Eddy current heating for the bored pickup maker

[Mike Sulzer]

Sure, but the motor drawing more current still has noting to do with the disk warming.

It sure does. The energy to heat the disk comes from the motor. The eddy currents tend to slow the rotation, and it is not surprising that the motor responds by drawing more current in order to provide the energy for the heating.

[There is more than one way to respond to an increased load. What happens when you cover the end of the hose of your vacuum cleaner, thus increasing the load on the system? It does not burn up because that system responds by drawing less current not more.]

[David Schwab]

It sure does. The energy to heat the disk comes from the motor. The eddy currents tend to slow the rotation, and it is not surprising that the motor responds by drawing more current in order to provide the energy for the heating.

[There is more than one way to respond to an increased load. What happens when you cover the end of the hose of your vacuum cleaner, thus increasing the load on the system? It does not burn up because that system responds by drawing less current not more.]

So if it were a gas powered motor and it used more fuel that would make the disk hotter? It's two separate things. The rotation is making the disk hot. The motor is making it rotate, but not making it hot. The energy to heat the disk comes from the magnets. Without the magnets the disk will rotate and not get hot.

Magnetically couple the motor and disk with an air space between the two. The disk will get hot because its rotating in the magnetic field, and if the motor draws more current and gets hot that has nothing to do with the disk getting hot. it's an indirect association. The motor is causing the rotation, and the rotation/magnets is causing the heating. The motor is not directly causing the heating.

If you cover the hose on a vacuum with your hand, and the motor gets hot, but your hand gets cold, is the fact that the motor is working harder making your hand cold? No. The air passing through your fingers is making your hand cold. This is regardless to how hard the motor is working.

[Steve Conner]

You guys need to brush up on your conservation of energy. :-P

The energy used to warm up the disk does not come from the magnets. That implies that the magnets would get used up and become less magnetic in the process, and they don't.

It comes from the mechanical power transmitted to the disk through the shaft. This power came from the motor, which converted it from electrical power.

That conversion from electrical to mechanical is not 100% efficient, and that's why the motor gets warm too, and more so the more power it's called on to convert.

How much power the motor is called to convert depends on a lot of other things, such as the braking torque exerted on it by the magnets, the torque/speed characteristic of the motor, the supply voltage, and so on.

[Mike Sulzer]

You guys need to brush up on your conservation of energy. :-P

What did I say wrong about about energy conservation? But carry this back as far as you want. The motor got the energy from the power line which got it from the power plant, which got it, for example, from coal. The energy in the coal came from the sun, which fused hydrogen into helium. Beyond that, the astrophysics gets more complicated.

David wrote: "So if it were a gas powered motor and it used more fuel that would make the disk hotter?"

Yes. In this case the increased resistance on the shaft would be countered by the engine slowing; the governor would feed more gas and the engine would speed up to nearly the same speed if everything is working right. The additional fuel that is burned would also heat the engine because it has limited efficiency and so only a fraction of the energy from the increased burning of fuel is transmitted through the shaft.

Suppose the disk were superconducting. There would be no energy dissipated in the disk. Would there still be drag on the disk? If so, where would the energy be going?

[David Schwab]

The energy used to warm up the disk does not come from the magnets. That implies that the magnets would get used up and become less magnetic in the process, and they don't.

The heating is being caused by the eddy currents. What's causing the eddy currents? The magnets, or the motor? The answer is the magnets. You can spin the disk all you like without the magnets, and unless you are causing some other breaking, such as by friction, it wont get warm from spinning.

The Permanent magnets produce their own magnetic fields and don't rely on the external energy of the motor, so there are two different systems at work here. Yes, you can't spin the disk without an applied force from somewhere. But the motor is going to get warm on its own with no load, and a string enough motor wont be effected by the drag of the magnets. So I still contend they are unrelated, yet intertwined.

[David Schwab]

David wrote: "So if it were a gas powered motor and it used more fuel that would make the disk hotter?"

Yes. In this case the increased resistance on the shaft would be countered by the engine slowing; the governor would feed more gas and the engine would speed up to nearly the same speed if everything is working right. The additional fuel that is burned would also heat the engine because it has limited efficiency and so only a fraction of the energy from the increased burning of fuel is transmitted through the shaft.

I made three examples to eliminate the heat being transfered through the shaft. One was cooling the motor and shaft so it doesn't get warm, the other was using a long enough shaft that it would air cool before it got to the disk (not as efficient). Even better, use a drive belt. And the last was to couple the motor to the disk using magnets separated by an air gap. In these cases (and in an ideal world), no heat would be transfered up the shaft from the motor to the disk. So would the disk still get warm? Yes, but not from the hot motor, from the eddy currents.

[Mike Sulzer]

The heating is being caused by the eddy currents. What's causing the eddy currents? The magnets, or the motor? The answer is the magnets. You can spin the disk all you like without the magnets, and unless you are causing some other breaking, such as by friction, it wont get warm from spinning.

And it will not keep spinning if you do not provide energy. The motor does this. The energy from the spinning is converted into heat when the electrons that are set in motion collide with the metal lattice. You must replace that lost energy to stop the disk from slowing down.

[David Schwab]

And it will not keep spinning if you do not provide energy. The motor does this. The energy from the spinning is converted into heat when the electrons that are set in motion collide with the metal lattice. You must replace that lost energy to stop the disk from slowing down.

Of course. I'm not disagreeing here as to the cause of the spinning, just the heating. I'm saying that the motor getting hot is not the cause of the disk getting hot.

If the disk was powered by you pedaling a bicycle, and you were getting hot and sweaty, that wouldn't be making the disk hot. You are making the disk spin, and the magnets are making the disk hot.

The direct cause of the spinning disk is the motor. The direct cause of the heat is the magnets. Remove the magnets and the disk wont get warm, yet the motor is still spinning and drawing current. That shows they are not directly related as far as the source of the heat, just that the motor is spinning the disk. The motor does not produce the eddy currents.

[David King]

The vacuum cleaner is a confusing case. When you block the air the motor speeds up, That should be the clue that's doing less work. (It's doing less work because the air resistance drops dramatically when there are fewer molecules to push around -same reason airliners fly at 40,000ft.) The question is then, "Why do stopped-up vacuums burn out much faster? They burn up because they can't draw any air through the motor to keep cool. Really good vacuum cleaners don't have this problem because they have a separate cooling system besides the suction and exhaust flow.

[Mike Sulzer]

The disk is harder to spin if the magnets are there than if they are not. That is because you must provide the energy that heats the disk.

[David Schwab]

The disk is harder to spin if the magnets are there than if they are not. That is because you must provide the energy that heats the disk.

Agreed. What I wasn't agreeing with it that the motor gets warm and that heats the disk. That was said a few times.

After all, if you set up a long track on a long steep incline and rolled something on wheels down the incline that had the disk and the magnets, the disk will still get warm, and eventually slow or stop the carriage it's attached too. Kind of the way a roller coater break works. And no motor was involved.

This is a fun forum!

Eddy current - Wikipedia, the free encyclopedia

Eddy currents, like all electric currents, generate heat as well as electromagnetic forces. The heat can be harnessed for induction heating.

[Steve Conner]

I didn't mean you personally, Mike.

If the disk were superconducting, the magnets would cause no drag. (The drag is caused by IR losses.)
YouTube - IFW-Dresden Superconducting Maglev Train Models

A similar effect to ponder: The better the conductivity of an induction motor's rotor, the lower the starting torque. (The motor's torque is just another kind of eddy current drag.)

[Mike Sulzer]

I didn't mean you personally, Mike.

If the disk were superconducting, the magnets would cause no drag. (The drag is caused by IR losses.)

What about the energy stored in the magnetic field resulting from the eddy currents?

Consider a superconducting inductor. If it initially has no current, and you then connect a voltage source across it, current through it increases according to the usual equation. The energy from the voltage source is stored in the magnetic field resulting from the current flow. If you disconnect the voltage source and complete the circuit, current continues to flow and the magnetic field holds on to its energy. These devices are used for energy storage in some applications.

I do not think that this is a very big effect in this case, but I would expect some drag on the superconducting disk while the currents are increasing. And they should keep increasing until something breaks down. (Super conductors can only take so much current.)

[Joe Gwinn]

To clarify things, let's replace the electric motor with a water wheel, like in the factories in the 1890s.

Or, like the display in the Franklin Institute (a science museum in Philadelphia) in the 1950s: It was a bicycle with the driven wheel replaced with a slotted copper disk which passed between the poles of a big electromagnet. The slots were radial. The bicyclist pumped away furiously, getting the disk up to speed, and then turned on the electromagnet. The disk stopped quite quickly, and pumping didn't have much effect. It was as if the disk were immersed in heavy grease, until the electromagnet was turned off. One could have gotten the same effect with a set of neo magnets that could be moved so their field did or did not go through the disk.

[rockin roy]

You can make the experiment a lot simpler. Just drop a strong magnet through a copper pipe. It takes extra time for it to fall because of the distortion of the magnetic field.
Some of the heat is coming from your magnets and they are getting weaker. The rest is from the motor distorting the magnetic field.