Drill It Into My Head
June 22, 2008 6:03 PM   Subscribe

Help me understand electricity and physics and how they apply to motors. I have my head around electricity basics and electromagnetism, but I have a few unanswered questions about the more subtle aspects.

Take a cordless / corded drill for example. When I press 'Go' on the drill, it is my understanding that a circuit is created, and in between sits the load / electromagnet(s). Is all of the electricity consumed by the electromagnet? If not, where does the rest go? Is the speed of the drill controlled mechanically through gears or through some type of electro-thingy? How does all that torque get created? Are there powerful magnets driving it? How is the motion reversed? Instead of asking a million questions, can someone explain how all the magic happens in a variable speed drill and where are those electrons eventually end up.
posted by kaizen to Science & Nature (11 answers total) 5 users marked this as a favorite

Here's what I remember from high school physics.

Well, the basic idea is that when electricity travels along a path (like a wire) it creates a magnetic field around that wire.

If you think about a regular magnet with 'north' and 'south' ends, you can imagine lines of a magnetic field traveling from the north end to the south end, as in this photo, or this illustration. (or this one But, when electricity is traveling in a straight line, the 'feild lines' go around the path in a circle, like in this picture.

Now, here's the cool part, when you take a wire and wrap it in a coil, the field lines created by the coil work just like a regular magnet! here is an illustration showing that.

So by sending electricity through a coil, you can create a magnet, when you turn the electrical signal off, the magnet goes away. If you reverse the flow of electricity, the magnet's polarity will change and it will attract where it repelled before.

So, if you send alternating current through a coil, you create a simple linear motor that will move something back and forth. In fact, that's exactly how a loudspeaker works.

To get a circular motor, you just arrange the coils and magnets so things go in a circle. I'm not exactly sure how that's done, though.
posted by delmoi at 6:16 PM on June 22, 2008

learn the left hand rule
posted by SatansCabanaboy at 6:37 PM on June 22, 2008

The cordless drill is easiest, because it's supplied with a constant voltage all the time from the battery. When a coil is connecting to the battery, current flows through it, turning it into a magnet until it's disconnected. The motor is just a set of coils arranged in a circle and a permanent magnet that's allowed to rotate (the rotor).

The coils don't all carry current at the same time, but do it in a time pattern such that the permanent magnet is always pulled in one direction (when one coil has pulled the permanent magnet as close to it as it gets, the next coil takes over, pulling the permanent magnet close, then the next again and so on until a turn has been completed). In simple DC motors, the position of the rotor determines what coils are powered and which are not. There are also stepper motors where an external electronic circuit takes care of the coil-powering sequence.

In your cordless drill, you probably don't have a stepper motor, but control the power by making the current through the coils higher or lower. You simply change the current passing through the motor as a whole, not caring which coil it's presently running through. You can do this in a simple way by putting a variable resistor in series with the motor. Much of the total power consumed goes to heat in the resistor, so this kind of control is only used in the very smallest and cheapest devices, not in power tools.

Instead, the power from the battery is switched on and off very rapidly, and the harder you squeeze the trigger, the more time is spent in the "on" state. The switching takes place maybe 10000 times per second, so you don't notice it. In fact, the motor doesn't notice it much either, because between the switching device (a transistor) and the motor is a small electron storage device (a capacitor), that can be charged and discharged much faster than a regular battery. When the switch is off, the motor gets some electrons from this reservoir and when the switch is on, some of the electrons go to replenishing it. As a result, the motor gets a fairly even current, but when the reservoir spends more time getting drained than refilled, the average current through the motor is low. The coil-magnets are then not so strong and therefore the torque is low.
posted by springload at 7:30 PM on June 22, 2008

Okay, here's an electric motors braindump. I hope it makes sense...
Is all of the electricity consumed by the electromagnet?
I'm not sure how to approach that question ... the electricity flows in a circle; exactly as much goes in one blade of the power plug as comes out the other blade. What gets used up is the electricity's ability to do work, which is proportional to the voltage (potential) of the wire the current's flowing through. It's analogous to water pressure in a pipe. So I think your question might boil down to "where does the voltage drop occur, and why?".

Understanding AC motors can be a little complicated because there are a bazillion different kinds in common use, and they make use of different techniques to produce rotation. So I'm going to describe this as if the drill were running on DC.

Simplifying down to the basics common to all motors, what's going on is that the electric current produces a magnetic field (a rotating or changing one), and this field pushes/pulls against another field (which comes from permanent magnets, or another set of coils, or (in the case of squirrel-cage motors) it is summoned from the vasty deep), and that pushing/pulling turns a shaft.

A magnetic field contains energy. If you turn on an electromagnet, for example, the current through it doesn't instantaneously go from 0 to its full value. It takes a moment to build up, because it's moving energy into the magnet's magnetic field. From an electrical point of view, the coil's inductance seems like the coil resists any change to the current flowing through it, because to make a change you have to put energy into or out of the field. In effect, there's an extra electrical voltage being applied by the field to oppose you, which is called the back-EMF.

There's a counterintuitive thing here (at least to me). While you're building up the field, the electromagnet appears to resist your attempt to put current through it. Once the field is at a steady state, though, the electromagnet doesn't oppose you (in a real-world electromagnet, there's some small resistance in the wire, which leads to heat, but hopefully that's a minor detail). So when you're no longer putting any energy in is when the most current is flowing. But this makes sense, because "no resistance" means "no voltage drop" across the coil, since the electricity is flowing through without doing any actual work.

If you then use the magnetic field to do work — say, to push a shaft around — then you take energy out of the field. From an electrical perspective, this looks like you've been set back to when you were still building up the field, and the coil will oppose you again: you'll see a back-EMF. This back-EMF is, basically, the electricity doing work to rebuild the magnetic field.

So, putting all that together: the electrical current does work to create magnetic fields, and the fields do work to turn the shaft.

If the motor is allowed to turn freely, it speeds up until the back-EMF equals the voltage applied to it from the wall. At that point no work is being done, in the physics sense: although the shaft is turning, it's not applying force to anything (well, there's a tiny bit of friction in the bearings); and although the current being sent through the motor is dropping a full 110 volts, there's actually little or no current being sent through the motor — the back-EMF is holding it off.

If the motor is stalled, then there's no back-EMF, a large amount of current flows, the fields are quite strong (lots of torque), and the various non-ideal resistances dissipate a lot of energy (the drill gets hot). But again, it's not doing any actual work, in the physics sense: the shaft isn't moving, and although there's lots of electrical current flowing, the voltage drop is "zero" (again, not quite zero in the real world, since the windings have some resistance: but if you had a superconducting drill...)

In between those extremes, it does actual work.
Is the speed of the drill controlled mechanically through gears or through some type of electro-thingy?
Electro-thingy. The torque the drill supplies is basically proportional to the amount of current going through it. (Which will depend on how much voltage you've supplied, and how much back-EMF you're seeing). More current means stronger magnetic fields. There are a bunch of ways to control the current. The old-fashioned way is a rheostat, which just burns some of the electrical energy as heat, and leaves only a fraction of the wall's voltage to be dropped in the motor. I'm guessing newer drills use PWM to avoid wasting power. Some industrial motors will switch multiple coils in or out of the circuit to adjust the motor's power. (Or use gears, I guess.)
posted by hattifattener at 7:36 PM on June 22, 2008

One extremely basic electrical concept to understand FIRST is that electricity always travels in a loop. A battery has two terminals (one at each end) because the electricity flows out one end and into the other. A wall outlet has two holes (plus an optional third hole for grounding) for the same reason. The electricity basically comes out of one of the terminals/holes eager for work, goes through the device to do work, and goes into the other terminal/hole exhausted. But it does always have to go back.

That should make the other stuff (e.g. the posts above) easier to understand.
posted by intermod at 9:24 PM on June 22, 2008

Response by poster: So if electricity goes out one terminal, through the electromagnet, and into the other terminal, where does it then 'go'? Into the ground?
posted by kaizen at 2:31 AM on June 23, 2008

Some people have already had some good answers but I'll try to give you some simple ones that address the same things I was curious about, hopefully it's what you're asking about in your post. apologies to those in the know if my explanation has mistakes in it, i'm sure you'll correct me :)

Check out this on batteries, here on current and here on motors, all from howstuffworks which seems to have gone downhill..

Batteries involve a chemical reaction, but there are plenty of different varieties out there, some of which the reaction is reversable. As well as having by-products from a reaction like heat, light, and weird coloured gunk, you can also have free electrons. Inside every battery is a reaction like this. Course, in a battery sitting on a shelf, they don't have anywhere to go, so the reaction doesn't progress very far.. but when you hook it up to a circuit, cool stuff happens.

A circuit will pull out what it needs from a battery. Imagine a battery as a full balloon with a valve hooked up to a pipe, only, the pipe is full of lots of air too. The air can't get out of the balloon into the pipe without something draining the pressure from the circuit. And yeah, every circuit is a loop, so.. just imagine the air getting back to the balloon eventually only now it's tired and wants to lie in it's air-bed and go to sleep.

Technically though nothing is "flowing" down the wires in the circuit, it's more like those executive desk toys with the marbles, or making a sine wave with a long rope. There is energy flowing down the path, but it's not about something physically moving.

Anyway. There are different types of magnets. Electric motors feature some fairly weak electro-magnets. Whenever you have current in a wire you have a small electromagnetic effect, but if you loop wire around a piece of metal you can create a decent one.
You can then use said electromagnet to pick up metal (you can do this with some wire, a paperclip and a 9v battery) but the tricky part is turning that effect into rotational motion.

If you pull apart an electric motor (go to some electronics store or hobby/model store, buy a cheap motor and pull it to bits, it's worth it) you'll see a central axle which rotates in a housing, and on the outside of the housing you'll have two magnets. The axle is wound with wire, and running a current through the contacts electrifies the wire causing the axle to become an electromagnet, repelling from the magnets on the outside, and rotate away from one side of the first magnet, be attracted to the next side of the other magnet, rotate some more, then be repelled by the far side of the second magnet.

The differing speeds of your drill are controlled by a variable resistor. Imagine the air in the pipe again, but suddenly halfway down your pipe gets very thin. Now, not much air can get through, and when it's moving through, it's all cramped and can't move around much.
A variable resistor is the same basic principle only you can change the resistance - a volume knob on your speakers is a variable resistor. If you're having trouble, imagine a speed bump for the electric current and a lever to change it's height. and/or some mechanical gearing.

It gets tricky when you have AC (current that changes direction constantly, like from your wall socket) and the invention of the AC motor was a significant turning point in the history of electronics, but your cordless drill uses a DC motor which is a lot simpler.

As for torque, well, most electric motors have very high rev counts but not much ooomph so you need some pretty heavy gearing just to turn a standard electric motor into anything vaguely able to drive a heavy load. If you've been in an electric car you might have noticed however that the torque is instant - going from 0-100% power is instantaneous.

Anyway. I know many people already have posted more accurate and in depth answers but hopefully this totally simplified explanation gave you some insight!
posted by Dillonlikescookies at 2:49 AM on June 23, 2008

Oops, bit of a copy and paste artifact there. Cordless drills use both a variable resistor (usually on the trigger) and some gearing at the end. You'll find at the faster gearing the drill has a lot less grunt.

As regard to your question, the electricity doesn't go anywhere., it's just the energy. Think a mexican wave - no one in the wave moves from their seat, but there's "something" traveling down. I have a few other metaphors if you are having trouble getting it. The depleted charge ends up back in the battery as the balancing out of the surplus created from the chemical reaction in the first place.
posted by Dillonlikescookies at 2:55 AM on June 23, 2008

Don't think of it as "the electricity" going places. Think of it as electrons - tiny little particles that have a physical existence, just like water molecules in a pipe, a river, or a cloud - going places. Voltage is the push that gets the electrons moving, current is a measure of how many electrons pass per second.

Where does rainwater go? Seeps into the ground, some evaporates into the clouds again, some finds it's way to creaks and rivers, etc. etc. Similarly, billions of electrons are everywhere all the time.. Electrical devices work by pushing those electrons around, but the electrons are never created or destroyed.
posted by Chuckles at 12:29 PM on June 23, 2008

When you buy a battery, you are buying an object with an electron-friendly side and an electron-unfriendly side. One way to make an electron-unfriendly side is to put some extra electrons there: this is what happens to you when you shuffle across a carpet and zap the doorknob. One way is to wave a magnet around. Generators usually use falling water or steam or exploding gasoline to turn a wheel with a magnet tied to it, which makes charge move around in some conductor. Batteries use chemistry. I had fun with the links I found for the last battery chemistry AskMe.

A motor is an anti-generator: you use an electrical current to move a magnet around, and tie a drill bit to the same wheel as the magnet.

The energy "goes" partly into pushing the electrons through the wire (that is, resistive heating) and partly into the motion of the magnet and its wheel.
posted by fantabulous timewaster at 3:50 PM on June 25, 2008

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