Magnetism
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- This topic has 14 replies, 6 voices, and was last updated 21 years, 2 months ago by
betty.k.
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November 13, 2003 at 3:20 am #11508
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. -
November 13, 2003 at 3:47 am #23763
aah, well done trash! very simple to understand and informative. but let me get this right. let’s say i attach a motor instead of a steering coil to the pcb. when i push left on the tx the motor spins one way, and when i push right the motor spins the other way. is this correct? if so, what voltage goes through the steering coils, the same as the motor? (VERY inspirational lecture!!:D):8ball:
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November 13, 2003 at 5:05 am #23766
Betty, the reply to that is as long as Trash’s first post above.
I’ll just say the the steerng coils only turn on and off, they dont reverse in polarity like the motor circuit does.
:)uA -
November 13, 2003 at 5:25 am #38962
bugger! i got excited for a minute there!:evil::8ball:
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November 13, 2003 at 7:06 pm #23396
Trash mentioned “The electro magnet attacts or repulses it steering the car” (in conjuction with a fixed permanent magnet attached to the steering), but I think he’s jumped one step too far – therein the confusion.
BCG technology only uses ‘attract’, doesn’t bother with ‘repulse’. ie you’ve got 2 coils for L & R and only one is turned on when you steer. The other one isn’t powered to reverse polarity to push the magnet away.
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November 13, 2003 at 11:49 pm #23402
Thanks Panda for the correction.
I was trying to be generic so that is understood that electromagnets can repulse as well as attract.
I wonder if uA can explain how he’d make it posible to drive a motor instead of the two coils.
🙂
Else I might have to write the next thread on diodes and bridge rectifiers ! -
November 14, 2003 at 12:26 am #23403
Underlying theory: “Opposite (poles) attract” 🙂
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November 14, 2003 at 12:47 am #23404
i’d be interested in knowing how to drive a motor instead of steering, if it’s not too difficult:8ball:
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November 14, 2003 at 12:52 am #23405
The Russians had a theory about the poles too.
When America created Disneyland, the Russians were jealous, so they put a fence around Poland.
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November 14, 2003 at 4:37 pm #23411
Very well compiled trash!!!
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November 14, 2003 at 11:15 pm #23708
Betty, I can think of a way of doing it with two diodes, two resistors and two transistors.
Might be tricky to draw it but I can send you a jpg via email if you like.From the L coil wire you solder a diode (Anode).
The Cathode you solder to one side of the motor.
From the Cathode you solder the Collector of a NPN
transistor (BC848). The Emmiter you solder to the ground wire of the coil. The base you solder to a 100 ohm resistor and the other side of the resistor you solder to the R coil wire.
You then duplicate this again but for the R coil wire.
This will let you control a motor instead of the steering coils. -
November 15, 2003 at 12:16 am #23425
no need for a pic, i understand perfectly. i’ll save that one for a rainy day. hmm, are those clouds i see?!:8ball:
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November 15, 2003 at 4:21 am #26021Quote:I wonder if uA can explain how he’d make it posible to drive a motor instead of the two coils.Quote:i’d be interested in knowing how to drive a motor instead of steering, if it’s not too difficult
Due to popular demand…….
Firstly, the drive circuit is controlled by 2 signals, forward and backward, these go into a H-bridge (4 transistors in a H formation) that reverses the polarity of the voltage going to the motor.
The steering controls also use 2 signals, left and right, if you were to couple these signals to a H-bridge which was connected to a motor, upon chosing ‘left’ the motor would spin one way, and chosing ‘right’ would make the motor spin the other way.
I have to refer to this pic again because it shows the connections of a simple H-bridge from a BCG.
Pins 10 and 11 are the forward and backward signals that turn on the transistors, but these could easily enough be substituted by the left and right signals from the steering control.
This is exactly the control method used in the micro tanks and some of the micro cars that have motor steering, eg the HSV cars.I know this isnt the definitive answer, but if you can understand the H-bridge diagram then you are half way to converting the steering control to a motor.
If you arent 100% sure you might want to do some homework first.Quote:From the L coil wire you solder a diode (Anode).Jamie, unfortunately most rectifier diodes have a forward voltage drop of around 1.2v, this kind of drop is too much in a low voltage circuit and would kill a large part of the current to the motor.
Quote:Else I might have to write the next thread on diodes and bridge rectifiers !Nooooo, please, no more……:smiley2:
:)uA
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November 15, 2003 at 5:22 am #22962
OoOh i jus could be bothered to read the first few lines of trash’s post..
I’m studying motors n generators in yr 11 physics at the moment 🙂 .. maybe i’ll take a read later..
Faraday’s law.. Lenz’s law..
Magnetic flux 😉 and all that stuff hehe
😀
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November 15, 2003 at 4:01 pm #38944
Flux is a four letter word ? 🙂
Good point uA, I forgot about the voltage drop across the diodes. I usually work at higher voltages, so it’s easier to ignore a few missing volts. I always thought it was about 0.7 Volatge drop across diodes, but again, I know it’s as low as 0.5 for some and higher for others.
Given than most SMD diodes come in the same package as transistors, it would make a lot of sense to use 4 transistors.
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