Here are Erector motors and gear boxes over the years. They are in right to left order. On the right, the first motor and gearbox, about 1918. The same motor is used with the "New Erector" gearbox, about 1925. This gearbox soon gave way to a new gearbox, then the more powerful 110 volt AC motor was added, the old 16 volt DC motor was still used in some sets. And one gearbox side plate was used as a dashboard for the White Truck model. Finally, about 1936, motors were made with integral gearboxes. The left motor and gearbox is from about 1950. There were several other motors and gearbox combinations made.
Many other construction set manufacturers made motors. On the left is a Steel Builder motor. See the Steel Builder system on another page, http://www.baileysteppe.com/Erector/Other/UnkSystem/unkset.htm. On the right rear is an unknown motor. In the right foreground is a homemade motor; made from what appears to be a phonograph turntable motor. All these motors run on 110 volt AC power, and will power erector models as well as the sets for which they were made.