overcurrent detection circuit

Posted on Feb 7, 2014

This motor overload circuit does not simply disallow overloading since it is permissible to overdrive a system but only if it is done for a short time (typically dependent on heat buildup). An overload detection circuit protects a motor against currents above rated current that persist for certain exposure times. The allowable exposure times are i

overcurrent detection circuit
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nversely related to the degree of overload current. The circuit has two overload current sensors with two respective overload thresholds. These are both connected to a pulse generating circuit and the pulse rate is accelerated when the second overload threshold is reached. The pulse generating circuit couples pulses to a pair of cascaded counters which generate a fault signal when a predetermined count is reached. The pulse generating circuit includes an integrator with a capacitor that must be discharged, before another pulse can be generated. The pulse generating circuit includes a comparator which operates a switch to discharge the capacitor shortly before coupling each pulse to the counters, to prevent a delay that would otherwise occur between signals from the integrator circuit. The design relates to a first improvement of multiple current-sensing circuits for an overcurrent protection device which are responsive in different degrees to various overcurrent conditions. Whereas the prior art provided a typical inverse response characteristic of overload trip time as a function of overcurrent, the present design provides a two-segment response characteistic in which the breakpoint between the segments and the shape of the response curve in the second segment can be controlled by the selection of components for the invented circuit. The design also relates to a second improvement in overcurrent detection devices of the...

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