13-09-2017, 10:33 AM
There is disclosed a solid state protection device which monitors an electrical quantity such as current in a protected circuit and which responds to abnormal conditions by activating a circuit breaker or warning device after a period of time which is related by a non-inverse function linear to the magnitude of the amount of circuit being monitored. The device includes a non-linear function generator which utilizes an operational amplifier to which a relatively low bias signal is supplied to approximate the non-linear operating characteristics of prior electromechanical protection devices.
Basically, such static overcurrent relays are composed of means for monitoring the current in the circuit being protected and for developing a voltage directly proportional to the circuit current, which is printed on a timing circuit which is designed to develop a level of output voltage substantially equal to the input voltage level after a predetermined time delay, thus providing an inverse time relationship. Since a normal current flow will also cause the timing circuit to be operative, a pickup circuit is provided to protect the operation of the timing circuit until a predetermined voltage magnitude is reached, at which time the synchronization circuit automatically activates to operate and produce the desired inverse time relationship. When the output of the timing circuit reaches a predetermined critical voltage level, it is used to control an electronic switching means for energizing relay means to automatically trigger a circuit breaker.
In many applications, it is highly desirable to provide means for significantly altering the reverse-current-to-current ratio to protect the circuit being monitored against the dilution forms of the overload current conditions. For example, when large motors and other similar devices comprise a load for the power circuit being monitored, the currents in said circuit are of such a nature that they require a batch or short duration time delay versus an intensity default.
Basically, such static overcurrent relays are composed of means for monitoring the current in the circuit being protected and for developing a voltage directly proportional to the circuit current, which is printed on a timing circuit which is designed to develop a level of output voltage substantially equal to the input voltage level after a predetermined time delay, thus providing an inverse time relationship. Since a normal current flow will also cause the timing circuit to be operative, a pickup circuit is provided to protect the operation of the timing circuit until a predetermined voltage magnitude is reached, at which time the synchronization circuit automatically activates to operate and produce the desired inverse time relationship. When the output of the timing circuit reaches a predetermined critical voltage level, it is used to control an electronic switching means for energizing relay means to automatically trigger a circuit breaker.
In many applications, it is highly desirable to provide means for significantly altering the reverse-current-to-current ratio to protect the circuit being monitored against the dilution forms of the overload current conditions. For example, when large motors and other similar devices comprise a load for the power circuit being monitored, the currents in said circuit are of such a nature that they require a batch or short duration time delay versus an intensity default.