Abstract: The present invention provides a circuit breaker trip unit configured to operate with a plurality of different plug-in display units. The display unit includes a memory which stores data representative of the particular display unit, and which allows the trip unit to automatically adapt to communicate with the display modules. The trip unit includes a processor and programming which permits a bi-directional flow of data between the units, such as display data and control data.

Abstract: A circuit for protecting the ac generator and/or load of a diesel-engine generator set is provided. Included is a circuit for measuring indications of the current in each phase of the ac generator, and a switching circuit for protecting the ac generator if the current in any phase is greater than a predetermined current for a predetermined length of time (overcurrent), or if the current in any phase is less than a predetermined function of the currents in each of the phases for the predetermined length of time (imbalance). The circuit which protects the ac generator preferably short circuits the field winding of the ac generator and thereby turns off the ac generator, and/or closes a fuel valve to effectively turn off the engine should the current imbalance or overcurrent be sensed. In conjunction with the protection circuit, a switch interlock and/or circuit breaker arrangement for shock protection is provided.

Abstract: System For Detecting A Breakage Of A Power Cable For An Elevator System Which can accurately detect the breakage of at least one copper wire in the respective power cables present between the inverter of the elevator system and a linear motor moving element and which is difficult to visually inspect.

Abstract: A method and a device for preventing overstabilization of longitudinal differential protections in case of internal faults on power lines, which may take place when a fault situation, which is indicated as an external faults, is in reality an internal fault. The invention comprises criteria which finally provide information as to whether the longitudinal differential protection is to enter into operation. The criteria comprise level and directional determination of the currents which are measured in the terminals of the power lines, checking whether these have the same direction, and so on.

Abstract: A fail-safe circuit for preventing an inadvertent communication of a signal from an input terminal to an output terminal includes a series switching network electrically connected between the input terminal and the output terminal. The series switching network contains two switching circuits electrically connected in series. One switching circuit is an electrical switching circuit and the other is an electromechanical switching circuit. The switching circuits are chosen such that defects causing the electrical switching circuit to short circuit do not affect the electromechanical switching circuit and defects causing the electromechanical switching circuit to short circuit do not affect the electrical switching circuit. The fail-safe circuit also includes circuitry for preventing inadvertent communication of the signal from the input terminal through the series switching network to the output terminal when one of the switching circuits is in a short circuited state or condition.

Abstract: A method and a device for preventing understabilization of longitudinal differential protections in case of external faults and current transformer saturation. The invention comprises an extension of the state of the art with regard to stabilization of longitudinal differential protections in such a way that the values (A1, A2, . . . An), (.phi.1, .phi.2, . . . .phi.n), obtained via current measurement (C1, C2, . . . Cn) and Fourier filters (4, 5, 6), for determining the function characteristic of the protection under certain conditions as regards current amplitude and current transformer saturation are given corrected values (A1k, A2k, . . . Ank), (.phi.1k, .phi.2k, . . . .phi.nk) (FIG. 2).

Abstract: In an apparatus for detecting turn-on state of plural heaters including an electric power source for supplying electric power to the plural heaters, a controller controls the electric power source to be turned on and off so that each of the plural heaters is turned on and off in a predetermined manner, and a current sensor measures the total current supplied from the electric power source to the plural heaters. Then, a comparator compares the total current measured by the current sensor with plural reference values different from each other and outputting signals for representing comparison results thereof, and a judging circuit judges turn-on or off state of each of the plural heaters based on the signals outputted from the comparators.

Abstract: A compound semiconductor device is provided which includes a thyristor region constructed by four continuous layers of p-n-p-n and an MOSFET region which is formed in the intermediate n layer of the thyristor region so as to be away from the intermediate p layer. The MOSFET is constructed by a p well layer, a source layer, and a drain layer. One main electrode of the device is in ohmic contact with the outside p layer of the thyristor region. While the other main electrode is in ohmic contact with the source layer and well layer of the MOSFET region. An arrangement is provided for electrically connecting the outside n layer of the thyristor region and the drain layer of the MOSFET region. Also, a first insulating gate is formed on the well layer between the source layer and the drain layer of the MOSFET region and a second insulating gate is formed on the intermediate p layer of the thyristor region; with the first and second insulating gates being electrically connected.

Abstract: A first transformer is coupled to a deflection yoke and coupled to a derived secondary voltage source for driving a load, for example, for generating an ultor voltage. An on/off switch is responsive to an on/off signal. A second transformer has a primary winding coupled to the switch and coupled to a voltage source and has a secondary winding coupled to the first transformer. A first sampling circuit generates a first feedback signal indicative of current flowing from the secondary winding to the first transformer. A first safety circuit responsive to the first feedback signal controls the effect of the on/off signal on the switch when the current flowing from the secondary winding to the first transformer exceeds a threshold, by limiting the on time of the switch to limit the peak value of the current. A second sampling circuit generates a second feedback signal indicative of the ultor voltage.

Abstract: A step-up converter is utilized to convert a first lower input voltage to a second higher output voltage. The circuit includes a soft start capability such that ringing due to excessive voltage and current is substantially eliminated. In addition, this converter includes an overcurrent protection mechanism if a load failure or other overcurrent condition occurs.

Abstract: According to the present invention, a pulse of short duration is generated from system voltage and is applied to a circuit branch being tested. The resulting current is measured and subjected to a comparative evaluation. The corresponding arrangement has a power semiconductor circuit for generating the testing pulse from the system voltage. Preferably, a so-called GTO (Gate Turn-Off thyristor) is used for this purpose.

Abstract: An inductive load drive device which has a switch defining an output terminal connectable to an inductive load between the output terminal and a reference potential line is provided. A first current recirculating branch is connectable parallel to the load and a control opens and closes the switch so that the load is supplied with current rising to a first predetermined value in a first increasing phase and falling to a second predetermined value in a first recirculating phase. The device detects a short-circuit between the output terminal of the control device and the reference potential line with a first current sensor for detecting current flow in the first recirculating branch during the first recirculating phase. A fault signaling circuit connected to the first current sensor produces a short-circuit signal in the absence of current in the first recirculating branch during the first recirculating phase.

Abstract: Plural electrical loads supplied with power from a single source are monitored to detect defective loads. Each load is periodically individually energized and current delivered by the source is measured. Measured current for each load is compared with limit values to detect abnormal values. To overcome consequences of supply voltage fluctuations, supply voltage is also measured and admittance of each load is determined from measured current and measured voltage. The admittance value thus determined for each load is compared with limit values to detect abnormal values. Load monitoring of loads energized according to a control algorithm proceeds by periodic momentary suspension of energization according to the control algorithm and energization of selected individual loads.

Abstract: A circuit and method for driving a power transistor device. The circuit for driving a power transistor device has a driver having an input and an output, the output coupled to a control input of the power transistor device and the input coupled to a primary control voltage source for driving the power transistor device. A current sensing device is coupled to the power transistor device for providing a signal proportional to the current in the power transistor device. An amplifier is coupled to the current sensing device for providing a substantially linear control signal proportional to the current in the power transistor device, the linear control signal being provided to the input of the driver as a secondary drive signal for driving the power transistor device when a current level in the power transistor device greater than a threshold level is detected. A detector is provided for detecting when the current in the power transistor device is greater than the threshold level.

Abstract: This invention concerns a system for the protection of electric circuits (60, 61, 62) which permits an analysis in real time of the evolution of electric currents circulating in such electric circuits, such analysis having as its purpose not only protection but also prevention. Such protection system comprises at least one circuit breaker (64, 66) including a first coil (70, 71) associated with a magnetic circuit and bearing the main current, as well as a second coil (74, 75) likewise associated with the magnetic circuit. Such second coil carries an induced current representative of the main electric current. Such induced current is processed by an electronic circuit (80) which supplies a processed signal representative of at least one characteristic of the main current, said characteristic being used for prevention in the electric circuits.

Abstract: A system including a plurality of processing units wherein one power source is connected to the plurality of processing units, is constituted such that when a total of maximum rated currents of the processing units exceeds an allowable current of the power source, an allowance or nonallowance of the power supplied to the processing units is determined from a state of the power source and the power supply is controlled thereby, and whereby the power source does not become overloaded. Thus, the system avoids overloading the power source if a sum of the maximum rated currents of respective connected processing units exceeds the allowable current of a power source.

Abstract: A low voltage electric power monitoring and circuit breaker system includes two processors. The first processor activates the circuit breaker when an overcurrent condition is detected and the second processor monitors the current sensed by the first processor and activates the circuit breaker for overcurrent conditions which should have caused the first processor to activate the breaker. If a current of this magnitude is detected, the second processor activates the circuit breaker as a backup protection for the first circuit breaker.

Abstract: An electrical apparatus for magnetothermal and differential protection, of the type comprising a device for sensing differential currents associated with a magnetothermal protection switch, comprises a differential current sensor connected to an input of a controlled gain signal amplifier, in turn connected to a plurality of light indicators arranged to go on progressively in dependence on the sensor. The apparatus provides a quantitative and qualitative measurement of the stray current.

Abstract: The overload protection circuit 12 comprises a resistor bridge 20 with a low impedance sensing resistor r and high impedance resistors R1 to R4 which generates voltages Va and Vb at the inputs 32 and 34 of a comparator 22. Voltage Vb depend upon the value of the load current so that the comparator provides at its output 36 a control signal for opening the switching device 24 comprising a field effect transistor when the load current exceeds a maximum value. Resistor R5 reinforces the effect of the control signal. This circuit can be used in a system which comprises a central power supply unit 8 which powers a plurality of devices such as 4. The status of the circuit 12 is reported to the system 8 though line 14 and circuit 12 comprises a control circuit 26 which is responsive to set and reset control signals on lines 14 and 18 from the unit 8 to close or open the switch 26.

Abstract: A lighting circuit for a vehicular discharge lamp having a DC voltage booster circuit for boosting an input voltage from a DC voltage input terminal, comprises a current detector, a voltage detector, an abnormality judging circuit, and a power cutoff relay circuit. The current detector acquires a detection signal concerning an output current of the DC voltage booster circuit. The voltage detector acquires a detection signal concerning an output voltage of the DC voltage booster circuit. The abnormality judging circuit compares the levels of the detection signals from the current detector and the voltage detector with each other to determine whether or not the lighting circuit is in an abnormal status. The power cutoff circuit inhibits power supply to the discharge lamp upon reception of a signal from the abnormality judging circuit representing an abnormality in the lighting circuit.

Abstract: A digital protective relay apparatus comprising an analog input section, a correction value-calculating section, a storage section, a correction section, a protective calculation section. A correction value for correcting the error of the analog input section is predetermined and stored in the storage section. Digital data obtained through the analog input circuit is corrected by the correction section using the correction value. The protective calculation section performs predetermined calculation using the corrected digital data, thereby determining whether or not a trip instruction should be supplied to a breaker.

Abstract: A power supply for generation of a mainly sinusoidal output voltage, the magnitude and frequency of which are adapted for driving of main power-operated apparatuses. The power supply includes means (10) for generating a DC voltage, a DC/AC converter (18) including controlled semiconductor switches (19,20,21,22) and an electric control device (30) for emitting to the semiconductor switches suitable commutating signals according to a predetermined program to keep the switches closed and open, respectively, such that the desired waveform for the output voltage is achieved. Sensing of a parameter associated with the load current of the power supply is associated with the load current of the power supply is provided and cooperates with the control device (30) such that upon the load current exceeding a predetermined value the commutating signals are cancelled. The commutating signals are thereafter reconnected after a period of time which is considerably less than half the period of the output voltage.

Abstract: A technique is described for improving the sampled current accuracy in current mirror circuits such as are often used to monitor the current output to a load. A reference voltage is established to feedback the load voltage conditions at the output to the current reference circuitry, thereby greatly reducing the error in the reference current over that which is produced by prior art circuits. The technique is shown as applied to a current limiting circuit for driving an output load. An alternative embodiment is also disclosed.

Abstract: A television apparatus has a horizontal deflection circuit with a transformer for developing a derived secondary power source. A circuit for disabling the horizontal deflection circuit is responsive to a fault condition in the horizontal deflection circuit. A driver stage in a vertical deflection circuit is energized by the secondary power source for supplying vertical deflection current to a vertical deflection yoke. An S-shaping capacitor is coupled in series with the vertical yoke. A circuit for sensing vertical yoke current includes a sampling resistor coupled in series with the vertical yoke and the S-shaping capacitor. The vertical yoke current sensing circuit generates a feedback signal for controlling the vertical deflection circuit during normal operation and generates a fault signal indicative of an overcurrent condition in the vertical yoke. Indication of the overcurrent condition is calibrated to be indicative of fault conditions in the vertical driver stage and/or in the S-shaping capacitor.

Abstract: A temperature compensated overcurrent and/or undercurrent detector monitors current through a solenoid or other load and signals when the load current exceeds an upper limit or falls below a lower limit. Such a signal may indicate a failure of another circuit which supplies the load current via a load transistor. This other circuit which supplies the load current is temperature compensated, and likewise the detector is temperature compensated so it can be set to signal an overcurrent or undercurrent condition when the load current varies a small amount from a predetermined range. The load transistor has an on-resistance which passes the load current and varies with temperature. The temperature compensation for the detector is provided in part by two pilot transistors which are integrated with the load transistor such that as the load transistor heats-up due to the load current passing through the on-resistance, the pilot transistors heat-up due to heat conduction from the load transistor.

Abstract: A circuit for protecting the ac generator and/or load of a diesel-engine generator set is provided. Included is a circuit for measuring indications of the current in each phase of the ac generator, and a switching circuit for protecting the ac generator if the current in any phase is greater than a predetermined current for a predetermined length of time, or if the current in any phase is less than a predetermined function of the currents in each of the phases for the predetermined length of time. For diesel-engine generator sets which generate three phase 460 V at up to thirty amperes continuously, the preferred predetermined overcurrent for any phase of the ac generator is thirty-five amperes, and the predetermined length of time (to permit inrush currents) is five seconds. Preferably the current in any phase should be less than fifty percent of the average of the current in the three phases.

Abstract: An electromagnetic contactor assembly with a sensing circuit for determining contactor state based upon the reluctance of the contactor coil. The coil has an electromagnetic coil and a movable contact separated from the coil by a gap when the coil is not energized. Energizing the coil causes the movable contact to close the gap and contact the coil. The determination of the relationship between the reluctance in the coil when the gap is in an open and closed state allows contactor gap status to be measured by measuring the coil inductance. A sensing circuit is added to the drive circuit to measure changes in AC current during the pulse cycle which holds the contacts in this closed position. The sensing circuit measures the AC current and is able to signal contactor status based on that value.

Abstract: A system and method for detecting partial discharge of gas-insulated switchgear wherein a high-frequency component and the fundamental wave component of the voltage applied to the conductor of the gas-insulated switchgear are detected, the zero-cross point of the fundamental wave component is set as the starting point for the voltage, and the presence of partial discharge and the reason for the occurrence of partial discharge are ascertained based on the phase difference between the phase of the detected high-frequency component and the phase of the starting point of the voltage.

Abstract: The interface circuit described comprises a plurality of control stages, each of which can detect and indicate a short-circuit in the corresponding load or the associated power transistor.In particular, the interface circuit is arranged to disable a transistor permanently when short-circuits occur repeatedly in the load in order to prevent damage to the transistor.

Abstract: A sensitive and fast low voltage lamp system controller to provide safe operation of "bare wire" systems that are particularly attractive for use in exhibition and tradeshow booth lighting. The high sensitivity is achieved by emphasizing high frequency noise present during shorts and by proportionately limiting the instantaneous current that tracks the line AC voltage waveform; fast response is provided by a power FET coupled to a bridge rectifier in series with the load and both then in series with a protection relay.

Abstract: An arc detection method and apparatus is provided for an electric circuit having an electrified conductor (46) connecting a voltage source (26) to a load (30), the conductor having a line side extending from the arc detector to the voltage source, and a load side extending from the arc detector to the load. The arc detector includes a field sensor (62) sensing the electromagnetic field established about the conductor by the occurrence of an electrical arc in the circuit, and generating a field responsive signal in response thereto. Direction sensing circuitry detects whether the arc is from the load side or the line side, and generates a direction indicative signal.

Abstract: An arc detection method and apparatus is provided for an electric circuit having an electrified conductor (46) connecting a voltage source (26) to a load (30). The arc detector includes a field sensor (62) sensing an electromagnetic field established about the conductor by the occurrence of an electrical arc in the electric circuit, and generating a field responsive signal in response thereto. Arc discrimination circuitry responds to the field sensor and generates an arc indicative signal in response to a random chaotic pattern of the field responsive signal.

Abstract: An arc detection method and apparatus is provided for an electric circuit having an electrified conductor (46) connecting a voltage source (26) to a load (30). The system senses the electromagnetic field established about the conductor by the occurrence of an electrical arc in the electric circuit, and generates a field responsive signal. Arc discrimination circuitry responding to the field responsive signal and generates an arc indicative signal in response to a given characteristic of the field responsive signal. The system includes an E field sensor (66) and a B field sensor (68).

Abstract: An arc detection method and apparatus is provided for an electric circuit having an electrified conductor (46) connecting a voltage source (26) to a load (30). A field sensor senses an electromagnetic field established about the conductor by the occurrence of an electrical arc in the circuit, and generates a field responsive signal. Arc discrimination circuitry includes frequency responsive circuitry responding to the field sensor and generating an arc indicative signal in response to a given frequency characteristic of the field responsive signal. The frequency responsive circuitry monitors a plurality of different frequencies of the field responsive signal, and generates the arc indicative signal in response to a given combination of field responsive signal frequencies. A comb filter passes the field responsive signal through a plurality of passbands, and the arc indicative signal is generated in response to a given frequency signature of the field responsive signal.

Abstract: A switching circuit providing detection circuit for detecting a current which flows through a main P-channel MOSFET detection of a floating voltage dependent upon a power supply potential, a reference-voltage generating circuit for generating a reference voltage which is a floating voltage dependent upon the power supply potential and has a constant value independently of variations in power supply potential, a comparator circuit operated on a supply voltage which is a floating voltage dependent upon the power supply potential, for comparing a detected voltage from the detection circuit with the reference voltage from the reference-voltage generating means, to convert the detected voltage into a logic voltage signal (i.e., a bi-level voltage signal and a conversion circuit for converting the logic voltage outputted by the comparator circuit into a voltage measured from a ground potential.

Abstract: An overcurrent trip circuit monitors the rate of rise of the current and the magnitude of the current in a power conductor and opens a circuit containing the power conductor when either the rate of rise of the current exceeds a first preselected limit value or the magnitude of the current exceeds a second preselected limit value. The inherent inductance in a metering shunt is used to produce a voltage signal that contains a component which is representative of the rate of change of current in a power conductor. This voltage signal is compared to a first reference signal to produce a trip signal when the voltage exceeds the first reference signal. In addition, the voltage signal is processed and the processed voltage signal is compared to a second reference signal to produce a second trip signal when the magnitude of the processed voltage signal exceeds the magnitude of the second reference signal. The power circuit is then opened in response to either of the first or second trip signals.

Abstract: An overcurrent protection device is provided for one or more phases of a circuit that generates a control signal dependent upon an adjustable trip level setting after a fault has occurred. The device includes a blocking level detector that blocks the generation of the control signal is the current in any of the phases exceeds either a first predetermined blocking level or the adjustable trip level setting. In a preferred arrangement, before control-signal generation, the device also requires that the current in at least one phase is below a predetermined load level.

Abstract: A high-speed high-voltage FET current interrupter which opens the cathode circuit of microwave tubes when internal tube arcing occurs. By removing the detrimental fault current which causes degradation to the microwave tube body and grid, the longevity of the microwave tube and the pulse energy storage capacitor is greatly increased over those transmitters which use dissipative crow bar circuits. Even more important is that the transmitter reset time after an arc occurs is orders of magnitude faster than transmitters using crow bars or primary power interrupters when arcing occurs--an important feature when engaged in a "dog fight".

Abstract: A zero-phase sequence current detector have three circular cores provided around three phases overhead lines of AC power lines, three magnetoptic sensors provided on respective cores for detecting currents flowing on respective power lines, and a processing circuit containing an adding circuit for adding outputs of the magnetoptic sensors and a compensation processing circuit for canceling erroneous components due to the differences of the temperature characteristics among the magnetoptic sensors.

Abstract: A hybrid current limiter comprising a first winding of non-superconducting conductive material having a large number of turns and electrically connected in series with a superconductoring coil of low self-inductance and placed in a cryostat, and a second winding of non-superconducting conductive material having a small number of turns, said second winding being closely coupled with said first winding in such a manner as to obtain low overall inductance, the second winding being connected in parallel with the series circuit constituted by the first winding and the superconducting coil.

Abstract: The invention relates to an output stage for an ac voltage switch, more particularly an ac voltage proximity switch, wherein a load is supplied from an ac voltage source via a rectifier which can be connected from an input stage via a control stage to a MOSFET connected to the output of the rectifier, wherein the internal voltage supply of the input stage and the control stage, when the rectifier is not connected the input stage and the control stage are connected via a source or an emitter-follower with voltage-stabilized input to the ac voltage source, and when the rectifier is connected the input stage and the control stage are connected to a voltage-stabilized electronic structural component at the output of the rectifier. This voltage-stabilized structural component is the MOSFET connected to the output of the rectifier, and when the rectifier is connected, the FET or transistor of the source or emitter-follower is connected through by the control stage.

Abstract: An arrangement for opening a circuit coupled to an electrical power source associated with a neutral circuit. The arrangement includes a bi-stable latch for electrically engaging and disengaging a pair of contacts in response to the value of the sum of currents in the circuit and the neutral circuit exceeding a predefined limit. The arrangement also includes a mechanism for opening and closing a pair of main contacts which are used to open and close the circuit. The mechanism can be operated by a shunt trip solenoid such that the solenoid causes the operating mechanism to open the contacts when the contacts of the bi-stable latch are closed.

Abstract: A circuit breaker includes a current detector for generator analog voltage signals in accordance with load current of respective phases and a control circuit for executing a tripping operation in response to the analog voltage signals. The control circuit includes an analog switch repeating a time division selecting operation that one of the analog phase voltage signals generated by the current detector is selectively allowed to pass through the same in a predetermined order, an analog-to-digital (A/D) converter for converting the analog voltage signal having passed through the analog switch to a corresponding digital signal, and a microcomputer receiving the digital voltage signal from the A/D converter and obtaining the effective load current value based on the digital voltage signals. The microcomputer generates a tripping order when the effective load current value exceeds a predetermined value.

Abstract: An air conditioning system comprises an inverter that inverts commercial power into a variable-frequency AC and supplies it to a compressor; a DC power circuit for converting the commercial power into DC current; a unit for detecting an analog value of current supplied from the commercial power; and a control unit to control inverted digital data by compensating the digital current data discrepancy due to scattering of the DC control voltage, when there is A/D conversion after an actual power supplied to the inverter has been detected via the detector.

Abstract: In a power MOS-type semiconductor device with a current detection terminal, a first number of main MOSFET elements are connected to one another in parallel, and a second number of detection MOSFET elements are similarly connected to one another. First and second terminals of the main and detection MOSFET elements are commonly connected. A third terminal of one of the detection MOSFET element is used as the current detection terminal. The current or voltage measured at this detection terminal is substantially proportional to the ratio of the first number to the second number. To this end, the parasitic resistance in the device is increased so as to inhibit a leak current from flowing therethrough. Moreover, the ON resistance of each of the detection MOSFET elements is designed to be lower than that of a main MOSFET element by an amount which is a function of the first number and the detection resistance connected to the current detection terminal.

Abstract: A current monitor includes a DC isolating amplifier circuit especially adapted for adjusting the set point of a comparator. The isolating amplifier circuit includes an operational amplifier generating a first current in response to an input voltage received by the operational amplifier. The first current is transmitted to a first optocoupler and a second optocoupler which are characterized by a first CTR and a second CTR, respectively. An output of the first optocoupler is fed back to an input of the operational amplifier while an output voltage of the second optocoupler is communicated to a set point circuit. The output voltage of the second optocoupler varies as a function of the product of the input voltage of the operational amplifier and a ratio between the first CTR and the second CTR.

Abstract: An underload protection system for an electric motor connected to first and second power supply lines, the lines being connectable to an AC power supply. The system comprises a line voltage sensing circuit connected to the first line and having a line voltage signal impressed therein, and an amplitude adjustment circuit connected to the voltage sensing circuit for adjusting the line voltage signal to produce an amplitude adjusted voltage signal. A line current sensing circuit is connected to the second line and has a line current signal impressed thereon, and a phase adjustment circuit is connected to the current sensing circuit for adjusting the phase of the current signal to produce a phase adjusted current signal.

Abstract: A power driver including a switch (17) that connects between a power input terminal (12) and a power output terminal (13). A current sense unit (22) provides an over current sense signal if an over current condition should exist with respect to the switch (17). A reference signal unit (23) utilizes a control signal as introduced at a control signal input (11) to provide a threshold current level signal. A comparator (21) compares the latter two signals and provides an enabling signal to a second switch (19) to disable the switch (17) upon detecting an over current condition. The comparator (21) is enabled by the control signal.

Abstract: An electric motor controller comprises a circuit for selecting between driving and braking an electric motor in accordance with the health or otherwise of a pair of main electric motor control switches (14 and 16). In a preferred embodiment the health of the switch is determined by the voltage (A) between the switches. The controller also comprises apparatus for detecting the sense of movement of the motor. In the event that the motor is moving in a sense opposite to that selected, the apparatus increase field current to reduce the armature current thereby increasing the braking effect of the motor. Also in the controller are apparatus for changing from armature to field current control and vice versa, in response to a given magnitude of control signal. In order to provide an unambiguous change-over, a lockout circuit is provided to inhibit change-over in a transition region. The lockout circuit maintains the mode of control to above/beneath the change-over level when changing from one level to another.

Abstract: Excess currents due to faults in an electrical power system are generally larger than excess currents due to the magnetization of distribution transformers. Discrimination has hitherto relied on a threshold detector. Magnetizing currents are, however, asymmetrical, whereas fault currents are symmetrical. A detects symmetrical current waveforms on a conductor 1 and thus discriminates between a fault and a magnetizing excess current. A resistor 4, placed across the output terminals of a bridge rectifier 3 has a resistance selected so that the voltage across it exceeds the breakdown voltage of trigger diodes 7 and 8 when the current in conductor 1, in any half cycle, exceeds a desired maximum. Capacitors 13 and 14 will thus only both charge up in response to excessive symmetrical current. A logic control 23 is connected via lines 21 and 22 to detect such a condition.