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I connected a 12V DC motor to a battery of 12V only. The motor rotated perfectly fine.

But my question is that if you don't have a resistor attached to the circuit, as per theory the current is infinite and this should burn the motor but it doesn't happen like that in reality. Can someone explain why and how ?

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    \$\begingroup\$ Look up 'back EMF'. \$\endgroup\$
    – Oldfart
    Jan 27, 2018 at 14:48
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    \$\begingroup\$ The 'near infinite current' characteristic you speak of is more of a warning to people designing equipment which controls motors. \$\endgroup\$
    – Harper - Reinstate Monica
    Jan 27, 2018 at 23:48
  • \$\begingroup\$ I will add that it's pretty common for very large motors to use a start up resistor bank. The big ones take a while to go fast enough to generate back EMF, and the resistor bank makes the startup much less brutal on both the machine and its power supplies. \$\endgroup\$
    – Nohbdy
    Feb 7, 2018 at 14:52

2 Answers 2

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Two reasons.

  1. The motor is itself a resistor. Look at all that wire inside it. This limits the current it takes when stalled. Not a lot; but a motor that takes 1 Amp when running may take 10-15 Amps when stalled, due to that resistance alone.
  2. The motor is also a generator. When it's running, it generates a voltage proportional to its speed. That opposes the driving voltage, leaving only the difference between these voltages across its internal resistance. Which reduces the current still further. (In a motor, this generated voltage is called "back EMF". In a generator, where you turn the shaft mechanically, it's just called EMF)

Load a motor and you'll slow it down. That decreases the generator voltage, (back EMF) increasing the voltage across its internal resistance, increasing the current, to produce enough torque to drive that load.

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    \$\begingroup\$ Yes, but it's a bit strange to list the points in this order. The Ohmic resistivity is not really an important effect, in fact it just wastes energy... the perfect motor would use superconducting coils. \$\endgroup\$
    – leftaroundabout
    Jan 27, 2018 at 21:54
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    \$\begingroup\$ @leftaroundabout I couldn't adequately explain the relationship between input and generated voltage without introducing the resistance issue first. Can you? Yes it wastes energy BUT it's NOT unimportant, it's fundamental. A longer (better) answer would point out its consequence : to run an electric motor efficiently, run it at high speed, lightly loaded. Because that minimises the I^2*R loss per unit of useful power. \$\endgroup\$
    – user_1818839
    Jan 27, 2018 at 22:02
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    \$\begingroup\$ 2. should really be 1. though, since the resistance of the wires can be "idealised away", while the back EMF certainly cannot. \$\endgroup\$
    – Noldorin
    Jan 27, 2018 at 22:40
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    \$\begingroup\$ eh, you can't please everybody. \$\endgroup\$
    – user_1818839
    Jan 27, 2018 at 22:41
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    \$\begingroup\$ @Noldorin It was the asker's "idealizing away" the wires of a motor that caused them to believe that a motor should draw infinite current. To me, this answer takes things in the best possible order: first, it explains why the current isn't infinite, as the asker thinks it should be; then, it explains why the current is even less than what it would be if that resistance was all that was limiting current. \$\endgroup\$
    – David Richerby
    Jan 28, 2018 at 11:48
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Take an ohmmeter and measure the resistance across the motor. You'll find the windings have some resistance.

Further, as the motor speeds up, it generates a back EMF (negative voltage) that eventually nears the supply voltage, partially canceling out the supply voltage and slowing down the current.

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