Universal series motor with speed limiting circuit to protect the motor from overspeeding during relatively-small loads

A speed limiting circuit for protecting a universal series motor from overspeeding when the motor is lightly loaded, comprising a motor having an armature and a field winding, said armature having first and second ends and said field winding have first and second ends, said second end of said field winding being connected to said first end of said armature to form a series circuit, a source of power for said motor, said power source including first and second conductors, said first conductor being connected to said first end of said field winding, said second conductor being connected to said second end of said armature, a shunt element connected in parallel with said armature, said shunt element being connected from the junction between said second end of said field winding and said first end of said armature to said second end of said armature, said shunt element being normally non-conductive and being responsive to a control voltage input to become conductive whenever said control voltage input exceeds a pre-determined level, a voltage detection circuit connected in parallel with said armature and with said shunt element, said voltage detection circuit having first and second input terminals and an output terminal, said first input terminal of said voltage detection circuit being connected to the junction between said second end of said field winding and said first end of said armature, said voltage detection circuit producing a control voltage at said output terminal thereof proportional to the speed of said armature, and means for applying said control voltage to said shunt element to control the conduction of said shunt element, whereby said control voltage will rise to a value sufficient to render said shunt element conductive whenever the speed of said armature rises to a pre-determined value and current flow through said field winding is thereby increased, thereby tending to limit the speed of said armature to said pre-determined value.