Abstract: A synchronous machine is protected from closely spaced instances of excessive field current and an excessive volts/hertz ratio by a control system which models rotor and stator cooling to provide memory of recent periods of excessive heating. Maximum excitation limiting and volts/hertz ratio limiting modules in a microprocessor based automatic voltage regulator for the machine generate time delayed limit signals which override voltage feedback control when the respective parameters exceed threshold values. The limiters each increment a count when the associated threshold value is exceeded and generate the associated limit signal when a predetermined count is reached. The predetermined counts and the counting rates are selected such that the delay intervals are inversely proportional to the amount by which the threshold values are exceeded. The counts are decremented at a rate which models rotor and stator cooling when the respective parameters return below the threshold values.

Abstract: Apparatus for protecting a synchronous generator from under-excitation, includes a limiter implemented as part of a microprocessor based voltage regulator. The limiter selectively characterizes one or more limiting parameters, such as, system stability, machine capability and loss of field protection, as straight line segments in the real/reactive power plane to provide versatile, easily modified limits on excitation.

Abstract: An inverse-time timer for a volt/Hertz ratio relay includes a ladder of level detectors associated with a like series of counters connected in cascade with a variable voltage-to-frequency connector responsive to the v/Hz ratio signal for controlling the counters to establish the time delay for tripping.

Abstract: Signal conditioning apparatus for reducing the noise frequencies of a speed sensing means signal being fed into a compensating means for compensating the output of a synchronous generator for the effect of torsional oscillation between masses in the main AC generator system. The signal conditioning apparatus includes two speed sensing transducers mounted 180.degree. apart on opposite sides of a tooth gear wheel and summing the two FM output signals from the transducers to eliminate odd harmonics of the synchronous frequency in FM noise due to eccentricity. The preferred embodiment of the invention reduces the remaining noise from the sensed speed signal by the use of a three-pole Butterworth low-pass filter in combination with a notch filter tuned to the turbine-generator synchronous frequency to thereby reduce the unwanted noise to a large degree while providing a minimal phase shift to the sensed speed signal.

Abstract: A lock-out control logic circuit for a feedback inverter system, which senses the load current in each load switching element of the inverter system and utilizes the sensed current through the presently conducting load switching element to inhibit the next firing of the presently non-conducting load switching element. Current transformers are inserted in series with each switching element and coupled to a CMOS logic circuit which inhibits the firing pulse to a non-conducting load switching element in the event a current conduction condition is sensed.

Abstract: A phase imbalance logic module for a solid state motor protection system for a three-phase motor. Three-phase line currents are converted to corresponding voltages and rectified. The resultant output voltage is a poorly regulated DC voltage having a ripple of approximately 4% during normal operating conditions. In the event of a phase failure, even if not total, the corresponding ripple percentage increases. The phase imbalance logic network senses ripple and provides an output signal to trip a protective circuit breaker if the component of ripple becomes significantly large.

Abstract: An electrical control system which is utilized for opening a circuit breaker which is connected to a load to be protected. The control system comprises a unitary current sensor module which senses current flowing in the load to be protected. The sensor is adapted to have connected at the output thereof a load resistor which is capable of cooperating with the sensed current to provide a range of voltage over which a set of plug-in modules are operable to either provide an indication of characteristics associated with the current or the load or to provide an output function which is adapted to open the circuit breaker. The load resistor is replaceable with other load resistors so that the voltage range may be maintained even though the expected value of currents to be sensed by the sensor may change depending upon the load and other circuit characteristics.