Abstract The specification discloses a system for preventing current overload in a multi-phase alternating current motor. Current transformers are coupled to each of the phases of the motor and voltage outputs of each of the transformers and are connected to a summing point. A differential amplifier is connected to the summing point for detecting the occurrence of a current overload on any of the phases. A digital circuit is connected to the differential amplifier for generating a logic level in dependence upon the output of the amplifier. A timing circuit including a normally conductive field effect transistor is coupled to the digital circuit. Upon the occurrence of an overload signal, the timing circuit initiates a timing cycle during which current may not be reapplied to the motor. A transistor switch normally maintains a triac in conduction and the triac operates the contactor relay to normally supply current to the motor phases. Upon the occurrence of an overload current, the transistor switch turns the triac off and terminates current flow to the phases of the motor. Light emitting diodes are provided to indicate when the timing cycle is in effect and when a current overload condition exists.
A system for protecting a multi-phase alternating current motor from current overload comprising: a current transformer coupled to each of said phases of said motor, means for connecting the output of each of said transformers to a summing point, a differential amplifier connected to said summing point for detecting the occurrence of a current overload on any of said phases, a digital circuit connected to said amplifier for generating a logic level in dependence upon the output of said amplifier, a normally conductive field effect transistor coupled to said digital circuit for becoming nonconductive upon the occurrence of a current overload, a capacitor connected to said field effect transistor, a pair of normally nonconductive transistors coupled to said field effect transistor, one of said pair of transistors coupled to said capacitor in order to initiate a timing cycle before said field effect transistor may be again rendered conductive after the occurrence of a current overload, a first light emitting device, the other of said pair of transistors operable to energize said first light emitting device to indicate the occurrence of said timing cycle, a triac, switch means for normally maintaining said triac in conduction and operable in response to the occurrence of a current overload to render said traic nonconductive, and relay means operable by said triac to terminate current flow through said phases when said triac is nonconductive.
Claim 2
Independent Claim
229 words
A system for protecting a multi-phase alternating current motor from current overload comprising: a current transformer coupled to each of said phases of said motor, means for connecting the output of each of said transformers to a summing point, a differential amplifier connected to said summing point for detecting the occurrence of a current overload on any of said phases, a digital circuit connected to said amplifier for generating a logic level in dependence upon the output of said amplifier, a normally conductive field effect transistor coupled to said digital circuit for becoming nonconductive upon the occurrence of a current overload, a capacitor connected to said field effect transistor, a pair of normally nonconductive transistors coupled to said field effect transistor, one of said pair of transistors coupled to said capacitor in order to initiate a timing cycle before said field effect transistor may be again rendered conductive after the occurrence of a current overload, a first light emitting device, the other of said pair of transistors operable to energize said first light emitting device to indicate the occurrence of said timing cycle, a triac, switch means for normally maintaining said triac in conduction and operable in response to the occurrence of a current overload to render said traic nonconductive, and relay means operable by said triac to terminate current flow through said phases when said triac is nonconductive.
Claim 3
Dependent Claim
29 words
The system of claim 1 wherein said switch means comprises: a pair of intercoupled normally conductve transistors responsive to the occurrence of a current overload to be rendered nonconductive.
Claim 4
Dependent Claim
45 words
The system of claim 2 and further comprising: a second light emitting device, and a normally nonconductive transistor responsive to one of said pair of normally conductive transistors in order to energize said second light emitting device to indicate the occurrence of a current overload.
Claim 5
Dependent Claim
26 words
The system of claim 1 and further comprising: means responsive to the operation of an interlock circuit on said motor to visually indicate an interlock malfunction.
Claim 6
Dependent Claim
17 words
The system of claim 1 wherein said triac is powered from one of said alternating current phases.