0.8 = 960 watts.

A power factor of 1.0 means a purely resistive load, where voltage and current are in phase. A low power factor is costly because more current has to flow for the same useful power; plants with large motors, which are inductive, add capacitors to cancel some of that inductive reactance. Transformers are rated in volt-amperes rather than watts for the same reason: the transformer cannot know the power factor of whatever it will feed.

Three-phase power

Three-phase supply is three AC waveforms on separate wires, each a third of a cycle apart. Because the phases peak at different times, their power deliveries overlap and the total is smooth rather than pulsing, which is why industry uses three-phase for large motors. A three-phase motor starts on its own, because the three phases naturally create a rotating magnetic field.

A house gets a single-phase three-wire service: two hot legs and a neutral. Each hot leg is 120 volts to neutral, and the two hot legs are 240 volts to each other because they are out of phase - which is how a dryer or range gets 240 volts from the same service.

Why the grid uses AC

The reason is transformers, and it follows from the power lesson.

A transformer can step AC voltage up or down with almost no loss, and it works only on AC because induction needs a changing field.

Power lost in a transmission line is I squared R, so it depends on the square of the current. Transmitting the same power at a higher voltage means a proportionally lower current, and the losses fall with the square of that reduction.

So: step the voltage far up for transmission, send it a long distance with small current and small losses, and step it back down near where it is used. That sequence is the whole argument, and DC could not do it simply when the grid was built.

Large motors also run well on AC, which is the other half of the reason.

Converting between them

A rectifier converts AC to DC, using diodes. Nearly every device with a power adapter contains one, because electronics need DC while the supply is AC.

An inverter converts DC to AC, which is what lets a vehicle or a solar system run household equipment.

A transformer does not convert between AC and DC. It changes AC voltage only. That is a question, because all three components appear together in the choices.

What you can skip

Across all 1,358 Electronics Information questions in our bank:

• Phase angle trigonometry. No question asks you to compute an angle or use cosines. The only calculations are the 3-4-5 impedance triangle and the power factor multiplication shown above.
• Complex numbers and phasor notation never appear.
• Three-phase wiring configurations such as wye and delta are not asked. Know why three-phase is smoother and that swapping two leads reverses a motor.

Where people lose points

Saying AC is always higher voltage than DC.

Saying a transformer converts AC to DC. A rectifier does that.

Confusing peak and RMS. A 120-volt supply peaks near 170.

Saying 60 hertz means the current reverses 60 times a second. It completes 60 cycles, which is 120 reversals.

Thinking batteries supply AC.

Saying transformers work on DC.

Work one in under a minute

Why is electrical power transmitted at very high voltage over long distances?

Power lost in the line is current squared times the line's resistance.

For a fixed amount of power delivered, raising the voltage lowers the current proportionally, and because the loss depends on the square of the current, a tenfold voltage increase cuts the loss to a hundredth.

And the voltage can be raised and lowered easily only because it is AC, which is the reason the grid is alternating rather than direct.

Where this leads

Transformers, rectifiers and the components that behave differently on AC are the next several lessons.