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  1. #1
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    Ever use back EMF?

    Ive seen some pretty impressive hillbilly engineering, But this takes the cake. http://m.youtube.com/watch?feature=f...;v=e6eKDsDpup8

  2. #2
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    So he's not using BEMF per-say (I was picturing him using a second motor to offset the voltage and simulate a third phase), he is using a 3 ph motor as a generator.

    I'm sure this works fine for a lathe on a farm that only gets used once in a while. I think this may lead to premature motor failure on continuous duty applications, not to mention the fact that you need 3 motors instead of one lol. Also, he may have a hard time getting 60hz out of it, although his lathe motor likely doesn't care.

    I always love farmer engineering!

  3. #3
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    Quote Originally Posted by LKJoel View Post
    So he's not using BEMF per-say (I was picturing him using a second motor to offset the voltage and simulate a third phase), he is using a 3 ph motor as a generator.

    I'm sure this works fine for a lathe on a farm that only gets used once in a while. I think this may lead to premature motor failure on continuous duty applications, not to mention the fact that you need 3 motors instead of one lol. Also, he may have a hard time getting 60hz out of it, although his lathe motor likely doesn't care.

    I always love farmer engineering!
    That voltage he is generating is known as back EMF. I looked at it as one hand washing the other. As far as the 60 hz the polarity of the voltage generated would be opposite of the voltage generating it. I see no way it could be anything but 60 hertz but, im up for discussion im no expert.

  4. #4
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    That small motor is just to help start the 3 phase motor. In a deenergized state their is no magnet therefor no voltage generated. An ECM motor can generate voltage by turning the shaft as it has a permanent magnet.

  5. #5
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    All you really need to do is make sure the motor is wound in delta formation and add a capacitor and possibly a start cap to one winding and a 3 phase motor will run on single phase. I've experimented with it a little. The start motor and belt he is using is doing the work the capacitor would do.

  6. #6
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    Ill admit that i dident really analyze the video, but it looked to me as if he is powering the small single phase motor the entire time, driving the large 3 phase motor via belt and pulley, then using the voltage generated by the large 3 phase motor to power his 3 phase lathe.

    If you supply the small motor with 60 Hz it will run synchronous speed, however because if slip, it runs slightly slower, add drive losses into that, and slip of the 3 phase motor, and you will get an output frequency lower than 60 Hz. I guess this could be corrected by adjusting the pulley ratio, and not simply using 1:1, it would take some guess work though, the lathe motor likely can run right down to low frequencies with no issues though

  7. #7
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    Quote Originally Posted by LKJoel View Post
    Ill admit that i dident really analyze the video, but it looked to me as if he is powering the small single phase motor the entire time, driving the large 3 phase motor via belt and pulley, then using the voltage generated by the large 3 phase motor to power his 3 phase lathe.

    If you supply the small motor with 60 Hz it will run synchronous speed, however because if slip, it runs slightly slower, add drive losses into that, and slip of the 3 phase motor, and you will get an output frequency lower than 60 Hz. I guess this could be corrected by adjusting the pulley ratio, and not simply using 1:1, it would take some guess work though, the lathe motor likely can run right down to low frequencies with no issues though
    Again turning the shaft on an AC motor will not produce voltage if the windings are not energized. Am I wrong about this? The small motor is there to start the three phase motor under a single phase condition.

  8. #8
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    seems like a rather complicated setup when he could just use the smaller motor to run the larger 3 phase motor and be done with it. it is just me or can i tell on YouTube that his belt is out of alignment...i suppose that since he doesn't own a broom, he probably doesn't own a piece of string.
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  9. #9
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    He's using the 3 phase motor as a phase converter. It won't start on it's own without a big capacitance boost so he's using the smaller motor to get it up to speed where it will run on 1 phase then the back emf from that motor will run his lathe motor

  10. #10
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    Quote Originally Posted by jayguy View Post
    seems like a rather complicated setup when he could just use the smaller motor to run the larger 3 phase motor and be done with it. it is just me or can i tell on YouTube that his belt is out of alignment...i suppose that since he doesn't own a broom, he probably doesn't own a piece of string.
    As the video poster stated the belt slipping was of little importance. Assuming the lathe has a gear box I would assume a small motor could not support the load placed on it. Im not posting to defend the video posters actions. Really, I just wanted to hear your thoughts on its operation electrically.

  11. #11
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    Quote Originally Posted by Core_d View Post
    As the video poster stated the belt slipping was of little importance. Assuming the lathe has a gear box I would assume a small motor could not support the load placed on it. Im not posting to defend the video posters actions. Really, I just wanted to hear your thoughts on its operation electrically.
    The lathe motor could be made to run on single phase as long as its either delta wired or the winding ends could be accessed and changed from wye to delta. Then all you would need is a capacitor or a run cap, start cap and pot relay. If the motor was a wye and couldn't be changed to delta then you would need an idler motor to act as a phase converter. You could also use a vfd to run a 3 phase motor off of 1 phase power.

  12. #12
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    Yea a capacitor is a possibility, would the video posted supply a smoother voltage? Discharge would be varied depending on capacitance rating thus changing frequency? Im just using this video as a standing point, I would like to throughly understand the circuit described in the video.

  13. #13
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    The more idler motors you have running the "cleaner" and "even" the legs become. For example, coming out of the idler motor (the one he started with the small motor) the voltage between L1-L2 = 237, L2-L3=268 and L1-L3=248 if he had another motor running it would "clean up" to 235, 250, 242 and so on

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