Edison dynamo: Field winding math.

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
I intend to repurpose the motor armature pictured for the construction of a miniature Edison dynamo.

The only difficulty I am encountering is the requisite math for determining the size, length and number of turns of wire needed to form each of the field poles.

The armature was removed from a damaged vacuum cleaner motor. It was the universal type, being series wound for 120 volts, having a nameplate current of 7 amps and having 71 turns of wire around each field pole for approximately 500 ampere-turns. It presumably ran at several thousand RPMs.

I wish to derive 125 volts from the finished dynamo armature, and I wish to have the field poles shunt wound for slightly less than that (i.e. 90 volts) so I can regulate the armature voltage via PWM or rheostat. I would prefer to run the dynamo within the 1750-3500 RPM range by belting it to a small 4-pole induction motor.

Can someone point me in the right direction?



Joined Jan 23, 2018
The dynamo that I have seen at a science museum was hand cranked with a RPM step-up of about 1:10 A rheostat was available to adjust the field current, and the load was a 120 volt incandescent bulb. What became very obvious was that as the output increased the effort also became quite a bit.