A dynamo is a simple current generator. It looks very much like an electric motor. Turning the permanent magnet reverses the magnetism through the coil every time the magnet is rotated by 180 degrees. The changes in the magnetic flux through the coil induce an alternating current in the wires. The frequency of the electricity depends on the speed of the bicycle.
In a bicycle dynamo, the magnet rotates and the coil is fixed.
Power station generators produce alternating current. Power stations use electromagnets rather than permanent magnets to create the magnetic field, and then pass the magnetic field through the rotating coils. The generator rotates at a fixed rate, producing a.c. at 50 hertz or 60 hertz, depending on the country.
When a coil of conducting wire is rotating in a magnetic field, by means of any external energy source, the electromagnetic induction results in an induced current through the loop. In this way, mechanical energy is converted to electrical energy. The device is called a Generator.
The generator will produce an electromotive force that will change as sine wave with the changing angle made by the coil with the magnetic field lines. Thus the direction of the current will vary, and the current so produced is called an alternating current.
The AC generator has two slip rings. The function of slip rings is to rotate with the coil and provide the alternate positive and negative induce current to the output carbon brushes. The carbon brushes are fixed with the body of the generator to maintain the contact with the rotating coil.
Note that this is similar to DC electric motor: the motor converts electrical energy into mechanical energy, while the generator converts mechanical energy into electrical energy.
The graph shows the changing alternating current with time during the one cycle of rotation. Whenever the rotating coil is in vertical positions, the size of alternating current or voltage is minimum (i.e zero). The current or voltage is maximum whenever the coil is in horizontal positions. The first half of cycle gives positive current output and second half negative current output.