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Discussion Starter · #1 ·
hi!why do shunted motors have teflon tube on the brush springs. i know it insulates the springs from the brush and endbell, but why? john.
 

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The insulators are fitted to stop the current going through the spring, could overheat the wire and cause a loss in spring tension.
[oneofwos]
 

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Current always takes the way of lowest resistance. So it will take the shunt because this has the bigger cross section. Nevertheless - the insulation is on the spring to prevent the heat transfer esp. from the brush into the spring and thus possibly cuasing the mentioned loss of tension.

Roland
 

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Discussion Starter · #4 ·
thanks guys its obvious now, should have thought about it harder. so why not shunt all motors if its the best solution. john.
 

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There is another reason for shunts where the brush runs in a live metal brush holder. Some current flows from the metal brush holder to the brush, this can cause pitting of the sides of the brushes which stops them sliding smoothly in the brush holders. This is reduced by brush shunts.

QUOTE (Schackel @ 12 Jun 2011, 17:47) <{POST_SNAPBACK}>Current always takes the way of lowest resistance. So it will take the shunt because this has the bigger cross section.
That's not quite how electric currents work.
With two parallel paths the current is shared between the two.
So for example if the shunt has half the resistance of the spring, 2/3 of the current goes through the shunt and 1/3 of the current goes through the uninsulated spring. (Neglecting whatever current flows from the brush holders.)

QUOTE (stoner @ 13 Jun 2011, 07:53) <{POST_SNAPBACK}>thanks guys its obvious now, should have thought about it harder. so why not shunt all motors if its the best solution. john.
In high level competition, virtually all the competitors do run insulated springs and brush shunts (except where the rules don't allow it).
They are not used lower down the performance spectrum. It only costs pennies if you spend the time to do it yourself, but the time taken to do it would put up production costs. Guess that's one reason why it isn't done on other motors. Also it's less important at lower current.
The brush gear at the lower end of the market isn't designed to allow insulated springs and shunts.
 
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