This is not a physics forum, and as such I don't expect answers to explain Faraday's law of induced emf (electromotive force), which is the basic physics principle behind how an alternator is able to change mechanical energy (from the cam belt or chain) in to electrical energy that is needed in any modern car for many reasons. This works basically by creating a change in magnetic flux.

This video has a relatively good explainaton of the alternator and slip rings as I understand them.

  • What are slip rings with respect to a cars alternator?
  • What is their purpose in the alternator?
  • How are they able to rotate freely without getting the wires all tangled up?
  • Do they even rotate or are they fixed?

One of the slip rings will be positively charged and the other will be negative. I assume that they are made of some sort of metal or alloy so as to conduct a current through the alternator (do correct me if I'm wrong here) but bearing this in mind;

  • How are the slip rings insulated or kept separate so as to avoid a short circuit?

1 Answer 1


In an alternator there are two major components; the stator and the rotor. The rotor has a coil wound on it. By applying a current to that coil a magnetic field is formed and then spinning the rotor electricity is excited in the stator. Because the rotor needs to spin while mainaining electrical contact for the coil slip rings are used.

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There are normally two rings made out of copper or some kind of copper alloy. The rings are mounted on some type of plastic or composite material to insulate them from the shaft of the rotor. The wires that come off the slip rings to the coils are attached to the rings on the underside. Then there are channels in the plastic to route the wires keeping the wires away from moving parts. To keep contact with the slip rings there is a pair of carbon brushes. The brushes are made out of carbon so as to be both conductive and not too abrasive for the rings. The brushes are held is place tight against the slip rings with springs.

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Because the electrical load of the vehicle and the RPM of the engine are constantly changing the output of the alternator constantly needs to be adjusted. That adjustment is done by controlling the current in the rotor. Those adjustments are preformed by the voltage regulator.

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