I thought this would make a good subject, and it’s not so hard to describe it without using formulas. I hope it’s not too heavy reading for some of the younger guys. If they are re-winding their own motors and actuators, then it might be of some use to them.
AC and DC and magnetic fields
You’re familiar with DC from a battery. You turn it on and the current flows in one dirrection.
So why then do we use AC (Alternating Current) ? Why make electricity flow backwards and forwards in the wires instead of just in one dirrection ?
Well I all has to do with Magnetic Fields.
We know how to make a magnet with electricity. We take a coil of wire
wrapped around a nail and connect the wire to a battery. Vola !
The nail becomes a magnet.
But when we hold a magnet to the nail, electricity is not produced in the wire. If you have a multimeter, connect it to the wire and try it.
No voltage or current in the wire ?
BUT, if you take the nail/coil in one hand and wave the magnet in front of it with the other hand, them multimeter starts to move.
The difference is the magnetic field is changing in strength.
I guess you can think of it like bicycle pedels. Putting you feet on the pedals doesn’t make the bike move, but putting you foot on one pedal after another does !
So now if we take two coils of wire and put them on the nail.
Connect the battery to one and the meter to the other.
Nothing registers on the meter. But, when we connect the battery,
the meter jumps, and when we disconnect the battery, the meter jumps again ! We’re turning the magnet on and off, the faster we do it, the more the meter jumps.
This is a transformer. So electricity is getting from one coil to the other without the two actually being connected.
Even better, if we make one coil bigger than the other, we will find that we get more or less voltage out of the other coil.
Your small wall plug pack steps 240 volts down to something safe like 6 volts. The ignition coil in your car steps 12 volts up to something like 5000 volts to power the spark plugs.
But this is just the start, magnetic fields are EVERYWHERE !
The steering actuator in your little BCG car is a little electro magnet with a permanent magnet. The electro magnet attacts or repulses it steering the car.
Tiny little electromagnets are in the stator of your motor and little magnets on the outside. As the electromagnets spin around, they turn themselves on and off and are attracted to the magnets as they fly around.
An electromagnet is in your stereo speakers. As electricity is applied it moves the cone forward and backward against a fixed magnet in the back of the speaker.
I was asked a few years ago what purpose does it serve to make eletricity go around and round in circles like in a coil.
The answer is that it creates a temporary magnet.
Whenever current is flowing in a wire, it has a magnetic field around it. The dirrection of the field is clockwise around the current flow.
It is sometimes called the “right hand screw rule”. If your thumb on your right hand points the way current is flowing in the wire, your fingers represent the magnetic field wrapping around it.
If you wrap the wire up in a coil, the magnetic field is concentrated into the center of the coil. Depending on which way the coil winds and which way the current flows, causes which end of the coil is north or south.
A coil with positive winding clockwise to the negative will have its north pole at the end with the negative end of the coil.
(gee I hope I have that right ?)
If the coil winds the other way, the magnetic polarity will be reversed. The same can be said for reversing the current.
Further reading you might like to look for “left hand rule”.
I’m not sure it will be of much use to anybody here though.
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