View single post by Keinokuorma
 Posted: Wed Dec 6th, 2006 03:43 pm
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Joined: Mon Jun 26th, 2006
Location: Finland
Posts: 1248
Here again the brushes and springs. In the upper pic you will see how the curvature on the old brush heads have settled to the radius of the commutator. On the new brushes, the curvature is much shallower. This kind of formation is normal, but the old brushes have worn in length over time so much that the spring cannot give enough pressure. The contact is weak and the function will become flaky.

Also you will see the material looks like copper. these brushes contain copper as well as anthracite. The material is harder than the usual motor brush. It will take longer to wear down and it can take higher temperatures and more friction.

In the lower pic, there is a shot of the commutator, which the brushes come into contact with. Note that this area in the alternator is different to most motors. Here there are two circular, insulated copper contacts, that connect to the ends of the rotor winding. The usual layout of the motor has a lamelled commutator, and the brushes aren't laid on top of each other like here, but right on opposite sides of the commutator.

Note that although this is an alternator from a car, and not a motor, very smilarly designed motors are used in some cases. The design consists of a stator very similar to an induction motor, and a rotor very similar to this. It can be used as a synchronous or asynchronous motor. In asynchronous mode, the brushes are shorted together by a high current switch, or there is a variable resistor between them, that can be used to adjust the output torque (and speed) of the motor. In synchronous mode, the brushes are supplied constant DC voltage... torque and speed can be adjusted by altering this, although mostly synchronous motors are adjusted by the stator field frequency.

Also this motor type can be switched between the modes easily, and often it is started in async mode, then swithched to sync mode when it is close to top speed. On some really geek systems, during async startup, power is taken out of the rotor circuit, rectified, and fed back to the power network through an inverter. This isn't exactly a perpetual motion machine, but probably one of the closest electrical solutions you can get... anyway it will give smooth startup while saving a lot on peak startup load on the network.

Attachment: brushes2.jpg (Downloaded 248 times)

Last edited on Wed Apr 18th, 2007 08:32 pm by Keinokuorma

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