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While doing some research about dynamos, I came across a situation wherein there was a question about the validity of my dynamo design. The counter to the design I suggested was that there would be back EMF generated by the design, but the design I have under consideration is that of a dynamo, where we generate electricity. I understand Back EMF is used in cases of motors, and not dynamos, is this the correct picture? The design of the dynamo is briefly described as follows: an outer rotating magnet that has a groove in which a fixed stator can fit in closely, of course, there is an air gap, and so on, as with standard dynamo design, but I did not see how anything like Back EMF, could really destroy the design under consideration, is this the correct?

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Is it being suggested that the current being generated will create a field opposing the generating field? –  Spoon Jul 26 '14 at 9:19

3 Answers 3

There is a back-emf even in dynamos that can be considered. A perfect dynamo will produce a terminal voltage related to the speed that it was spinning and this also is the same for an imperfect dynamo.

However, under load, the imperfect dynamo has an extra factor to take into consideration; sure, as the electrical load increases, the dynamo shaft is harder to keep turning at the same speed (conservation of energy) but the current taken from the dynamo also flows through a few percent of copper wire turns that are not generating a voltage - this is called leakage inductance (also applies to transformer windings) and the current will produce a back-emf in those turns that reduces the forward emf due to the perfect dynamo effect.

Maybe this is what the OP is considering?

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Whenever there is a field is built up in a coil, it eventually has to collapse. This collapse results in the magnetic field being converted to "power". This "power' will flow into every conductor connected to the coil. Diodes are typically used to block this "power" from going where it shouldn't. Read up on inductors. There are well documented ways of dealing with the issues you appear to have. Also, I am blind to the design you speak of, as I cannot see it.

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In a dynamo it's not called "back EMF" - just EMF!

It's the same thing; but that's the whole point of a dynamo...

To expand on this a little : any machine rotating a coil in a magnetic field will act as a voltage source across that coil, and that voltage is dependent on the speed. (Also on the magnetic field and the construction, number of turns on the coil etc, but the speed is easiest to vary).

Now imagine there is also a voltage on the machine's terminals, and consider the current flowing between these voltage sources. This current will be the difference between the two voltages, divided by the coil's resistance.

If the external voltage is greater, the current flows into the machine Then we call it a motor. Current times voltage dropped across the resistance is wasted heat; current times "back EMF" is work done by the motor.

If the external voltage is less, then current flows out of the machine, and we call it a generator. Now current times "back EMF" is work done by you turning the shaft, and again that dropped across the resistance is wasted heat. The rest (current * external voltage) is the power output of the generator.

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exactly what the thought was, why would a dynamo have back EMF apart from inductance losses? –  user3046538 Jul 26 '14 at 16:39

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