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.
Electrical – Using hard disk motor as generator
generatormotorstepper motor
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I haven't even bothered watching after "only DC motors can be used as a generator".
As far as I am aware, a motor can be of the following families:
- Permanent magnet DC brushed. DC back emf.
- Coiled stator DC brushed (as a separate winding, or internally wound as series or parallel). DC back emf IF the stator is powered. Single phase universal motors are a subset of series connection types for which, regardless of the polarity of the voltage, torque is always generated (needs moveable brushes or a different wiring to change direction though).
- Permanent magnet AC synchronous (three phases). Three phase AC back emf.
- Coiled rotor AC synchronous. I think those generally are not brushed but rather rectify the current induced by the stator. If brushed, no back emf unless the rotor is powered.
- DC brushless. This one is basically a permanent magnet AC synchronous with hall sensors built in, to be able to electronically switch the phases. The back emf is however square or trapezoidal to maximise flux linkage.
- Stepper motor (2, 3, 5 phases). Close to the PM AC synchronous in its construction, except that the motor is made to maximise the number of stable equilibrium positions of the rotor (many alternating magnetic poles at the rotor or variable reluctance). Back emf depends on how it's driven.
- AC asynchronous (3 phases). The rotor is a closed loop (a coil, or a squirrel cage made of bars) which creates its field from currents induced by the stator. Can only be used as a generator beyond the synchronous rpm (+voltage at stator). AC back emf (TBC).
- AC asynchronous (single phase). The motor cannot be self-started unless an out-of-phase auxiliary supply is created via a reacting capacitor, and fed to windings 90° from the main windings. Can only be used as a generator beyond the synchronous rpm (+voltage at stator). AC back emf (TBC).
There are many more (e.g. hybrids), but I think those represent 95% of the production. I'm sure I've missed a few important ones, please feel free to comment and I'll update the list.
The biggest clue to the type of a motor is the number of wires, but as you can see this is not enough. Some motors cannot generate power without an excitation, some not at all, and even if they do, the back emf is funny sometimes (trapezoidal for example) depending on its construction.
You could plan to try the various types of supplies on the motor, ramping up the voltage, and see if it does anything, but what's your "OK that's not it, better cut the power before I smoke it" point? If you don't know what type of motor it is, I assume you don't know anything about it. Including the voltage and current ratings, Max rpm. You could get that from eyeballing it, but there is no guarantee then.
For your specific problem though, if you are certain your motor is a DC bruhless but you don't know if the inverter+control circuit are integrated, look at the number of wires. Generally the motor does not have a circuit built in, and an ESC must be connected to it. You will have to identify which wires are the hall sensors.
ESC might or might not be used for current generation, it depends on how they are made. I don't think there can be any harm in hooking up a resistive load compatible with its current range at the input and test it.
A typical motor will behave as an ideal motor in series with a certain amount of inductance and resistance. At all times, the rotational speed of a motor will be a certain multiple of the applied voltage, and the torque will be a multiple of the current flowing through it. These relationships are bidirectional, so whether something acts like a motor or a generator will depend upon whether relative directions of the applied voltage and the current flow.
Based on the above description, I would estimate that your the voltage on the ideal motor will be roughly 1 volt per 1500rpm (look at no-load speed divided by rated voltage). I'm not sure whether the stall current is measured at 2 volts or 6 volts, but the short-circuit current from a motor spinning at a given speed will be roughly equal to the stall current of a motor driven at the open-circuit voltage that would correspond with that speed. To get maximum power output, you should draw less than half of the short-circuit current,.
Best Answer
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:
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:
As you need DC, you could use three rectifiers:
simulate this circuit – Schematic created using CircuitLab
or six diodes, as @jms pointed out:
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.