Machine 27Build an electric motor← → step through the guidebaseboard · 102 × 40.5 cm
At motor0.0 V
Current0.0 A
Armature0 rpm
Fan0 rpm
From battery0 W
What this step adds; in part B, the current through brushes and commutatorParts already in placeA permanent-field DC motor: the same armature, commutator and brushes as a dynamo, fed instead of driven. The battery's 12 V pushes a current I = (V − k·ω) / R through the brushes; the turning armature makes its own back-voltage k·ω, so with no load the speed is simply proportional to the voltage, about 125 rpm per volt. The model assumes k = 0.0764 V·s/rad (1,500 rpm at 12 V with no load), 1.0 Ω in the armature and brushes, 0.06 N·m of friction, a fan whose drag grows with speed squared, and a 12 Ω rheostat. Rheostat cut out, it runs at 1,160 rpm on 2.7 A and the fan at half that, 580 rpm; halfway, 499 rpm on 1.1 A. Swapping the two armature leads reverses the current in the armature but not the field, so it turns the other way. This is a guide to how the parts go together, not engineering drawings. Keep fingers, hair and sleeves clear of the belt and fan, and fuse the battery lead: a shorted 12 V battery can melt wire.
