A self-starting dc motor

The armature consists of three arms formed by bending soft iron strips (three required, see photo).   These are then assembled on a wood dowel to serve as a shaft and to hold the commutator, using transparent tape which also provides insulation.   One hundred turns of 0.315 mm enameled copper wire are wound on each limb in the same sense.   The end of each winding is joined to the start of the next and the joint connected to the commutator segment lying between the two.   The motor will work with a permanent magnetic field, but in this case a stator with two hundred turns of the same wire is wound.   Such a motor is more suited for experiment with series and shunt windings and variable field from the stator.   I will be happy to demonstrate this model to students and discuss it and other variants.   Just visit the Science Museum in room 452.

Raymond Libreri,  Science Museum 15/4/2002