Electricity and Magnetism Explained

Whenever there is relative motion between a conductor and a magnetic field, a current flow will be induced in the conductor. This and other mysteries are revealed here now!

As an airline employee and college professor, I’ve lived most of my life in large urban centres, such as Toronto and Montreal. The bustle of big city living and international airport employment in a reasonably high-tech, electronically based profession has meant that I’ve always been “tuned in, turned on, and wired up.” A recent (and wonderful) change of circumstances now sees me living in a small hamlet on the north shore of Lake Erie.

Although not quite as low tech as my many Amish neighbours, I now live without television or telephone, and with the expectation of multiple power outages each month. During a recent 4.5-hour blackout, the only sounds I could hear from my front porch were the crashing of waves against the Erie shoreline, and the distinctive “whoosh …whoosh … whoosh” of the 77-metre long blades on a nearby wind turbine (calculate the tip speed of THOSE babies turning at 20 rpm!).

This got me thinking about aircraft generators and alternators, and Transport Canada’s requirement in AWM 566 for AME students to “Explain:  Magnetism, electromagnetism, and Electromagnetic induction.”

The relationship between electricity and magnetism is a mysterious and fascinating thing. It is the phenomenon that enables us to create and harness electricity as well as convert it to mechanical energy in the form of motors and actuators. In order to understand how this “magic” works, we need only to know two very fundamental facts. These are:

1. Whenever electrical current flows through a conductor (wire) a magnetic field will be created around the conductor.
2. Whenever there is relative motion between a conductor and a magnetic field, a current flow will be induced in the conductor.

So let’s look at what these facts actually mean, and how we can use this information to better understand the practical applications as they apply to aircraft maintenance.

The fact that current flowing through a wire causes a magnetic field to be created around the wire means that we can easily create a magnet or more correctly an “electromagnet” by simply connecting a wire to a power source and allowing current to flow through the wire.  The more current we pass through the wire, the stronger the magnetic field will be. We can further increase the strength and effectiveness of the magnetic field by winding the wire into a series of coils.

A common application of this principle would be the operation of a relay or solenoid. A small amount of current passing through the coil of the relay/solenoid creates a magnetic field that is used to move the contacts. A more elaborate example of the same principle is the antenna of a radio transmitter.  When radio frequency (RF) current is coupled to the transmitter’s antenna, an electromagnetic field is created around the antenna, which is radiated in the form of a radio transmission signal.

The second fundamental fact states, “Whenever there is relative motion between a conductor and a magnetic field, a current flow will be induced in the conductor.” The term “relative motion” simply means that either the conductor is moving relative to the (stationary) magnetic field, OR the magnetic field is moving, relative to the (stationary) conductor. This is the principle upon which the operation of generators and alternators is based. In the case of an alternator, the mechanical energy of the engine is used to move, (in fact rotate) a magnet. This rotating magnetic field spins in close proximity to a number of conductors that are wound into coils known as “stators” or “stator windings”.  Since the magnetic field is moving relative to the stators, a current flow is induced into the stator windings. We can now use this current to power our various aircraft electrical systems.

In the case of a generator, the magnetic field is stationary, and mechanical energy from the engine is used to rotate the coils of wire (called the “armature”) within the magnetic field. A current flow is then induced into the armature windings, and this current is used to power the aircraft’s electrical systems.

The amount of electrical power required for the operation of a large aircraft will of course vary throughout the course of any given flight. Requirements for thermal anti-icing, lights, passenger accommodations and so on will constantly be changing and so then will the amount of electricity we need our alternators or generators to produce. The amount of current being generated is dependent upon the RATE at which the conductors are cutting the lines of magnetic flux. This rate can be changed in three ways:

1. Speed of rotation. If we rotate the field or the armature more quickly, we will induce more current.
2. Number of windings. By increasing the number of stator or armature windings, we will increase the amount of current being produced.
3. Magnetic field strength. By increasing or decreasing the strength of the magnetic field, we can control the amount of current output from the generator or alternator.

It is obviously impractical to consider re-winding the generator/alternator to meet current demands, and adjusting engine speed for that purpose is equally unsuitable for aircraft requirements, so that leaves us with controlling the strength of the magnetic field. This is indeed how aircraft voltage regulators control the output of the generators and alternators, to meet the constantly changing demands placed on the electrical system.

The magnetic field used by aircraft generators and alternators is actually an electromagnetic field powered by the generator/alternator’s own output. By passing more current through the field windings, we create a stronger magnetic field, and therefore increase the amount of current being induced into the armature/stator windings.

Whether your aircraft uses a simple belt driven alternator, similar to that on your car, or a 100 KVA, three-phase, 400 Hz, brushless monster, the principles of operation can be understood and problems diagnosed using the two basic facts:
1. Whenever electrical current flows through a conductor (wire) a magnetic field will be created around the conductor.
2. Whenever there is relative motion between a conductor and a magnetic field, a current flow will be induced in the conductor.
Now if you’ll excuse me, I’m off to the local pub to make some phone calls, and watch Monday Night Football.

Question:  How does a voltage regulator control the output of a generator?
Answer to previous question:
Q: If a voltage of 24 volts is applied to an eight-ohm resistance, how much current will flow in the circuit?
A: The formula for calculating current is I = E/R therefore current is equal to 24 volts divided by eight ohms = three amps.