Magnets, Electricity You and Me.
By Bernie Willis
Some days I wish magnetism was as simple as moving metal filings around on a piece of paper with a magnet underneath. But if it were simple we wouldn’t have radios, lights, LED’s, cars, trucks or airplanes, just sail boats lost at sea out of sight of land.
Our understanding of magnetism and electricity go back to some adventuring Viking who with a rock suspended from a rope cruised to North America and an American printer /statesman who got shocked from a key attached to a wet string during a storm. Together these discoveries contain elements that have been engineered for our benefit.
Roald Amundsen’s replica brass head sits on a cement monument in Nome. He flew over the north geographic pole and is credited with finding that the magnetic North Pole had moved from it position discovered in 1831 by James Clark on the Boothia Peninsula. He was attempting to sail through the North West Passage in Canada’s archipelago and found that the pole had moved to King William Island. As I was nearing his overwintering harbor at Gjoa Haven in the Bonanza one beautiful summer day I noticed my compass tilt increase until it was rubbing on its case. It made me think of the pictures in the flight training books about magnetic lines of flux entering the earth at the poles. It’s true the trusty and required compass becomes useless at high latitudes. It’s also useless in turbulence, while turning or accelerating. But it can save your life when you know its secret habits. They’re for another day.
There’s a monument where the magnetic North Pole was years ago. But due to the molten metal down deep inside the earth the North Pole is not stable. Currently its about 800 miles North East of its first discovered location. That’s the basic reason behind the change of runway numbering and sectional chart magnetic variation changes from time to time.
Inside our aircraft we have a variety of magnets. There are the permanent type magnets in the compass or generator and the magnets that are temporary, made with electricity inside our alternators and solenoids. A piece of steel can become magnetic when rubbed with a magnet or it can become a magnet when wrapped with insulated wire as long as electric power is supplied. The same wrapped piece of steel when passed close by a permanent magnetic will produce its own electricity. In this situation you have a generator. If you have two pieces of steel each with its own wrapping and can switch electric power to one or the other you have a motor or a generator or a starter/generator, but not a starter alternator. In the alternator the permanent magnet is replaced with an electric magnet. I believe any steel can be magnetized but hard steels make better permanent magnets while softer steels are used for temporary or electro magnets.
This difference is important to us because of the strength of the permanent magnet vs the electric magnet. For the generator to produce useable power it must turn at about 1500 RPM, way above idle speed. Long taxi routes before parking can deplete the battery make the next start a problem. Long waits for clearances deplete our battery reserves requiring a longer time for it to recharge after departure. This is one reason why the FAA requires aircraft for IFR flight to have 20% excess charging capacity beyond the normal amperage draw of continuous use equipment such as radios, lights etc. However, if an alternate method of starting is used the generator will upon reaching enough speed recharge the battery.
Your alternator equipped aircraft will starting charging around 800 RPM. Soon after engine start the battery will be fully recharged. However, if you discover a dead battery and use an alternate method of starting you will still have a dead battery. If your ignition system is powered by magnetos the engine will run fine but nothing electrical will work, no radios, no electric flight or engine instruments, or lights unless powered by their own back up batteries. Just leaving the master switch on will deplete a battery in a few days because of the electro magnet in the master solenoid.
Most modern cars and light aircraft use alternators because they are lighter and produce more power than generators. There is a way to get them charging with alternate battery power. Two 9 volt batteries in series with wires to the appropriate terminals on the aircraft battery or cigarette lighter will sometimes initiate enough current in an alternator to energize its electro magnets and allow it to start charging. Firewall mounted batteries are not the place to try this. Pictured are four magnet powered devices in most of our aircraft. Can you identify them? There are at least a couple more important electro-magnetic switches. What do they do and where are they located in your plane?
We’ve come a long way from the sea faring Vikings and the tri cornered hat statesman/scientists but they made our modern electro -magnetic world possible. Now that silicon has been discovered to control electrons it seems low electric power/amperage consuming electronics will continue to bring consumer rewards for a long time.