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So I have this multi-tap transformer as a project. I bought the transformer, the only thing I did is the connections and the casing. When I tried to connect a 12v DC motor, the motor will vibrate like it is spinning but the rod/tip of the motor is not. Sometimes it spins clockwise or counter clockwise. What is the problem? Help please.enter image description here

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    \$\begingroup\$ Are you running a DC motor from an AC source? \$\endgroup\$
    – jonk
    Commented Jul 8, 2018 at 4:28
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    \$\begingroup\$ Wiring diagram? DC motors won't work when supplied with AC, or with wrong voltage. \$\endgroup\$ Commented Jul 8, 2018 at 4:56
  • \$\begingroup\$ Oh shoot. I just realized that I dont know if this transformer converts AC-DC. Nothing on the label. So guys you think this is step down AC-AC? About the wiring im pretty sure its correct. \$\endgroup\$
    – rlmarco
    Commented Jul 8, 2018 at 5:06
  • \$\begingroup\$ That's a pretty case, by the way. \$\endgroup\$ Commented Jul 8, 2018 at 5:22
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    \$\begingroup\$ @rlmarco oh man. A transformer, by the very definition of what a transformer is, is AC/AC... \$\endgroup\$ Commented Jul 8, 2018 at 5:23

2 Answers 2

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A transformer, by the very definition of what a transformer is, is an AC/AC converter.

So, you can't attach a DC motor directly to a transformer. You need some kind of rectifier after the transformer, or you need an AC motor.

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    \$\begingroup\$ Up-voted so you fell less silly :-) \$\endgroup\$
    – Oldfart
    Commented Jul 8, 2018 at 7:09
  • \$\begingroup\$ don't forget that the peak DC voltage after the rectifier will be √2 times higher than the AC input, you may need to add a voltage regulator or use a lower AC input voltage to run whatever DC device within its specs. \$\endgroup\$
    – dlatikay
    Commented Jul 8, 2018 at 12:09
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To expand on Marcus Muller's answer, you can excite a DC motor with an AC signal, but it won't do what you're expecting. Exciting a mid-range DC motor with mains frequency (50 or 60 Hz) is likely well into the stop band of the motor's frequency response. This explains why the motor was humming but no motion was observed on the output shaft.

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