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What happens to a generator when we rotate it in open circuit(no load)? Let's say we have an old bicycle's dynamo without regulators, capacitors and battery. What happens if we remove the lamp and turn the wheels? Do we transform mechanical energy into electrical energy? And then what happens to this energy?

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A perfect generator exhibits no mechanical resistance when electrically unloaded. Real generators have losses due to friction, eddy currents, and magnetic core losses. These will still be there without a load (no current). There will therefore be some mechanical load on the shaft, and the resulting mechanical power ends up getting dissipated as heat by the generator.

If you allow output current to flow by connection a electrical load, the shaft torque goes up at the same speed. The additional mechanical input power gets converted to electrical power and is delivered to the electrical load.

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  • \$\begingroup\$ Is the heat by the generator caused by the voltage? Could I damage the generator if the voltage is too high? \$\endgroup\$ – sotirios Jul 3 '16 at 12:31
  • \$\begingroup\$ @sot: No, voltage by itself doesn't cause heat. As I said, the heat is due to friction, eddy currents, and magnetic core losses. When the generator is loaded, there will be current thru the windings, and additional heat created proportional to the square of the current due to the non-zero resistance of the windings. \$\endgroup\$ – Olin Lathrop Jul 3 '16 at 12:35
  • \$\begingroup\$ But voltage causes the eddy currents, right? Can these currents damage the generator if they are too high? \$\endgroup\$ – sotirios Jul 3 '16 at 21:31
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If you have a true "open circuit" on a generator, then it produces no electrical power. This can easily be seen by the definition of electrical power:

\begin{align} P = V I \end{align}

When you have an open circuit, \$I\$ is by defintion 0, so \$P = 0\$, no matter what \$V\$ is.

If you actually had an the bicycle as you've described, you will notice that it is harder to turn the pedals when the lamp is connected vs. not connected. The only extra energy you have to add is associated with the mechanical friction and any leakage currents which violate the open circuit assumption (for example, eddy currents in the magnetic components of the generator).

In an ideal bicycle where you have no mechanical friction and an ideal open circuit, you can then turn the pedal up to some speed and by Newton's laws, they will keep spinning at the same speed indefinitely with no additional energy since there is no energy lost to any sinks (mechanical friction or electrical power output).

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