My understanding of a motor is that when a supplied current is fed through the coils, it rotates due to the motor effect. However, as the coil rotates it also experiences a change in magnetic flux resulting to an induced emf, seen in a generator. This emf is in the opposite direction to the supplied emf explained through Lenz's Law.

My confusion is that if a generator rotates at the same speed as a motor, shouldn't the energy output of the generator be equal to the energy input of the motor for the same rotational speed? This is the law of conservation of energy. And if what I said above is true, then shouldn't a motor experiencing back emf have its current canceled out by the back emf?

I know this sounds wrong but I can't seem to understand why. Please help me by removing my ignorance.

  • $\begingroup$ Duplicate? physics.stackexchange.com/a/233101/104696 $\endgroup$ – Farcher Jan 15 '17 at 7:52
  • $\begingroup$ That question asks about eddy currents. Im asking about back emf. Although that question contains answer about emf it doesn't answer my question which is about conservation of energy and emf. $\endgroup$ – masterwarrior123 Jan 15 '17 at 8:16
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    $\begingroup$ The answer I provided does answer your question. If there are no losses in your motor the back emf exactly equals the applied voltage, no current flows and the motor rotates for ever with no consumption of electrical energy. As soon as the motor is asked to do work it slows down, the back emf drops and a current flows thus the voltage source is providing power for the motor to do external work. physics.stackexchange.com/a/233101/104696 $\endgroup$ – Farcher Jan 15 '17 at 8:34
  • $\begingroup$ Ok fair enough. Thx for your help. $\endgroup$ – masterwarrior123 Jan 15 '17 at 8:53

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