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AC and DC generator structure question (1 Viewer)

el_manu

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Um, I have a question regarding the structure of both generators. In DC generators, the brushes are stationary and it creates friction with the commutator.............In AC generators, do the brushes turn with the generator, and what are the terminals connected to...look at picture (the bottom diagram in the link) ac_gen5.jpg

I thought the brushes turn with generator, so does whatever the terminals are connected to have to spin or not, because I can see the terminals getting tangled if the object they are connected to are stationary. And if whatever is connected to the terminals spin around with the generator, how does that help us do anything with the electricity generated.
 

anomalousdecay

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Slip rings and brushes do not move at all in any case. Commutators however can be moving or not moving (ideally it works much better if they are moving though as it actually is less prone to problems with perfecting the wiring contact like as how must be done with slip rings, so I will only consider the case where commutators are moving below).

In AC generators the slip rings provide contact with the coil at all times. The slip rings do not move. Only the coil does (theoretically the ends of each coil is touching the respective slip ring at all times). This means that as the flux through the loop changes (it occurs sinusoidally), the emf will always be dependent on the flux through the loop by Faraday's Law, and hence you will have a complete sinusoidal output for every period.

In DC generators, the commutator provides the contact to the ends of the coil. The split ring commutator has a split in the middle to make sure that the ends of the coil are actually separately connected to different brushes at all times and so that never is there a time where both brushes are touching the same commutator. So while rotating, it follows a half a sinusoidal input. After the orientation of the loop (and hence the commutator sides) changes, the magnetic flux threads through giving emf in the opposite direction now and the commutator sides also switch over to different brushes. So if the commutator contacts switch over and the flux is in the opposite direction, the emf is induced as the same direction as before. Hence, you get an output from a DC generator where it is a rectified sinusoidal output (that is, an absolute value of a sinusoidally shaped function).

This may help if you have more trouble with this:

 

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