Singer 15-91 power

[CowboyBob]

I used to work for an ex-Singer salesman & he told me when they demonstrated them they would sew through a wooden yardstick to impress people with the power.They were such a good machine that when they started selling the newer Touch & Sews (with plastic gears that break) they took them in on trade, then out in the alley & busted them up w/a sledge to get them off the market.

[Ole South]

Re: Drawing power when idle:

One of the early to mid-20th century foot pedals were a stacked carbon disk controlled device, I've often seen these on the 15-91s.  So yes, the potential for any cast iron (black) Singer drawing power when unused is present.

How they work (ie: WHY they may still pass current when idle)
Rather than a rheostat/variable potentiometer or variable transformer, these use a stack (or most often dual stacks) of carbon disks working as a pressure resistor.  The individual disks are slightly thicker but about the diameter of a dime.  Pressure is offset/removed by a spring (think return to zero pressure) and as you press the foot pedal down the stack of carbon disks are compressed reducing resistance and thus increasing current flow in a fairly linear manner.  Since these motors were AC/DC capable (I believe down to about 50vdc) this was an effective and inexpensive control solution for either power source.   Effectively, the tighter the disks are pressed together the more juice flows.  Even so, there was no insulation or air gap created between the disks when the spring returned the pedal thus eliminating the possibility of Zero current (yes, this is a double negative).  

Usually over the years the spring(s) would weaken, and dust (both carbon and common household) would build up between the disks reducing its efficiency by not allowing current flow which was the most common reason for replacement.  BUT... I have seen these as the apparent precipitating failure* causing the wires exiting the Bakelite power connection (to motor) fry in a rare case or two.  Probably why my Grandmother never converted her Red Eye model 66 from treadle to motor. 

Rewiring these machines is not impossible as most of the Bakelite connectors were threaded brass thru-hole posts with screws forming the connection... better than the "wind around the screw" we see today imo.  If you take this path (re-wiring): current flow to a vintage motor is/was, on the average, .75 amp at 120vac.  Modern replacement motors are more in the range of 1.0 to 1.5amp @ 120vac.  Original wiring was typically cotton or treated linen covered rubber which is prone to cracking and fraying over time.  Choose your replacement wire accordingly (I.e. for the motor used)

So yeah.... if you are using a vintage machine and foot pedal I'd err on the side of safety and unplug the bugger. (or at least switch off the surge suppressor/extension cord it's plugged into and you never know then a grandkid is gonna explore under the sewing table.)

 

* Burnt open wiring and evidence of severe arcing between the carbon disks. Sometimes holes were blown completely through adjacent carbon disks or they were vaporized all together.

Edited November 2, 2022 by Ole South