Upside down generator help.

[large_ghostman]

HI

Emergency reconsideration for my sound and light ART project, I finished the laser mirror one and it OK, but I look at it and go .

So I have until Monday morning to do something better, I am thinking of something similar to a POV with hardrive platters (details later), rather than mess with slip rings etc I would like to space small wire coil inductors around the edge of the underside of the platter and have them pass over a ring of magnets (upside down generator), so if we assume a rough RPM of 6500, how many coils of what inductance (roughly) would I need to get 300mA at 7V?

The magnets will be horseshoe shape neo's from hard drives. The harddrive motor is driven separately, my intention is to generate the power for the chip and leds on the platter

 

[Les Jones]

Hi LG,
I do not thing that generator design will work very well as you will not have a closed magnetic circuit. By the way what is a POV ?

Les.

 

[large_ghostman]

Persistence Of Vision, like the HDD clocks etc that spin, dont tell me it cant work!!! FFS that isnt what I want to hear with less than a week to go! We have to bend the laws of physics if it helps, after all the quantum world does it all the time so why cant I?

 

[Les Jones]

Hi LG,
I did not see from your first post that you are trying to get power to the rotating part without the use of slip rings. Is this understanding correct ? If the disk drive motor is driving the device I doubt that it will provide enough mechanical power to generate 7 volts at 300 mA even if you find an efficient generator design.

Les.

 

[large_ghostman]

Hi LG,
I did not see from your first post that you are trying to get power to the rotating part without the use of slip rings. Is this understanding correct ? If the disk drive motor is driving the device I doubt that it will provide enough mechanical power to generate 7 volts at 300 mA even if you find an efficient generator design.

Les.

Yes your correct, I was hoping a large number of small coils might do it , might be worth a quick try to see what I would get, the current is kind arbitrary as I could use a extra low power micro etc.

 

[Colin]

Build it and test it with 4 coils of 30 turns and the diameter is the same size to cover the magnet.
Use the largest diameter you can get and put 4 or 8 magnets on the part that spins. Make sure the magnets pass close to the coils.
This will produce the highest voltage and current. Use 0.5mm wire

 

[large_ghostman]

I am going to draw this out as I have not explained this well, I will try again.

So HDD platter then magnet stuck to bottom of platter, then pcb with coils or those SMD coil inductors on then off the platter on the base and not connected to the platter another circle of magnets. If the top and bottom magnets are different poles should this not induce a electric field inside the inductors? As for the HDD motor these are pretty old HDD's I have read a few times that these are weak but the two I have used seem to have a fair bit of torque? I am not adding much weight and can bring the platters upto speed at a steady rate

 

[large_ghostman]

Build it and test it with 4 coils of 30 turns and the diameter is the same size to cover the magnet.
Use the largest diameter you can get and put 4 or 8 magnets on the part that spins. Make sure the magnets pass close to the coils.
This will produce the highest voltage and current. Use 0.5mm wire

The coils have to be on the part that spins, hence the inverted generator. but I figured as my above post, I will try and knock it up in paint Colin

 

[alec_t]

7V x 300mA =2.1W. You would have to provide that + whatever the HDD requires for normal running. I smell smoke .

 

[large_ghostman]

7V x 300mA =2.1W. You would have to provide that + whatever the HDD requires for normal running. I smell smoke .

Ah see I knew I hadnt explained it well! The HDD motor is powered completely separately.

The generator just powers a chip and a few bits

 

[cowboybob]

... the two I have used seem to have a fair bit of torque? I am not adding much weight and can bring the platters upto speed at a steady rate ...

I am concerned that the HDD drive won't have the torque to get the platter/generator mass off a dead stop. Throw in any magnetic attraction (however slight) and that ups the ante... .

Also concerned about centrifugal balance at 6500RPM.

Although, LG, you do seem adept at rabbit pulling... .

 

[Les Jones]

I think Alec is making the same point as I made before. The disk motor will have to provide the additional mechanical power to drive the generator so it will have to take more current than it did in normal operation. The motor or the motor driver may overheat with the extra load or just not be capable of driving the generator. Would it be possible to put batteries on the rotating part ?

Les.

 

[strantor]

Have you looked up all the youtube videos for DIY wind turbines? Been a while for me but IIRC they should give you an idea how many turns per rpm per volt.

P.S. I also suspect the hd motor can't support the extra load.

 

[large_ghostman]

I am concerned that the HDD drive won't have the torque to get the platter/generator mass off a dead stop. Throw in any magnetic attraction (however slight) and that ups the ante... .

Also concerned about centrifugal balance at 6500RPM.

Although, LG, you do seem adept at rabbit pulling... .

Bootstrap 18V through it for ~200ms or whatever then slowly ramp up over 4 seconds or so, I dont know but has 3 platter that are metal I can remove 2 and use 5x2mm small magnets, weight should be much higher. I will try it and worse case scenario use the plate mounting and use a different motor to turn it, not sure yet but working out the inductance needed is first step the rest is..............details

 

[large_ghostman]

I think Alec is making the same point as I made before. The disk motor will have to provide the additional mechanical power to drive the generator so it will have to take more current than it did in normal operation. The motor or the motor driver may overheat with the extra load or just not be capable of driving the generator. Would it be possible to put batteries on the rotating part ?

Les.

Batteries are possible but not cool! Please lets no over simply a complex problem!! lol

 

[cowboybob]

You'll burn some serious time going through the process of creating a generator. Time, I take it, you don't have, so...

I had reasonable success using a 12VDC, 6" "whisper" fan assembly as a generator.

I cut off the blades, pulled the hub and rewired the motor to bypass the electronics. Then glued the hub to the bottom plate of a savonius wind rotor and, wahla, AC juice.

Never got anywhere near 6500RPM, but at about 600-800RPM generated about 1-2VAC. There was, also, a very strong magnetic attraction "bump" to overcome. The coils had iron cores.

 

[strantor]

Why not draw some slip rings on it with graphite pencil?

 

[large_ghostman]

Why not draw some slip rings on it with graphite pencil?

I have broken the lead drawing a faraday cage

 

[dr pepper]

I too share the concern whether the motor will have the torque, a generator will brake the motor.
Run the motor up and measure the power consumed, maybe slow the disk down with your finger to speed a which is still ok to run at, then measure current and calc the wattage, it will need to be at least double maybe triple what your circuit needs, or it will run slow or even stall.
Maybe a better way would be to arrange a coil around the centre of the disc and the centre under the disk on the hdd frame then apply 50 khz or so from a 555 to the stationary coil, then rectify the o/p from the rotating coil on the disk for your pov board supply.
At least hdd discs are usually aluminium, which wont impede either a gen or coupling coil.
I did something like this for a project cant recall the no. of turns but from the size of the coil I'd say it was around 100.
If you do build a gen, program the pov code to delay a while at power on to give the motor time to spool up.

 

[large_ghostman]

I tested the motor today with around 370g on the plate, absolutely no problems spinning up and only 120mA in extra current. Its a old HDD which makes me wonder if newer ones are wimpy compared to older ones. This one is from around 2002, so I defiantly have the torque and power, I need to test it the opposite way I will use it so I can measure the voltage output etc in the coils before committing to building. Good job I have glasses now as I can actually see the motor connections without a scope .

The chip will be on the platter, so it will be all upto speed before it turns on etc anyway. I have a large number of new smd electrolytic caps so I will make sure there is plenty on the board to smooth everything out