| 摘要 |
An electric motor armature current control circuit includes an electric motor with an armature in series with a protective device comprising a pair of polymer PTC elements connected in electrical series with the motor armature windings and in electrical parallel and thermal contact with each other. The first polymer PTC element, a temperature compensating element, has a very low cold resistance and a trip temperature higher than the maximum expected ambient temperature to which it is to be exposed in motor operation and generates heat in motor operation to trip to a much higher resistance and become self-regulating at that trip temperature. The second polymer PTC element is maintained at least at the trip temperature of the first through thermal contact and has a lower resistance at that temperature than the cold resistance of the first. Its trip temperature is higher so as to be reached only with excessive armature current. Due to the temperature compensation element, the protective device limits excessive armature winding current after a period of time independent of ambient temperature.
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