Dad killed the paper shredder. At least, we think he’s the culprit. Mom and I were always very careful about minding how many sheets could go in at a time and letting it cool off if it got too hot. Probably one place we failed was not running the lubrication sheets through like we should.
Dad seems to have tried to shred an entire catalog at once. I spent two hours digging compressed paper out of the machine shredding rotors, but the machine remains dead. I know from experience that if pushed too hard, the motor running the shredder can burn out.
But what exactly does that mean? My knowledge of motors is limited to this:
A motor is a device which turns electrical energy into kinetic energy. It involves magnets and conductive wire. The electricity creates a current in the wire, which creates a magnetic field, which makes the magnets spin in a circle. That motion is then transferred to . . . something else? Which makes the machine go?
I know ball bearings and bushings(?) and maybe brushes of some sort are also involved, but I have no idea how.
It’s not literally burning, is it? Because there was no sign of fire or smoke. Who will explain this to me?
These copper conductors are covered in a clear varnish so they are insulated from each other. Essentially it is a single piece of wire wound over the rotor or stator of the motor. If a motor is overheated the varnish can melt and the winding is short-circuited.
If that happens the motor has to be rewinded, ie the old winding removed and a new one put in place. This is an expensive and time consuming process and generally it is not cost effective on small home appliance motors
An electric motor has a copper coil and magnets (either electromagnets or permanent). Part of the motor rotates to extract energy from the opposing magnetic fields.
If the motor is prevented from rotating, this energy is not removed. The (now static) magnetic field resists the continuing flow of electrons through the wire and so the resistance in the coils goes up, increasing the current draw and dissipating the increasing electrical energy as heat in the wire. Eventually the heat destroys the insulation (often shellac, a type of varnish) in coil leading to a dead short, fatally destroying the coil or electromagnet. It does usually smell, but it may not take too much damage to kill the motor so this may not be noticed.
The other thing that may happen before this is that the brushes may burn out. The brushes are rotating contacts that supply electricity to the rotating part of the motor, and are usually carbon pads on a metal ring. If too much current goes through the brush it can heat up and burn out. Burned out brushes can be replaced more easily than a shorted coil.
A great demonstration of how resistive electromagnetic forces work is to drop a strong magnet down a tube of nonmetallic metal (aluminium or copper). The moving magnet induces a current in the tube, which causes a magnetic field that resists the motion of the magnet. It slows down as it drops, but it has to actually move (if it stopped, the induced field would disappear). With a really strong magnet, it can take a long time for the magnet to drop out the bottom of the tube. It is really odd to watch.
The other trick demonstrates the relationship between work out and in. If you hook up a motor with a crank as a generator and use it to drive a resistive lightbulb, you can turn the crank to make the light go. Adding more lights to the system increases the load (resistance) and makes the crank harder to turn.
According to Ohms law, true. I’ll plead inductive loads, plus thermal change of resistance, plus …
Oh, too complex (really, I’m sure that there are imaginary parts to the math). Certainly as the coils short out the resistance goes down and the current goes up.
Anyhow, jammed motor, bad thing, lets the magic blue smoke out.
One way to look at this is that if there are, say, 12 coils in the rotor, then each coil is only expected to have a 1/6th duty cycle at most (2 x 1/12th as the contacts pass the brushes one way, then the other at 180 degrees rotation) - the rest of the time, there’s no current flowing in the coil.
So in this scenario, jamming the motor means that in any given period, you’re running it at six times its expected duty - with no cooling interval, etc.
There are a lot of different motor designs, but generally speaking you’ve got a rotor (a part that moves) and a stator (a part that doesn’t move, or is static). The rotor is generally one or more coils attached to a shaft. There will typically be a gear or a belt attached to the shaft so that the motor can move or spin something else, but sometimes the shaft is directly attached to something with no gear or belt in between.
One of the simplest motors is called a Beakman motor (you can google that for more info). This is the type of motor kids often make in school because it is easy to construct and is really simple in design. You take some insulated wire and wrap it into a small coil, with the ends of the wire sticking out. You then unbend a couple of paper clips and stuck them in some sort of base, and use them to hold up the coil. The paperclips are then attached to a battery, and a permanent magnet is placed under the coil. If you are having trouble picturing it, it should look something like this: http://www.simplemotor.com/images/Conv4.jpg
On one side, you strip the enamel insulation off of the wire completely. On the other, you only strip the insulation off of one side of the wire. As electricity flows through the coil, a magnetic field will be formed. This will interact with the magnet underneath the coil, and the coil will rotate. If you completely stripped the wire on both sides, the coil would rotate until the magnetic fields were aligned with each other (north to south) and then it would just stay there, which isn’t useful as a motor. Since only half of the insulation was stripped though, the coil rotates, and then the un-stripped part of the wire comes in contact with the paperclip base and breaks the current, removing the magnetic field. The coil keeps spinning due to momentum, until the stripped part contacts the paperclip again, and the magnetic field forms again, rotating the coil again, and this keeps repeating over and over, spinning the coil.
That’s about as simple as you get for a motor. Most motors are based on the same basic principle, with a lot of variation. There will be a magnetic field formed by one or more coils, and the interaction of magnetic fields will make a shaft spin. Many motors use coils of wire with current going through them in place of permanent magnets.
Bearings and bushings are used to make the rotating parts rotate more easily. Without bearings, the rotor just slides across the surface of the stator which creates a lot of friction. If you instead put a bunch of bearings around the rotating shaft and hold them in place with bushings, the shaft can rotate with much less friction.
If you have a coil of wire on the rotor, how do you get electric current to it? You can’t just attach wires because they will get all twisted up as the rotor rotates. So you use “brushes”, which are just electrical contacts that “brush” against the rotor. The brush will touch electrical contacts on the rotor shaft when you want current to flow through that particular coil. The Beakman motor has only one coil, but real world motors can have many coils, so there may be several contacts placed around the motor shaft.
In the simple Beakman motor, the paperclips serve not only as the base that the coil sits on mechanically, but the also serve as the brushes to get the current into the coil.
As the others in this thread have already explained, if there is mechanical resistance to the motor spinning, the current in the coils increases. This causes the coils to heat up. The insulation around the wires can melt, causing the coil to short out. The wire itself can also melt and break. If the insulation is flammable it can catch fire and make smoke. If enough current flows the wire can also burn, making smoke. So something very well could be burning.
It is possible to design a motor that can withstand a locked rotor condition. This requires heavier wire and thicker insulation, which adds weight and expense, which is why most motors will self destruct if the rotor is physically prevented from turning.
It’s slightly more complicated than this. A DC electric motor, when rotating, produces its own counter EMF that varies with RPM. This counter EMF acts against the power supply voltage, limiting the total current flow through the motor. An unloaded DC motor spins at some characteristic speed where the back voltage almost completely cancels the power supply voltage, resulting in just enough current/power flow to counteract friction and windage in the motor.
(BTW, counter EMF is why a DC motor can also be used as a DC generator: supply mechanical power to the rotor, and you can measure a voltage being produced at the electrical terminals; that voltage is the counter EMF. Power tools (such as chop saws) that have an “electric brake” feature take advantage of this behavior: when you release the power switch, the motor terminals are disconnected from wall voltage and instead are connected to a power-dissipating resistor. Now the tool’s motor is generating electrical power that gets dumped in that resistor, rapidly slowing the motor to a stop.)
When a DC motor is prevented from rotating (a locked-rotor condition), it can’t generate any back EMF. Now the windings in the rotor are subjected to the full power supply voltage, and the current flow becomes very large because the windings don’t have much resistance. The motor will heat up pretty quickly, and as others have noted, eventually the insulation on the windings will melt and you’ll develop a short-circuit. That lowers the circuit resistance even more, resulting in higher current flow. At some point an individual metal wire in the windings melts completely through and opens the circuit, or a fuse pops. Or the circuit breaker in your basement opens. One of these things has to happen to shut down current flow through the motor, otherwise it will keep heating up and start a fire.
If the shredder never started to smoke or stink, there’s probably a thermal fuse or circuit breaker somewhere on it that can be reset.
What about an RC motor? Does the wind increase resistance and lower the amps? As long as the prop doesn’t stop turning and the initial voltage is low enough not to cause the current to exceed the amp limit, is it safe to put whatever resistance on the prop you want?
shredders usually use series-wound universal (works on AC or DC) motors. These have field coils wired in series with the armature, and pass current to the armature via brushes and a commutator. When you kill a series-wound motor, either the windings overheated and opened/shorted, or the brushes and commutator overheated and burned.
Mangetoute and si_blakely thanks for your post taught me alot, and im grateful now to buy a cheap motor from China/Korea and try to repair my cordless drill-for no reason other than i don’t like throwing away stuff and it is possible/hopefully cost effective (ebay) + ill learn something.
any thoughts on the best place to purchase cheap electronic motors fit for my purpose also what kind of specs must i note when looking for a replacement motor on ebay etc?
Sometimes they quite literally overheat and catch fire (warning: coarse, but hilarious, language. Probably shouldn’t watch at work without headphones unless you work in an oilfield of the like.)
Buy a broken model similar to yours with a good motor. Whether its cost effective depends on a lot, replacing a motor on an old or low end cordless probably wouldnt be. Afaik most cordless replacement parts are manufactured overseas.
