Electric motor armature current control circuit

An electric motor armature current control circuit comprising, in combination:

• an electric motor having a field and an armature with armature windings;
• an electric power supply;
• a motor protection device connected in series with the motor armature windings and electric power supply, whereby any armature winding current also flows through the motor protection device, the motor protection device comprising (a) a first polymer PTC element connected in electrical series with the motor armature windings, the first polymer PTC element having a resistance effective to generate heat with armature winding current flow therethrough and characterized by a first trip temperature higher than the maximum expected ambient temperature to which the first polymer PTC element is to be exposed in motor operation, above said first trip temperature said resistance increases greatly from a low resistance to a high resistance, the heat generated by normal armature current being effective to raise the tempeature of said element above the first trip temperature and the resulting high resistance being effective to reduce electric current therethrough sufficiently as to make the first polymer PTC element self regulating at the first trip temperature, and (b) a second polymer PTC element connected in electrical parallel with said first element and in thermal contact therewith for being maintained at least at the first trip temperature, the second polymer PTC element having a resistance effective to generate heat with armature winding current flow therethrough and characterized by a second trip temperature sufficiently higher than said first trip temperature and requiring an excessive armature winding current to raise the temperature of said second element to the second trip temperature, and above said second trip temperature said second element resistance increases greatly from a low resistance lower than that of the first polymer PTC element and essentially negligible in its effect on the armature winding current to a high resistance protectively limiting the armature winding current, the first polymer PTC element also having, at the second trip temperature, a high resistance protectively limiting the armature winding current, whereby the protective device limits excessive armature winding current after a period of time independent of ambient temperature.