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I'm planning on connecting an elliptical trainer to an AC motor in order to charge some batteries and use the energy later. So I found an old 300 W juicer which I disassembled. Then I connected the bars to a multimeter and a drilling machine to the shaft.

So when I measure the voltage with the drilling machine on (at maximum power), I get maximum 1.8 V (see picture).

enter image description here

This is a very disappointing value for me, as I expected more...How much more? I don't know, but definitely not such a low, useless value.

The voltage of the drilling machine battery is 14.4V and the energy is 21.6Wh. Not sure if this has any relevance.

Can anyone help me to understand why do I get such a low voltage from a 300W motor? And if it is normal, why is it so low and what to do to increase it, considering that I want to connect it to the elliptical trainer.

Edit: Please see the picture with the specifications of the juicer. enter image description here

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    \$\begingroup\$ Did the original juicer have a reverse function for backing out something it got stuck on? It kind of looks like your motor may have a second set of contacts for a field coil, the idea being that the direction of rotation on A/C would depend on how those are wired compared to the brushes. To operate as a generator you would have to (carefully) inject current to the field, or possibly wire it to borrow some from the brushes (counting on residual magnetism to start things off). \$\endgroup\$ – Chris Stratton May 27 '17 at 16:09
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    \$\begingroup\$ you can NOT get max power transfer unless you match load impedance to generator, just like MPPT on a PV. A battery is like a short circuit ESR. So use a PWM with choke to regulate Z with a DC motor just like regulating PV with variable sun. Peddling torque is a current source with no current Voltage proportion to RPM like Voc on a PV ...but different curve. \$\endgroup\$ – Sunnyskyguy EE75 May 27 '17 at 16:32
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    \$\begingroup\$ s18.postimg.org/51h8ma8xl/efficiency_load_chart.jpg \$\endgroup\$ – Sunnyskyguy EE75 May 27 '17 at 16:39
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    \$\begingroup\$ Odds are the motor is either an "induction" motor or an "AC/DC" motor. An induction motor (as stated elsewhere) will not function as a generator at all (except in very carefully planned circumstances). An AC/DC motor will function as a generator, but the stator must somehow be "energized" to do this, requiring a special circuit, or at least careful planning. (A hint as to which can be gotten by reading the nameplate of the unit -- does it say "AC" or "AC/DC"?) \$\endgroup\$ – Hot Licks May 27 '17 at 21:12
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    \$\begingroup\$ But note that if the motor is AC/DC you might get more voltage from it by simply turning it the opposite direction. If the electron gods are with you this could be sufficient for your purpose. \$\endgroup\$ – Hot Licks May 27 '17 at 21:22
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The rotor of AC asynchronous motors is simply a lump of aluminium and iron. As there isn't any magnetically hard material involved, there is little to no residual magnetism stored and so, the motor, as it comes from the shelf, has no field.

When connected to AC power, the AC in the stator first has to induce a field inside the rotor. Then the rotor starts to run slowly. Because its field has smaller frequency than the stator field due to the overlay of rotation speed and outer field, the rotor has a driving torque in result and goes up to a speed slighty smaller than the sychronous speed (for a two-pole motor: ≈2950rpm on 50Hz, ≈3500rpm on 60Hz)

M/n diagram of an AC asynchronous motor

M: momentum (torque); n: revolution speed

The only way to make an AC asynchronous motor into a generator is by connecting it to AC and connect a drive to its rotor which runs that one at a slightly higher than synchronous speed (for a two-pole motor: >3000rpm on 50Hz, >3600rpm on 60Hz). Then, the current direction (in relation to voltage) on the AC input reverses and the motor actually delivers electrical power.

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    \$\begingroup\$ Would it be possible to label or explain what M and n are in this graph? \$\endgroup\$ – user2813274 May 27 '17 at 14:12
  • \$\begingroup\$ Thank you for the answer. So let me see if I understood it correctly. I would need a power supply to speed up the motor until it reaches the speed necessary for generating higher electrical energy? And the extra energy will come from the trainer? That is, I have to have always an additional power. Is that correct? \$\endgroup\$ – Physther May 27 '17 at 14:34
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    \$\begingroup\$ @Physter: As written as a comment to Olin's answer, I'm now pretty sure your motor is an AC/DC universal motor. With that one, you need to separate the field winding from the rotor winding first, then put a battery to the field winding to form a field. It would then function as a generator for any speed. \$\endgroup\$ – Janka May 27 '17 at 15:29
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    \$\begingroup\$ @Physter: If you had an asynchronchrous AC motor, it would consume electrical energy below 3000(3600)rpm and generate electrical energy above 3000(3600)rpm. To wrap your mind around this, remember the thing is in torque equilibrium at 3000(3600)rpm. Not at 0rpm. \$\endgroup\$ – Janka May 27 '17 at 15:33
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    \$\begingroup\$ @Physter: You may think of it as a bike running downhill. You have to pedal faster than the idle downhill speed to apply any torque and to put any more energy into the movement. \$\endgroup\$ – Janka May 27 '17 at 15:41
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This depends on the type of motor. Not all motors work like generators without any electrical power applied. For example, if this is a AC induction motor, then it is only due to accidental residual magnetism that you are getting anything at all.

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  • \$\begingroup\$ Oh, didn't think about that. Is there any way I can check that? \$\endgroup\$ – Physther May 27 '17 at 12:29
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    \$\begingroup\$ @Phy: Try applying current-limited DC to the motor, like from a bench supply. If it doesn't do anything, even just feeling more torque in one direction than the other when spinning the shaft by hand, then it's a AC motor. If it's AC and only has two leads, then it won't work as you want. \$\endgroup\$ – Olin Lathrop May 27 '17 at 12:33
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    \$\begingroup\$ A universal motor will run on DC but won't run as a generator (needs a current in the field windings) \$\endgroup\$ – sstobbe May 27 '17 at 13:00
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    \$\begingroup\$ @sstobbe: Please make this into an answer, as from the photo, this very much looks like an universal motor, because of the brush holders visible. (Now I regret jumping on the AC induction motor train.) \$\endgroup\$ – Janka May 27 '17 at 14:00
  • \$\begingroup\$ So I connected some batteries and I can hear a click in the motor. I think it pushes it a bit, but there is not enough power to make it move. \$\endgroup\$ – Physther May 27 '17 at 14:32
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Such appliances are mostly made with asynchronous AC (induction) motors with cast rotor and higher gap between rotor and stator. There are 3 reasons - the first is that it requires fewer (zero) external components to run, the second is that this motor is relatively cheap to produce, and the third (and most important) is that if the motor stalls mechanically, it will not draw much more current than its nominal (depending on design, but not more than twice, which is acceptable for a certain period until the operator turns it off) and will not require any additional protection. After removing the reason for which it got stuck, it will work again.

These motors cannot be used as generators.

To charge batteries you will need a DC motor. If you take a DC motor with a permanent magnet from some toy it will do. Not perfect, but will do.

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  • \$\begingroup\$ Thank you very much for your answer. So you are saying that can't make it generate proper power with an asynchronous motor? I just read about this kind of motors and I can see that they are used for windmills and hydro power. What is the modification for that? Can I have that too? \$\endgroup\$ – Physther May 27 '17 at 14:37
  • \$\begingroup\$ @Physther Well, as Janka said AC voltage with lower frequency should be applied, but this is way too complicated for a DIY gym battery charger ;) This is done using a special controller. \$\endgroup\$ – Todor Simeonov May 27 '17 at 14:41
  • \$\begingroup\$ Darn! I had a feeling that it is not so straightforward \$\endgroup\$ – Physther May 27 '17 at 14:46
  • \$\begingroup\$ While some juicers use induction motors, many do not. This does not look like an induction motor. \$\endgroup\$ – Chris Stratton May 27 '17 at 16:06

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