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I have disassembled 20 broken hard disks and taken out the motors, thinking they might be useful. However, they didn't work using DC current, because they required some kind of control circuit. Now I have a project (electrolysis) that might require to use those motors as generators. I want to know what kind of current these generate (DC or AC)? They were able to power an LED but as LEDs are diodes, I couldn't tell whether that motor generated AC.

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  • \$\begingroup\$ If you have DC motors it will generate only give DC as output. The theory is, in DC motor produced AC is converted to DC with the commutator within the motor circuit. So no need to worry just run the rotor it will give DC output. \$\endgroup\$ – Honeybee Mar 24 '16 at 6:27
  • \$\begingroup\$ @Honeybee However, this motor only vibrates when given direct DC current, but it came from hard disk, so I am assuming it requires control circuit to operate. \$\endgroup\$ – sadljkfhalskdjfh Mar 24 '16 at 6:33
  • \$\begingroup\$ If its a stepper motor you need a stepper motor driver for that. Its available in the market you can buy it. \$\endgroup\$ – Honeybee Mar 24 '16 at 6:42
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    \$\begingroup\$ The question was about using it as a generator, if I wanted to use it for powering something I could just use the original HDD control circuit. \$\endgroup\$ – sadljkfhalskdjfh Mar 24 '16 at 6:43
  • \$\begingroup\$ refer this links makezine.com/2012/01/25/… and instructables.com/id/Wind-Generator-from-old-scanner/?ALLSTEPS \$\endgroup\$ – Honeybee Mar 24 '16 at 6:46
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A HDD motor usually is brushless, i.e. it has no commutator. Basically, it consists of three coils arranged around a magnetic rotor. There are three terminals to power the three coils and one common current return terminal:

enter image description here

You can drive the motor by a 3-phase AC voltage, and when you turn it by hand, it will generate a 3-phase AC voltage.

Here is a youtube video of someone who measured the voltage on all three terminals while spinning the motor by hand. This screenshot from the video shows a wonderful 3 phase voltage:

enter image description here

As you need DC, you could use three rectifiers:

schematic

simulate this circuit – Schematic created using CircuitLab

or six diodes, as @jms pointed out:

schematic

simulate this circuit

However, I'm not sure how powerful those motors are, or how much power you get out when using them as generator. @JRE's experience (see comment) is that the power is very low. This may not be enough for your experiments, but if you want to try, choose diodes/rectifiers with low voltage drop (schottky diodes) to reduce the losses in them.

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    \$\begingroup\$ They don't generate much. You have to crank REALLY fast to get any power out of them. I've done this using a toy steam engine. With the steam engine running full out, you could light a couple of LEDs. At lower speeds, the voltage wouldn't get high enough. The load on the steam engine was almost nothing. Shorting the output of the generator motor barely slowed the steam engine, so there wasn't much power being converted to electricity. \$\endgroup\$ – JRE Mar 24 '16 at 8:05
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    \$\begingroup\$ You don't need three full bridge rectifiers to rectify three phase AC (as produced by the motor), three diode half bridges (6 discrete diodes) will suffice. If you really want to use those full bridge modules, you just need two of them. There is no need to connect the neutral in a star wound motor. \$\endgroup\$ – jms Mar 24 '16 at 8:32
  • \$\begingroup\$ @JRE If load on steam engine was very low, why not just add a gear or something? \$\endgroup\$ – sadljkfhalskdjfh Mar 27 '16 at 15:48

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