On Thu, 23 Apr 2009 04:20:31 -0700, "William Sommerwerck"
wrote:

No one has said anything about replacing the pot with an SCR or triac
controller.

Sacrilege! It's probably an antique Singer sewing machine. Trends
towards modernization of antiques are highly discouraged by convention
among collectors. The desecrations of cherished antiques never seems
to end. The slippery slope of modernization and evil upgrading will
soon lead to the total replacement of the entire sewing machine. Such
things cannot be tolerated. Maintaining an old machine is an art.
Doing the same to a modern plastic equivalent is often a waste of
time. Best to restore the old Singer to its original condition than
to butcher it into some manner of hybrid contrivance.

It's a marginal idea anyway. Small sewing machine motors use solid
carbon brushes. That's fine for low duty cycle, variable speed, and
minimal load motors. Add an electronic speed control and the brushes
now see a higher frequency component in the wave form. The result is
a slight increase in arcing of the brushes, which causes increased
wear of the copper commutator sections. Modern brushes are a sandwich
of graphite (for lubrication) and carbon (for conductivity) to prevent
this type of wear. My guess(tm) is that an electronic speed control
will take about 25% off the life of the motor.

Some instructions and clues on rebuilding sewing machine motors:
http://reviews.ebay.com/HOW-TO-OVERHAUL-YOUR-SEWING-MACHINE-MOTOR-IN-30-MINUTES_W0QQugidZ10000000004665359
"The next step is to clean the armature commutator. This is where
the carbon brushes make contact. 90% of motor's troubles comes
from a bad commutator. Heat, oil, carbon accumulation and dirt
are the ideal mix to provocate(sic) sparkling when electricity
is applied to the motor. Under load, the sparkling will increase
and some motors will start smoking..."

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558