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I stripped an old Epson scanner and found a 4 wire stepper motor inside (EM-489 from Minebea.) I decided to play around a little bit with it and measured that one pair of wires has ~2.9 ohms and the other pair ~2 ohms.

Then I decided to connect one of the pairs to 3x1.5V Duracell batteries in series to make it run (that was before I knew that a stepper motor doesn't work as a DC motor and needs a controller) and noticed some sparks.

Connecting to 1x1.5V batteries makes the spark less bright/noticeable but still there.

Why do I get the sparks? AFAIK the generation of a spark is related to the voltage difference and 4.5V and 1.5V should not be enough to generate a spark

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    \$\begingroup\$ the voltage difference and 4.5V and 1.5V should be enough to generate a spark I think you mean: "4.5 V is not enough to create a spark. And you would be correct. To get a spark you need a much higher voltage. \$\endgroup\$ Sep 17, 2021 at 21:11
  • \$\begingroup\$ Corrected. Thanks! \$\endgroup\$
    – Jon
    Sep 17, 2021 at 21:11
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    \$\begingroup\$ One can create a visible spark (or arc, depending on the definition) by just shorting and opening an 1.5V battery with a piece of wire. \$\endgroup\$
    – fraxinus
    Sep 18, 2021 at 7:49
  • \$\begingroup\$ @fraxinus How come? I tried it and didn't get anything. AFAIK a high voltage is needed to genetare an arc \$\endgroup\$
    – Jon
    Sep 18, 2021 at 13:08
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    \$\begingroup\$ @Jon try when it is dark. \$\endgroup\$
    – fraxinus
    Sep 18, 2021 at 21:22

2 Answers 2

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The motor windings have high inductance.

  • Inductors are coils of wire on an air, steel or ferrite core.
  • They tend to resist change in current. When you connect your battery the current rises from zero to the final value in an exponential curve.
  • When you switch off the current (disconnect the supply) the inductor does its best to keep the current going. (Remember that it resists changes in current.) To keep the current going it causes the voltage to rise as high as required to breakdown the insulation of the air until all the stored energy is dissipated. This can be hundreds of volts and explains the spark.

This little experiment should help you understand later on why snubber diodes are required to protect the stepper motor drivers which switch the current at every step.

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  • \$\begingroup\$ Thanks for your explanation. That sounds like a sensible explaination! I didn't reach inductance in my studies yet so I'll come to revisit this answer in the future and hopefully, it will make more sense then \$\endgroup\$
    – Jon
    Sep 17, 2021 at 21:14
  • \$\begingroup\$ The fact that the spark is bigger/brighter when providing more voltage is because there's more current and therefore the spike of voltage when switching off is higher? \$\endgroup\$
    – Jon
    Sep 17, 2021 at 21:15
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    \$\begingroup\$ Jon, thanks for accepting my answer but I always recommend that you wait until the earth has rotated once or twice to give the whole of humanity a chance to answer. You'll encourage others to answer and get a range of different insights to your question. Then accept the best and upvote others that are useful. \$\endgroup\$
    – Transistor
    Sep 17, 2021 at 21:18
  • \$\begingroup\$ Thanks for the suggestion. \$\endgroup\$
    – Jon
    Sep 17, 2021 at 21:28
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Energy saved in inductor will be "discharged" in capacitor parasitic.

So \$Energy L = 1/2 * L * I^2\$ will be changed in \$Energy C = 1/2* C* U^2\$.

So voltage of spark : \$U = I*sqrt (L/C)\$.

If \$I = 1 A\$, \$L= 10 mH\$, \$C=100 pF\$ ... \$U\$ should be : \$ U = 10 kV\$ ...

So, don't make a short with your hands to try a inductance (especially transformer primary) with a DC power supply ... even if low voltage ...

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