# Scrapped Motors Don’t Care About Direction

Spinners built into games of chance like roulette or tabletop board games stop on a random number after being given a good spin. There is no trick, but they eventually rest because of friction, no matter how hard your siblings wind up for a game-winning turn. What if the spinning continued forever and there was no programming because there was no controller? [Ludic Science] shows us his method of making a perpetual spinner with nothing fancier than a scrapped hard disk drive motor and a transformer. His video can also be seen below the break.

Fair warning: this involves mains power. The brushless motor inside a hard disk drive relies on three-phase current of varying frequencies, but the power coming off a single transformer is going to be single-phase AC at fifty or sixty Hz. This simplifies things considerably, but we lose the self-starting ability of the motor and direction control, but we call those features in our perpetual spinner. With two missing phases, our brushless motor limps along in whatever direction we initiate, but the circuit couldn’t be much more straightforward.

This is just the latest skill on a scrapped HDD motor’s résumé (CV). They will run with a 9V battery, or work backwards and become an encoder. If you want to use it more like the manufacturer’s intent, consider this controller.

Thank you for the tip,[Itay].

## 18 thoughts on “Scrapped Motors Don’t Care About Direction”

1. With some experimentation, it’s possible to add a capacitor to allow it to self start.

1. Hmm. The capacitance is normally 70uF/kW, so 70nF/W, so about 2uF for a 25W harddisk motor….

1. SparkyGSX says:

25W sounds awfully high for a hard drive spindle motor, and that 70nF/W assumes 230V. A 25W motor at 230V would draw about 100mA (if we assume W = VA for simplicity so no reactive current), while at 12V, a 25W motor would draw just over 2A. It seems inlikely those motors would need the same capacitance. The 12V motor needs 20x the current, and only has 1/20th the voltage across the capacitor to get that current going.

Even with the full 12V across a 2uF capacitor (so without the motor coil in series), you’d only get about 7.5mA of current. That doesn’t sound like a whole lot for a motor that would draw 2A in it’s main winding.

I’d think you’d need a capacitor about 400 times larger (20x the current and 1/20th the voltage). 800uF at 12V and 50Hz gives 3A, which sounds like a lot, but remember this is without the coil. By comparison, 70uF at 230V and 50Hz gives just over 5A, for a 1kW. Again, without the coil.

2. Jan says:

the text in the article “Fair warning: this involves mains power.” is really misleading.
The motor isn’t driven from the wall socket directly! The fact that is uses a transformer and mains AC doesn’t make it dangerous (who wrote this article, Edison?). The video doesn’t show the bare wires of the transformer directly pushed into a wall socket, therefore I assume it has a decent plug connected to it and it therefore perfectly safe.
In other words, this project isn’t any more dangerous then most other project around here. How many times have we seen people connecting to mains via a regular power supply… yep… lot’s of times. If the author of the article really wanted to protect his readers it would have been better to state NEVER CONNECT A HARDDRIVE MOTOR DIRECTLY TO MAINS! (sorry for shouting).

Anyway, although this article isn’t really surprising from a technical point of view, it is nice to see the concept of AC motors (low voltage) seen from a straight forward hands-on perspective. The way I learned about AC motors hands-on was using a dynamo from a bicycle. I used it to sync my nipkov disc to the net. Although it wasn’t strong enough to drive the disc, in combination with a DC motor is kept the whole thing running and the dynamo more acted like a synchronizing brake. But without the disc is worked just like the hard drive motor. But back in the 80’s I never heard of harddisks.
Regarding the nipkow disc project, I was too inexperienced to finish that successfully but now I’m older and have more access to harddrive motor then bicycle dynamos I perhaps should give it a try again. Considering that it is easier to find an harddrive motor then a bicycle dynamo. But also because harddrive motors are much smaller and therefore easier to work with.

So… thanks for posting.

1. > The fact that is uses a transformer and mains AC doesn’t make it dangerous (who wrote this article, Edison?).
But with the transformer being built to be enclosed inside a housing of some sort, having the transformer “bare” on the table and then pointing at it… Made me cringe.

1. That’s completely safe as long as the primary terminals are insulated (e.g. with a good layer of thick heatshrink). The outside of the transformer is insulated, there is normally nothing there that you could touch that is energized, except for the terminals. The primary winding of a mains transformer is typically the deep-most one, so there is little chance to get to it without causing a lot of damage to the transformer.

Of course, if someone starts to poke a screwdriver or something into the windings or tear out the cable they will likely get shocked but you could equally poke a screwdriver into the wall outlet as well … Some common sense needs to be used.

So there is no need to overreact here as long as the mains wiring is done properly and kids are not playing with it.

1. sneakypoo says:

And as we all know, there are no kids that watch youtube videos and attempt to replicate anything…

Overreacting this much over a small warning to be careful when working with mains is a bit silly.

2. I think he meant that the transformer needs to wired to a cable and a plug – the video shows a “naked” type transformer, not some pre-assembled power brick or power supply (which in most cases have a built-in rectifier and thus produce DC, making them unusable for this project). And that involves mains wiring. So I think the warning isn’t misplaced there, even though it is not anything super dangerous with a bit of care neither.

BTW, “dynamo” from a bicycle doesn’t produce AC but DC (because it has a commutator). There are some that do produce AC but those are strictly speaking alternators and not dynamos.

1. Sine says:

Actually most (if not all) bicycle “dynamos” use a rotating magnet and stationary windings, there is no commutator involved.
So yes, they should be called alternators

1. RÖB says:

Except for the ones that are actually “dynamos” which are by definition DC generators.

3. alfcoder says:

edison :) now that was a good one :)

1. Phrewfuf says:

\$2.85 for a RC model ESC off aliexpress.

3. Best part of hdd motor it it’s rotor is isolated during run.

4. Luke says:

It’s very simple to make an oscillator to generate the sine wave(s).

http://www.wseas.org/multimedia/journals/circuits/2014/a405701-233.pdf

Second page, top left diagram. The output will be three phases with an amplitude of about 1.4 Volts peak to peak. The op-amps will be able to deliver a couple milliamps of current so put a small value (47 Ohm) resistor between the motor and the output.

The motor should spin weakly. You can switch the direction by swapping any two phases. The speed is controlled by the value of the resistor R1 – changing all three in unison is a bit tricky, but if all you want is for the motor to self-start and spin in a certain direction, that’s acceptable.

1. Luke says:

Oh, and since you’re probably going to build it with a single side power supply, you can use the fourth op-amp in a 4x chip to generate a virtual ground rail around half-way through the voltage range. Just split the voltage supply in half with resistors and wire it up as a voltage follower.

5. snaslund says:

You could get the other two phases by using discreet components like inductors and capacitors to get the phase shift +- 120 degrees. Changing speed is just adjusting the frequency. The controllers do not need to be very fancy unless you are trying for A/D conversions, braking, soft start, or stuff like that.

6. Modern hard drive spindle motors usually have a preferred direction of rotation, actually. If you try to turn one by hand, you’ll notice that it turns more easily (and spins for much longer after you give it a spin) in one direction than the other. This is because they have internal air bearings, which are directional. I’ve read, but not tried to confirm myself, that they’ll wear out rapidly if turned much in the non-preferred direction.

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