I believe that the electric motors used for sewing machines were of "series wound" configuration. That is, they have their armature and field windings in series. That allows the use of such "universal" motors with either AC or DC.
The overall resistance of the field and armature winding plus the resistance of the foot petal rheostat together with the applied voltage determine the current flowing through the series circuit (I=E/R). The current flowing through the windings produces a torque between the armature and field which causes the armature to rotate. The speed of rotation is limited by the load applied, too much load and the motor stalls, too little load and the speed increased until limited by bearing friction.
This is not a problem in small (fractional horsepower) motors. However series motors are also used for traction drives on Diesel electric locomotives, electric buses, hoisting engines, &c. If you lose the mechanical load under power the armature will accelerate until the centrifugal forces lifts the windings from the slots in the armature, or the commutator pads will pull lose and jam under the brushes. Either will pretty much destroy the motor unless you want to spend two or three days learning how to rewind a blown series motor. Don't ask me how I know that.
The motor runs on either AC or DC because in a series configuration running on DC there is no time varying component to the voltage and thus the current is steady and so is the torque. Running on AC, the varying voltage will add a time varying component to the current which will change polarity twice every 1/60th of a second. Since the torque is produced by the slots in the armature and field being S to S as they approach and S to N as they repel. One 1/20th of a second later they will be N to N on approach and N to S as they repel. The resultant torque will have a time varying ripple but the momentum of the armature will allow the mechanism to coast through the zero current intervals before accelerating back to full speed. Most machinery connected to a series motor will not respond to this small variation.
Now back to the sewing machine, it is true that you could run your machine on either AC or DC. But the voltage would have to be 120volts in either case to get the same output power. Running on 12volts DC (one battery) would only give one tenth the power and most likely fail to drive the mechanism. Ten car batteries in series would work but would constitute a serious problem designing a charging circuit and would be a serious safety hazard as 120VDC from batteries (very low internal resistance) can kill people dead as a door nail.
It would be much easier to find a small series motor with about one tenth of the internal resistance (nameplate rating of 12 to 20 volts). I would look via Google for places like "Science and Surplus". They buy old equipment and break it down for tinkers. I misspoke in my earlier post about the foot control. There is little likely hood of using the old controller on 12 volts. The only part that needs be changed is the variable resistance element contained inside the control box. A wire wound rheostat with a maximum value about 10% of the high voltage setup currently driving your machine (measure across the pins on the power plug w/ an ohmmeter while lightly stepping on the foot controller.
Regards,
GtG
Thanks. I guess I’ll just stick with running it on AC the way it is.