A technician sets a multimeter to DC volts, puts the probes on a live 115 volt alternating current (AC) bus, the bar that carries power to the circuits, and reads 0.03 volts. He writes the bus up as dead. A second technician connects an oscilloscope, an instrument that draws a voltage as a line on a screen, and sees a wave that swings 325 volts from bottom to top. The placard, the label beside the bus, says 115 V, 400 Hz.
FIG 1 One bus, three readings.
An AC voltage takes more than one number to describe. Read the wrong one and you call a live bus dead, or you miss the highest voltage the wiring has to hold.
When you finish this lesson you can do the four things below. The check at the end asks about each one.
A magnet moving in a coil makes a voltage. A magnet sitting still makes none.
Period is 1 divided by frequency. 400 Hz is 2.5 ms.
The meter on AC volts shows RMS. A 115 V bus peaks at 162.6 V.
A transformer raises the voltage, so the same power travels on less current and loses less in the wire.
Watch the lecture. Continue appears when it ends. You can skip it for now and come back to this page later.
In the Voltage lesson you learned the voltages on an aircraft. Light aircraft run on 14 V or 28 V direct current (DC). Larger aircraft add an AC bus.
Which two numbers describe the AC bus on a larger aircraft?
Before you can see where AC comes from, you need magnetism. Magnetism is the property of an object to attract iron and steel. Every magnet has two ends, called poles: a north pole (N) and a south pole (S).
The invisible force around a magnet. It is drawn as lines of force that leave the north pole and enter the south pole. The lines never cross. Together they are called magnetic flux.
A coil of wire, many turns wound side by side, with current through it. The current makes a magnetic field, and the coil gathers it into a north pole and a south pole, like a bar magnet.
The switch is open. No current flows, so the coil has no magnetic field. The compass points north.
In 1820 the Danish scientist Hans Christian Oersted saw a compass needle swing when he held it near a wire carrying current. It was the first proof that a current makes a magnetic field.
A current makes a magnetic field. It also works the other way: a magnetic field that changes inside a coil makes a voltage in the coil. This is electromagnetic induction, and the voltage it makes is called an induced voltage.
Drag the magnet into the coil and out again. You can also use the buttons.
The magnet is out of the coil and still. The needle reads zero.
Galvanometer. A meter with its zero in the middle of the scale, so the needle shows current in either direction.
The magnet moves down into the coil. Current flows, and the galvanometer needle swings one way.
The magnet sits inside the coil. Its field passes through the coil but does not change, so the needle reads zero.
The magnet moves up out of the coil. The current reverses, and the needle swings the other way.
FIG 6 Inducing a current flow. Source: FAA-H-8083-30B, Figure 12-104.
The English scientist Michael Faraday discovered electromagnetic induction in 1831. The unit of capacitance, the farad, which you meet in the next lesson, is named for him.
Faraday was the son of a blacksmith and had little schooling. As a bookbinder's apprentice in London he read the science books he bound, and in 1813 he became an assistant at the Royal Institution. In 1831 he moved a magnet in and out of a coil of wire and measured a current in the coil. The same year he built the first generator, a copper disk turned between the poles of a magnet.
FAA-H-8083-30B, Aviation Maintenance Technician Handbook, General. Chapter 12
Michael Faraday discovered that by moving a magnet through a coil of wire, a voltage was induced across the coil. If a complete circuit was provided, then a current was also induced. The amount of induced voltage is directly proportional to the rate of change of the magnetic field with respect to the coil.
Source: FAA-H-8083-30B, page 12-56. Text as printed.
Select a highlighted phrase to read what it means.
Select a phrase in the handbook text.
A generator is a machine that turns motion into voltage by induction. The simplest one is a loop of wire turned between the north and south poles of a magnet. A generator that makes alternating current is called an alternator. The alternator the engine turns on a light aircraft works on the same principle.
Drag to turn the view. Select a number to read about that part.
The loop is stopped. Select Turn the loop.
Select a number on the model.
FIG 7 A simple generator, drawn after FAA-H-8083-30B, Figure 12-108. The meter under the model has its zero in the middle.
More wire cutting the field makes more voltage.
A stronger field makes more voltage.
The sides cut the field faster and make more voltage.
As the loop turns, its voltage rises and falls in a smooth curve called a sine wave. The FAA handbook follows one side of the loop, side A, through one full turn of 360 degrees (°) in five positions. A quarter turn is 90°.
0°. Side A moves along the lines of force and cuts none of them. Zero voltage.
Source: FAA-H-8083-30B, Figure 12-109.
The curve a turning loop makes. The line across the middle is time, or degrees of rotation. The height above or below the line is the voltage at that instant, positive above and negative below.
Direct current is electricity that only moves in one direction. Alternating current reverses direction in cycles: the electrons shift back and forth in place while the energy moves along the wire. You call a supply AC or DC by the way its current flows, not by where it comes from.
One direction
Tap the bucket where the card belongs. On a computer you can also drag the card.
Reverses in cycles
Which bucket does this card belong in?
One complete pattern of the wave, up, down and back to the start, is one cycle. The number of cycles in one second is the frequency. The time one cycle takes is the period.
One complete pattern of the wave. It has two alternations, a positive half and a negative half.
Cycles per second, in hertz (Hz). The FAA handbook and older manuals also write "cycles", as in 400-cycle power.
The time for one cycle, in seconds or milliseconds (ms). One millisecond is one thousandth of a second.
The screen shows 20 milliseconds from left to right. Select 60 Hz, then 400 Hz, and count the cycles that fit in the same 20 ms: just over one at 60 Hz, eight at 400 Hz.
Frequency and period are opposites: the higher the frequency, the shorter the period. Divide 1 by the frequency to get the period.
A ground power unit, the cart that powers a parked aircraft, supplies 400 Hz AC (FAA-H-8083-30B, Chapter 1).
Building power in Europe is 50 Hz. What is its period in milliseconds?
An AC voltage changes every instant, so it takes more than one number to describe it. The FAA handbook names three values: instantaneous, peak and effective, also called RMS. Peak-to-peak is a fourth number you read on a scope.
Select each value to mark it on the wave.
Select a value above.
Move the peak-to-peak slider. The wave grows taller, and the three numbers follow it.
RMS stands for root mean square, the math used to find it. The FAA handbook calls it the effective value, because it measures the heating effect of the wave. The RMS value of an AC voltage is the DC voltage that makes the same heat in the same resistor.
Two identical 100 Ω heaters. Heater A is on a 115 V AC bus whose wave peaks at 162.6 V. Set the DC supply until heater B glows exactly as hot as heater A.
Set the DC supply until heater B glows exactly as hot as heater A.
P = E² ÷ R, from the Ohm's Law lesson: 115 × 115 ÷ 100 = 132 W.
FAA-H-8083-30B, Aviation Maintenance Technician Handbook, General. Chapter 12
The effective value is also known as the RMS value or root mean square, which refers to the mathematical process by which the value is derived. Most AC voltmeters display the effective or RMS value when used. The effective value is less than the maximum value, being equal to .707 times the maximum value.
In the study of AC, any values given for current or voltage are assumed to be effective values unless otherwise specified.
Source: FAA-H-8083-30B, page 12-49. Text as printed.
Select a highlighted phrase to read what it means.
Select a phrase in the handbook text.
For a sine wave, and only a sine wave, you convert with two numbers, 0.707 and 1.414. The 115 V on the placard is RMS.
Check any conversion. If an RMS answer comes out larger than the peak, you multiplied when you should have divided.
Many aircraft also have a 26 V, 400 Hz AC bus for instruments. Work each card, then select it to check.
A scope shows an AC sine wave measuring 200 V peak-to-peak. What does a meter on AC volts read?
A transformer is two coils of wire wound on one iron core, the frame the coils sit on. AC in the first coil, the primary, makes a magnetic field that keeps changing. The core carries the changing field to the second coil, the secondary, and induces a voltage there. More turns on the secondary steps the voltage up; fewer turns steps it down.
Select a source for the primary.
Every wire has resistance, so current through it turns some power into heat. The loss depends on the current. For the same power, a higher voltage needs less current, because current is power divided by voltage, I = P ÷ E.
1,000 ÷ 28 = 35.7 A
1,000 ÷ 115 = 8.7 A. Less current rides on smaller, lighter wire.
A generator sends 12,000 W to a town through a feeder, the pair of wires between them, with 0.2 Ω of resistance in all. Select a line voltage.
Voltage up 100 times, current down 100 times, loss down 10,000 times. The shortcut P = I² × R from the Ohm's Law lesson gives the same answer: 100 × 100 × 0.2 = 2,000 W.
In the 1880s Thomas Edison's direct current (DC) and George Westinghouse's alternating current (AC) fought to light American cities. The contest is called the war of the currents, and the transformer decided it.
1882Edison, DC
1884
1886Tesla and Westinghouse, AC
1888Tesla and Westinghouse, AC
1888Edison, DC
1892Edison, DC
1893Tesla and Westinghouse, AC
On 4 September 1882, Thomas Edison (1847 to 1931) started his Pearl Street Station in lower Manhattan. Steam engines turned its dynamos, the generators of the day, and sent 110 V DC to about 400 lamps for fewer than 100 customers. At 110 V the current was high, so the copper wire was costly and lost power to heat. A DC station could serve customers only within about a mile.
The dynamo room at Pearl Street Station, engraving, Scientific American, 26 August 1882. Deutsches Museum. Public domain, via Wikimedia Commons.Map: drawing made for this lesson, simplified. The district ran from Wall Street to Spruce Street and from Nassau Street to the East River. Portrait: Thomas Edison, photograph by Emil P. Spahn, 1880. Library of Congress. Public domain, via Wikimedia Commons.
Edison Machine Works, Goerck Street, New York, photograph by Charles L. Clarke, 1881. Gilder Lehrman Collection. Public domain, via Wikimedia Commons. Portrait: Nikola Tesla, photograph by Napoleon Sarony, about 1890. Public domain, via Wikimedia Commons.
Induction coil (a transformer), patent drawing by William Stanley Jr., US Patent 349,611, 21 September 1886. Public domain, via Wikimedia Commons. Portrait: George Westinghouse, photograph by Joseph G. Gessford, about 1900 to 1914. Library of Congress. Public domain, via Wikimedia Commons.
Electro-magnetic motor, patent drawing by Nikola Tesla, US Patent 381,968, 1 May 1888, Figure 9. Public domain, via Google Patents. Portrait: Nikola Tesla, wood engraving by T. Johnson after a photograph by Napoleon Sarony, The Century Magazine, 1894. Library of Congress, no known restrictions on publication.
A Warning from the Edison Electric Light Co., cover, 1887. Columbia University Libraries copy. Public domain (published before 1931), via the Internet Archive. Portrait: Harold P. Brown, Electric Railway Gazette, 16 November 1895. Public domain, via Wikimedia Commons.
Auburn State Prison, stereograph by Trowbridge and Jennings, about 1865 to 1885. New York Public Library. Public domain, via Wikimedia Commons.
General Electric shops, Schenectady, New York, The Engineering Magazine, 1896. Public domain, via Wikimedia Commons. Portrait: Thomas Edison, photograph by J. M. White and Co., 1893. Library of Congress. Public domain, via Wikimedia Commons.
The Grand Court at night, illustration by Charles Graham, Harper's Weekly, 24 June 1893. Library of Congress. Public domain, via Wikimedia Commons.The Westinghouse exhibit of Tesla motors, 1893. Heinz History Center. Public domain, via Wikimedia Commons. Portrait: George Westinghouse, photograph by Joseph G. Gessford. Library of Congress. Public domain, via Wikimedia Commons.
Map: drawing made for this lesson, simplified, with Pearl Street's district drawn to the same scale.The Niagara power house with its first generator, Cassier's Magazine, 1895. University of California Libraries. Public domain, via Wikimedia Commons.
Transmission line, Washington State, photograph by the Bonneville Power Administration, 2010. U.S. Department of Energy. Public domain, via Wikimedia Commons.A Navy technician servicing an F/A-18 generator, photograph by Kenneth Abbate, U.S. Navy, 2011. Public domain, via Wikimedia Commons.
Edison's DC stations could serve only the area close around them. Which reason explains that limit?
Sources: IEEE Milestones, Pearl Street Station, 1882, Alternating Current Electrification, 1886, and Adams Hydroelectric Generating Plant, 1895; ETHW, "Early Electrification of Buffalo"; Thomas A. Edison Papers, Rutgers University; T. Reynolds and T. Bernstein, "Edison and 'The Chair'," IEEE Technology and Society Magazine, March 1989; N. Tesla, "My Inventions," Electrical Experimenter, May 1919; The Electrical World, 11 August 1888 and 28 November 1896; U.S. Department of Energy, "The War of the Currents"; Tesla Science Center at Wardenclyffe; FAA-H-8083-31B, pages 9-27 and 9-59; The New York Times, 14 November 2007.
A 26 V, 400 Hz instrument bus feeds a lamp. You read it with the meter on AC volts and on DC volts, at 400 Hz and at 60 Hz, and then with a square wave, a wave that jumps between its top and bottom values instead of curving. On the dial, AC volts is marked V~ and DC volts V⎓.
The meter is on DC volts, across this live 26 V, 400 Hz bus. What will it read?
Then work the four steps in the panel beside the circuit. Each step checks itself, and the lab records your readings.
The lab did not load. Open the Peak, RMS and the Meter lab from the room's lab list.
The bus on the first page was live. Follow these steps to check an AC bus and read it the right way.
On DC volts (V⎓) the meter averages the wave. The two halves cancel, and it reads 0.03 V on a live bus.
Five questions, three answers each. Each answer tells you why it is right or wrong.
A magnetic field that changes inside a coil makes a voltage; a turning loop makes that voltage rise, fall and reverse in a sine wave, which is alternating current. Frequency counts the cycles each second, and the period is 1 divided by the frequency. The meter on AC volts shows RMS, and the wave peaks at 1.414 times that. A transformer needs a changing field, so only AC can be stepped up for the long run, and the higher voltage carries the same power on less current with less loss.
You can now explain that a changing magnetic field makes voltage, and call AC and DC by the direction of flow.
You can now turn a frequency into a period.
You can now read a 115 V bus as RMS, peak and peak-to-peak.
You can now explain why the transformer and AC won.
Set the meter to V~ before you trust a reading on an AC bus.
Sources: FAA-H-8083-30B, Aviation Maintenance Technician Handbook, General (2023), Chapter 12 (Magnetism, Electromagnetism, Alternating Current and Voltage, Generator Principles, Cycle and Frequency, Values of Alternating Current, Inductance) and Chapter 1 (ground power units); Encyclopaedia Britannica (Michael Faraday, Nikola Tesla, George Westinghouse); U.S. Department of Energy, The War of the Currents.