Multipole ring magnet generator

[bmhiggs]

I've dissected a pig (flashlight) and found a hand crank generator that has a multipole ring magnet. I used a bar magnet to establish that it was a multipole magnet, because I was wondering if I could figure out the direction of the current based on the direction of the magnetic field.

My basis for understanding the direction of induced current is Lenz's law, which at the high school level simply describes the direction of the current in a loop given the direction of the magnetic field and whether it is increasing or decreasing. It applies the right hand grip rule according to the induced magnetic field in the wire.

What is the nature of the flux and the current induced in this circuit since it is a multipole magnet? It powers LEDs, so I would assume it is DC, but the multipole magnet seems like it would induce an AC because of the constantly changing direction of the poles of the magnet as it passes over the wire.

Here's the whole pig for context (it also has batteries, but in parallel), and the setup of the generator. It is located under the gray flywheel. Also an image of what I mean by multipole.

Thanks!

 

[Charles Link]

See https://www.comsol.com/model/homopolar-generator-14425
It would be interesting to see exactly what the design of your generator is=maybe others have more insight, but I find it difficult to assess it in very much detail, without having a more detailed description of the materials, and electrical points of contact, etc.

 

[bmhiggs]

It's just a coil of wire with a spinning magnet on top. The gear spins the flywheel, which houses the magnet, and rotates above the spool of fine copper wire. The copper wiring is connected directly to the lightbulb (no resistor, capacitor, etc.)

I guess this is more of an engineering question than physics, it's just I was struck by how closely related it is to what we are doing in class. The textbook has the following about Lenz's Law:

With the way the multipole magnet spins on top of the coil, the magnetic fields would seem to look like this:

It seems to me that it would produce an alternating current, but that doesn't make sense for an LED. The LED doesn't blink, it produces a relatively steady light, but that could be because the magnet is spinning fast enough that it doesn't seem like it (very likely, it spins fast).

Although, does alternating current even work like that? We are talking here about various segments of a wire that would be experiencing opposite fields at the same time... one particular segment would be experiencing alternating current, but it would be the opposite alternating current of it's two neighbors. What would that do to the current of the wire as a whole and as it extends out to the load?

Here's the circuit diagram in case it helps in any way. (Except the direction of the voltage source is obviously in question.) The switch toggles between the two voltage sources.

 

[bmhiggs]

I posted this thread over on the physics forum, and no one could help me...

https://www.physicsforums.com/threads/multipole-ring-magnet-generator.1057756/

Anyone here want to take a crack? It is probably more in the engineering wheelhouse.

 

[Baluncore]

A AC voltage will be induced in the coil, proportional to the number of turns on the coil, and the RPM of the magnetic rotor.
The AC current that flows, will be determined by the total resistance of the wire coil and the load.

 

[bmhiggs]

How does that affect the LED? Is it just a case of the frequency of the current being high enough that you can't tell the light is blinking?

Any idea why it would have been manufactured that way? Based on the other components of the circuit, a DC current from a regular (dipole) ring magnet seems like it would have been more practical.

 

[Baluncore]

A generator, with a permanent magnet rotor, generates AC voltage. It is an alternator. If DC is needed, a rectifier can be used.

You need to take a very close look at the circuit board, to see the rectifier, and any other components used. They are probably mounted with the LEDs. Get a close-up picture that shows the detail and part codes.

There may be no capacitor to smooth the DC voltage and current flow through the LED, your eyes do not notice the flicker.

Manufacturers save costs by reducing the number of components. The alternator coil inductance may be used to smooth the current, while the resistance limits the current.

 

[et_al]

I had such a flashlight, and I don't think it is a usefulness design. From other perspective, this kind of generater efficiently emits a sufficently high voltage to drive LEDs with such a simple structure.

You can note that there are two pieces of iron sheet on the both sides of the coil, and they extend the toothed structure upwards to the side of the magnet. In this way, the upper and lower iron plates are magnetized into corresponding magnetic poles and switch when the magnet rotates 45 degrees. The current is induced when the magnetic poles switch. So if use a regular ring magnet, no magnetic poles switch and no current. In fact, almostly all DC generator need a more complicated strcture, such as need a commutator. For the purpose of just light up, this structure is simple enough alough it may provide a high power for practical use.

For the question of flicker, we can carry on a rough estimate. If we consider the visual persistence of humans as 0.1 seconds, only a 2.5 cycle/s (150 RPM) rotational speed is enough to for you to ignore the blinking (because the direction of the current switch 8 times in a cycle).

I have a way that may help you observe this blinking. When making the flashlight work, you quickly swing the flashlight relative to yourself to observe the trajectory of the light (preferably in the dark). If there is blinking, then the trajectory you observe is dashed, otherwise it is continuous.

 

[Rive]

Any idea why it would have been manufactured that way?

Cheap. Also, an old design: some old style bicycle dynamos had this kind of structure.
Here is what they produce (voltage, without load).
Of course, due the LEDs doing their own rectifying and LED thing with this kind of load it'll be ... well: one sided and lot more messy...

Also, for this kind of thing you don't need efficiency or any finesse. What's not taken out as electicity will just remain in the flywheel and will be taken out later. As long as the flywheel can easily spin it's fine.

 

[Tom.G]

Probably no diode needed for the LEDs.
For instance reverse voltage rating is commonly 5V and up, but forward operating voltage for a White LED starts around 3V.

Cheers,
Tom