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MMD > Archives > February 2003 > 2003.02.13 > 11Prev  Next

Sparking of Ampico B Spool Drive Motor
By Bob Taylor

[ I wrote privately to Bob about this subject and he replied with this
 [ equally plausible explanation, which he approved for publication.
 [ -- Robbie

> Here's my intuitive explanation.  The average current to the motor
> flows through the points and the resistor, alternating between the
> two as the points open and close.  When the motor is playing a roll
> at normal speed the resistor becomes warm.  If the resistor fails or
> is disconnected then it stays cool.
> But, with or without the resistor, the total power to the motor
> stays constant, as confirmed with an ammeter.  Therefore if the
> resistor isn't heating up, something else is.
> If the resistor isn't working the motor current flows through the
> points alone.  When the resistor fails an oscilloscope will then
> indicate greatly increased voltage across the points, showing that
> the points are dissipating the power that normally is dissipated
> by the resistor.  Therefore I presume that the expensive ceramic
> wire-wound resistor was installed to increase the service life of
> the points, perhaps to two lifetimes.  ;-)
> Robbie Rhodes
> Etiwanda, CA

Robbie,  Yours is an interesting insight, but still not a complete
analysis, and consequently, the conclusion just a bit convoluted.

Of course the governor points are "saved" by the resistor -- that has
never been an issue.  They still survive quite nicely without the
resistance.  I have several boxes of theses resistors, all open
circuits, and the pianos from which they came had no noticeable
difference in point wear.

The argument could go on for ever, but who cares?  The resistor,
correctly working, makes the motor run smoother, and cooler, and that
is a known development criteria.

If the points are doing the heat dissipation in lieu of the resistor
as you suggest, why then does the tempo lever and surrounding area stay
cool when the resistor is burned out?  This is my intuitive input and
it seems to speak more loudly than your intuitive input.  Indeed, when
correctly operating, the entire cast housing becomes the heat sink, and
considerable energy is dissipated.

I think your data collection to prove a point, is lacking.


Bob Taylor

 [ I'll gather quantitative electrical data when my Ampico returns
 [ from the restorer's shop.  -- Robbie

(Message sent Fri 14 Feb 2003, 02:16:44 GMT, from time zone GMT-0600.)

Key Words in Subject:  Ampico, B, Drive, Motor, Sparking, Spool

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