Circuit breaker-like apparatus with combination current transformer

In a circuit breaker, a current transformer for fault powering trip unit electronics and sensing low currents and high currents includes a core with solid laminations and gapped laminations to sense a wide range of currents from locked-rotor currents to high, instantaneous short-circuit currents in a single current transformer. The current transformer can also fault power trip unit electronics without requiring an additional current transformer. The operating range of the circuit breaker is significantly enhanced compared to existing breakers that can sense only a limited range of current levels.

FIELD OF THE INVENTION

This invention relates to current transformer devices used for circuit breakers, motor control units, or the like, and more particularly, to current transformers for powering and sensing current over broad current ranges.

BACKGROUND OF THE INVENTION

As is well known, a circuit breaker is an automatically operated electro-mechanical device designed to protect a load from damage caused by an overload or a short circuit. A circuit breaker may be tripped by an overload or short circuit, which causes an interruption of power to the load. A circuit breaker can be reset (either manually or automatically) to resume current flow to the load. One application of circuit breakers is to protect motors as part of a motor control center (“MCC”). A typical MCC includes a temperature triggered overload relay, a contactor and a motor circuit protector (“MCP”). The MCP is a specialized circuit breaker that provides instantaneous protection against instantaneous short-circuit events. In the United States, these motor circuit protector devices must meet National Electric Code (“NEC”) requirements when installed as part of a UL-listed MCC to provide instantaneous short-circuit protection.

Mechanical circuit breakers energize an electromagnetic device such as a solenoid to trip instantaneously in response to a rapid surge in current such as a short circuit. Most existing MCPs protect only a limited range of motors, but should avoid tripping in response to in-rush motor currents that occur during motor start-up. MCPs that sense relatively low currents may not be suitable for motors having a relatively low in-rush current because tripping will occur during normal operation of the motor. On the other hand, MCPs that sense relatively high currents may not trip on relatively low current levels such as those corresponding to locked-rotor current levels. Because of their limited operating range, some existing MCPs cannot protect for both relatively low current levels and relatively high current levels. Other existing MCPs that can protect against a wider range of fault currents are very large and their current transformers require large volumes of steel to remain in their linear range of operation.

Some circuit breakers include a current transformer, along with other electrical components, to make up the breaker system. Presently, current transformers used in existing circuit breaker devices are designed to supply power to trip unit electronics, or to sense low current ranges, or to sense high current ranges, and have a limited operating range. Thus, current transformer devices designed to sense low fault currents cannot effectively sense high fault currents. An additional current transformer specifically designed for supplying power to the trip unit electronics must be incorporated into the circuit breaker, increasing its size, complexity, and cost. Similarly, current transformer devices designed to sense high fault currents cannot effectively sense low fault currents.

What is needed is a current transformer system for use in circuit breaker devices that operates over wide current ranges.

SUMMARY OF THE INVENTION

Briefly, according to an embodiment of the present invention, a current transformer that extends the range of a circuit breaker, such as a motor circuit protector, includes both solid and gapped laminations that are staked and stacked together to form a single core. The solid laminations produce secondary current sufficient to power electronic components of the circuit breaker and sense relatively low currents. The gapped laminations produce secondary current sufficient to power the electronic components and sense relatively high currents, thereby extending the range of sensed currents for the MCP. The gapped laminations decrease the amount of remnant flux or saturation in the current transformer compared to solid cores.

The number of solid laminations and gapped laminations as well as the size of the gap in the gapped laminations are selected to fault power the MCP electronic components and sense a range of currents corresponding to locked-rotor or in-rush motor currents as well as high instantaneous short-circuit currents. As the number of solid laminations are increased, the saturation knee threshold region of the core's transfer function is pushed higher, resulting in saturation at a higher peak current. Gapped laminations are added for higher current sensing based on remnant flux requirements. As each gapped lamination is added, the core's saturation region shifts to a higher peak current value. By adjusting the ratio of solid-to-gapped laminations, a variety of operating ranges can be achieved for the MCP, operating ranges that can be significantly extended compared to existing MCPs. Moreover, the linear region of the current transformer can be extended by increasing the ratio of solid-to-gapped laminations and/or by varying the number of turns wound on the primary coil of the current transformer, resulting in more accurate approximation of the primary current. In a specific implementation, the core includes sixteen solid laminations and eight gapped laminations, resulting in a current transformer that can sense locked-rotor currents in the range of 10 A as well as high fault currents in the range of 3000 A.

The above summary of the present invention is not intended to represent each embodiment, or every aspect, of the present invention. This is the purpose of the figures and the detailed description which follow.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Turning now toFIG. 1, an electronic motor circuit protector100is shown. The motor circuit protector100includes a durable housing102including a line end104having line terminals106and a load end108having load lugs or terminals110. The line terminals106allow the motor circuit protector100to be coupled to a power source and the load terminals110allow the motor circuit protector100to be coupled to an electrical load such as a motor as part of a motor control center (“MCC”). In this example the motor circuit protector100includes a three-phase circuit breaker with three poles, although the concepts described below may be used with circuit protectors with different numbers of poles, including a single pole.

The motor circuit protector100includes a control panel112with a full load ampere (“FLA”) dial114and an instantaneous trip point (“Im”) dial116which allows the user to configure the motor circuit protector100for a particular type of motor to be protected with the rated current range of the motor circuit protector100. The full load ampere dial114allows a user to adjust the full load which may be protected by the motor circuit protector100. The instantaneous trip point dial116has settings for automatic protection (three levels in this example) and for traditional motor protection of a trip point from 8 to 13 times the selected full load amperes on the full load ampere dial114. The dials114and116are located next to an instruction graphic118giving guidance to a user on the proper settings for the dials114and116. In this example, the instruction graphic118relates to NEC recommended settings for the dials114and116for a range of standard motors. The motor circuit protector100includes a breaker handle120that is moveable between a TRIPPED position122(shown inFIG. 1), an ON position124and an OFF position126. The position of the breaker handle120indicates the status of the motor circuit protector100. For example, in order for the motor circuit protector100to allow power to flow to the load, the breaker handle120must be in the ON position124allowing power to flow through the motor circuit protector100. If the circuit breaker is tripped, the breaker handle120is moved to the TRIPPED position122by a disconnect mechanism, causing an interruption of power and disconnection of downstream equipment. In order to activate the motor circuit protector100to provide power to downstream equipment or to reset the motor circuit protector100after tripping the trip mechanism, the breaker handle120must be moved manually from the TRIPPED position120to the OFF position126and then to the ON position124.

FIG. 2is a functional block diagram of the motor circuit protector100inFIG. 1as part of a typical MCC configuration200coupled between a power source202and an electrical load such as a motor204. The MCC configuration200also includes a contactor206and an overload relay208downstream from the power source202. Other components such as a variable speed drive, start/stop switches, fuses, indicators and control equipment may reside either inside the MCC configuration200or outside the MCC configuration200between the power source202and the motor204. The motor circuit protector100protects the motor204from a short circuit condition by actuating the trip mechanism, which causes the breaker handle120to move to the TRIPPED position when instantaneous short-circuit conditions are detected. The power source202in this example is connected to the three line terminals106, which are respectively coupled to the primary windings of three current transformers210,212and214. Each of the current transformers210,212and214has a phase line input and a phase load output on the primary winding. The current transformers210,212and214correspond to phases A, B and C from the power source202. The current transformers210,212and214in this example are iron-core transformers and function to sense a wide range of currents. The motor circuit protector100provides instantaneous short-circuit protection for the motor204.

The motor circuit protector100includes a power supply circuit216, a trip circuit218, an over-voltage trip circuit220, a temperature sensor circuit222, a user adjustments circuit224, and a microcontroller226. In this example, the microcontroller226is a PIC16F684-E/ST programmable microcontroller, available from Microchip Technology, Inc. based in Chandler, Ariz., although any suitable programmable controller, microprocessor, processor, etc. may be used. The microcontroller226includes current measurement circuitry241that includes a comparator and an analog-to-digital converter. The trip circuit218sends a trip signal to an electro-mechanical trip solenoid228, which actuates a trip mechanism, causing the breaker handle120inFIG. 1to move from the ON position124to the TRIPPED position122, thereby interrupting power flow to the motor204. In this example, the electro-mechanical trip solenoid228is a magnetic latching solenoid that is actuated by either stored energy from a discharging capacitor in the power supply circuit216or directly from secondary current from the current transformers210,212and214.

The signals from the three current transformers210,212and214are rectified by a conventional three-phase rectifier circuit (not shown inFIG. 2), which produces a peak secondary current with a nominally sinusoidal input. The peak secondary current either fault powers the circuits216,218,220,222, and224and the microcontroller226, or is monitored to sense peak fault currents. The default operational mode for current sensing is interlocked with fault powering as will be explained below. A control algorithm230is responsible for, inter alia, charging or measuring the data via analog signals representing the stored energy voltage and peak current presented to configurable inputs on the microcontroller226. The control algorithm230is stored in a memory that can be located in the microcontroller226or in a separate memory device272, such as a flash memory. The control algorithm230includes machine instructions that are executed by the microcontroller226. All software executed by the microcontroller226including the control algorithm230complies with the software safety standard set forth in UL-489 SE and can also be written to comply with IEC-61508. The software requirements comply with UL-1998. As will be explained below, the configurable inputs may be configured as analog-to-digital (“A/D”) converter inputs for more accurate comparisons or as an input to an internal comparator in the current measurement circuitry241for faster comparisons. In this example, the A/D converter in the current measurement circuitry241has a resolution of 8/10 bits, but more accurate A/D converters may be used and may be separate and coupled to the microcontroller226. The output of the temperature sensor circuit222may be presented to the A/D converter inputs of the microcontroller226.

The configurable inputs of the microcontroller226include a power supply capacitor input232, a reference voltage input234, a reset input236, a secondary current input238, and a scaled secondary current input240, all of which are coupled to the power supply circuit216. The microcontroller226also includes a temperature input242coupled to the temperature sensor circuit222, and a full load ampere input244and an instantaneous trip point input246coupled to the user adjustments circuit224. The user adjustments circuit224receives inputs for a full load ampere setting from the full load ampere dial114and either a manual or automatic setting for the instantaneous trip point from the instantaneous trip point dial116.

The microcontroller226also has a trip output250that is coupled to the trip circuit218. The trip output250outputs a trip signal to cause the trip circuit218to actuate the trip solenoid228to trip the breaker handle120based on the conditions determined by the control algorithm230. The microcontroller226also has a burden resistor control output252that is coupled to the power supply circuit216to activate current flow across a burden resistor (not shown inFIG. 2) and maintain regulated voltage from the power supply circuit216during normal operation.

The breaker handle120controls manual disconnect operations allowing a user to manually move the breaker handle120to the OFF position126(seeFIG. 1). The trip circuit218can cause a trip to occur based on sensed short circuit conditions from either the microcontroller226, the over-voltage trip circuit220or by installed accessory trip devices, if any. As explained above, the microcontroller226makes adjustment of short-circuit pickup levels and trip-curve characteristics according to user settings for motors with different current ratings. The current path from the secondary output of the current transformers210,212,214to the trip solenoid228has a self protection mechanism against high instantaneous fault currents, which actuates the breaker handle120at high current levels according to the control algorithm230.

The over-voltage trip circuit220is coupled to the trip circuit218to detect an over-voltage condition from the power supply circuit216to cause the trip circuit218to trip the breaker handle120independently of a signal from the trip output250of the microcontroller226. The temperature sensor circuit222is mounted on a circuit board proximate to a copper burden resistor (not shown inFIG. 2) together with other electronic components of the motor circuit protector100. The temperature sensor circuit222and the burden resistor are located proximate each other to allow temperature coupling between the copper traces of the burden resistor and the temperature sensor. The temperature sensor circuit222is thermally coupled to the power supply circuit216to monitor the temperature of the burden resistor. The internal breaker temperature is influenced by factors such as the load current and the ambient temperatures of the motor circuit protector100. The temperature sensor222provides temperature data to the microcontroller226to cause the trip circuit218to actuate the trip solenoid228if excessive heat is detected. The output of the temperature sensor circuit222is coupled to the microcontroller226, which automatically compensates for operation temperature variances by automatically adjusting trip curves upwards or downwards.

The microcontroller226first operates the power supply circuit216in a startup mode when a reset input signal is received on the reset input236. A charge mode provides voltage to be stored for actuating the trip solenoid228. After a sufficient charge has been stored by the power supply circuit216, the microcontroller226shifts to a normal operation mode and monitors the power supply circuit216to insure that sufficient energy exists to power the electro-mechanical trip solenoid228to actuate the breaker handle120. During each of these modes, the microcontroller226and other components monitor for trip conditions.

The control algorithm230running on the microcontroller226includes a number of modules or subroutines, namely, a voltage regulation module260, an instantaneous trip module262, a self protection trip module264, an over temperature trip module266and a trip curves module268. The modules260,262,264,266and268generally control the microcontroller226and other electronics of the motor circuit protector100to perform functions such as governing the startup power, establishing and monitoring the trip conditions for the motor circuit protector100, and self protecting the motor circuit protector100. A storage device270, which in this example is an electrically erasable programmable read only memory (EEPROM), is coupled to the microcontroller226and stores data accessed by the control algorithm230such as trip curve data and calibration data as well as the control algorithm230itself. Alternately, instead of being coupled to the microcontroller226, the EEPROM may be internal to the microcontroller226.

FIG. 3is a functional block diagram300of the interrelation between the hardware components shown inFIG. 2and software/firmware modules260,262,264,266and268of the control algorithm230run by the microcontroller226. The secondary current signals from the current transformers210,212and214are coupled to a three-phase rectifier302in the power supply circuit216. The secondary current from the three-phase rectifier302charges a stored energy circuit304that supplies sufficient power to activate the trip solenoid228when the trip circuit218is activated. The voltage regulation module260ensures that the stored energy circuit304maintains sufficient power to activate the trip solenoid228in normal operation of the motor circuit protector100.

The trip circuit218may be activated in a number of different ways. As explained above, the over-voltage trip circuit220may activate the trip circuit218independently of a signal from the trip output250of the microcontroller226. The microcontroller226may also activate the trip circuit218via a signal from the trip output250, which may be initiated by the instantaneous trip module262, the self protection trip module264, or the over temperature trip module266. For example, the instantaneous trip module262of the control algorithm230sends a signal from the trip output250to cause the trip circuit218to activate the trip solenoid228when one of several regions of a trip curve are exceeded. For example, a first trip region A is set just above a current level corresponding to a motor locked rotor. A second trip region B is set just above a current level corresponding to an in-rush current of a motor. The temperature sensor circuit222outputs a signal indicative of the temperature, which is affected by load current and ambient temperature, to the over temperature trip module266. The over temperature trip module266will trigger the trip circuit218if the sensed temperature exceeds a specific threshold. For example, load current generates heat internally by flowing through the current path components, including the burden resistor, and external heat is conducted from the breaker lug connections. A high fault current may cause the over temperature trip module266to output a trip signal250(FIG. 2) because the heat conducted by the fault current will cause the temperature sensor circuit222to output a high temperature. The over temperature trip module266protects the printed wire assembly from excessive temperature buildup that can damage the printed wire assembly and its components. Alternately, a loose lug connection may also cause the over temperature trip module266to output a trip signal250if sufficient ambient heat is sensed by the temperature sensor circuit222.

The trip signal250is sent to the trip circuit218to actuate the solenoid228by the microcontroller226. The trip circuit218may actuate the solenoid228via a signal from the over-voltage trip circuit220. The requirements for “Voltage Regulation,” ensure a minimum power supply voltage for “Stored Energy Tripping.” The trip circuit218is operated by the microcontroller226either by a “Direct Drive” implementation during high instantaneous short circuits or by the control algorithm230first ensuring that a sufficient power supply voltage is present for the “Stored Energy Trip.” In the case where the “Stored Energy” power supply voltage has been developed, sending a trip signal250to the trip circuit218will ensure trip activation. During startup, the power supply216may not reach full trip voltage, so a “Direct Drive” trip operation is required to activate the trip solenoid228. The control for Direct Drive tripping requires a software comparator output sense mode of operation. When the comparator trip threshold has been detected, the power supply charging current is applied to directly trip the trip solenoid228, rather than waiting for full power supply voltage.

The over-voltage trip circuit220can act as a backup trip when the system200is in “Charge Mode.” The control algorithm230must ensure “Voltage Regulation,” so that the over-voltage trip circuit220is not inadvertently activated. The default configuration state of the microcontroller226is to charge the power supply216. In microcontroller control fault scenarios where the power supply voltage exceeds the over voltage trip threshold, the trip circuit218will be activated. Backup Trip Levels and trip times are set by the hardware design.

The user adjustments circuit224accepts inputs from the user adjustment dials114and116to adjust the motor circuit protector100for different rated motors and instantaneous trip levels. The dial settings are converted by a potentiometer to distinct voltages, which are read by the trip curves module268along with temperature data from the temperature sensor circuit222. The trip curves module268adjusts the trip curves that determine the thresholds to trigger the trip circuit218. A burden circuit306in the power supply circuit216allows measurement of the secondary current signal, which is read by the instantaneous trip module262from the peak secondary current analog-to-digital input238(shown inFIG. 2) along with the trip curve data from the trip curves module268. The self-protection trip module264also receives a scaled current (scaled by a scale factor of the internal comparator in the current measurement circuitry241) from the burden resistor in the burden circuit306to determine whether the trip circuit218should be tripped for self protection of the motor circuit protector100. In this example, fault conditions falling within this region of the trip curve are referred to herein as falling within region C of the trip curve.

As shown inFIGS. 2 and 3, a trip module265is coupled between the trip circuit218and the voltage regulation module260. Trip signals from the instantaneous trip module262, the self protection trip module264, and the over temperature trip module266are received by the trip module265.

The following terms may be used herein:

DIRECT DRIVE—Initiating a trip sequence prior to achieving a stored energy trip voltage.

STORED ENERGY TRIP—Sending a trip sequence with knowledge of the stored energy trip voltage on the power supply voltage, VCAP,304.

REDUNDANT TRIP OUTPUT—Send both “trip output” to the trip circuit218and “FET off” output to the power supply circuit216if the digital trip output was not successful. This will eventually cause the over-voltage circuit220to activate the trip solenoid228.

OVER-VOLTAGE TRIP BACKUP—A trip sequence that uses the over-voltage trip circuit220to trip the breaker. This sequence is a backup for the normal “trip circuit” method. This sequence can be activated later in time due to a higher VCAP304activation voltage.

FIG. 4is a detailed circuit diagram of various circuits of the motor circuit protector100, including the power supply circuit216and other related components including the stored energy circuit304, the burden circuit306, a scaled current comparator current input404, an energy storage capacitor voltage input circuit406, and a voltage regulator circuit408. The power supply circuit216derives the secondary current from the secondary windings of the three current transformers210,212and214, which are rectified by the three-phase rectifier302. The output of the three-phrase rectifier302is coupled to the burden circuit306, which is coupled in parallel to the stored energy circuit304. The power supply circuit216also includes a peak current input circuit402that is provided to the microcontroller226, a scaled current comparator input circuit404that is provided to the comparator of the current measurement circuitry241of the microcontroller226via the scaled secondary current input240, a stored energy capacitor voltage input circuit406and a voltage regulator circuit408. The stored energy capacitor input232of the microcontroller226is coupled to the stored energy capacitor input circuit406, the reference voltage input234is coupled to the voltage regulator circuit408, the secondary current input238is coupled to the peak current input circuit402, and the scaled secondary current input240is coupled to the scaled current comparator input circuit404.

The burden circuit306includes a burden resistor410connected in series with a burden resistor control field effect transistor (FET)412. The gate of the burden resistor control FET412is coupled to the burden resistor control output252of the microcontroller226. Turning on the burden resistor control FET412creates a voltage drop across the burden resistor410and the burden resistor control FET412allowing measurement of the secondary current for fault detection purposes. The voltage drop may also provide an indication of current available to charge the stored energy circuit304.

The secondary current from the rectifier302is measured by the peak current input circuit402and the scaled current comparator input circuit404. The stored energy circuit304includes two energy storage capacitors420and422. The energy storage capacitors420and422are charged by the secondary current when the burden resistor control FET412is switched off and are discharged by the trip circuit218to actuate the trip solenoid228inFIG. 2.

The scaled current comparator input circuit404has an input that is coupled to the rectifier302. The scaled current comparator input circuit404includes a voltage divider to scale down the signal from the rectifier302and is coupled to the scaled secondary current input240of the microcontroller226. The voltage regulator circuit408provides a component power supply (in this example, 5 volts nominal) to the electronic components such as the microcontroller226in the motor circuit protector100. The microcontroller226includes an internal comparator in the current measurement circuitry241that may be switched to compare the input232or the input240with a reference voltage that is received from the voltage regulator circuit408to the reference voltage input234. The reference voltage is also a reference voltage level when the inputs232and240are configured to be coupled to analog-to-digital converters. When the internal comparator is switched to receive the input240to the self protection trip module264, the peak current is scaled for the comparator input by external hardware such as the scaled current comparator input circuit404. An internal comparator reference is set by the microcontroller226to control the comparator trip thresholds.

The stored energy capacitor voltage input circuit406includes the parallel-connected capacitors420and422and measures the voltage level of the stored energy circuit304, which is indicative of the stored energy in the capacitors420and422. The stored energy capacitor voltage input circuit406provides a signal indicative of the voltage on the capacitors420and422to the stored energy capacitor input232of the microcontroller226to monitor the voltage of the stored energy circuit304.

Upon startup of the motor circuit protector100(such as when the user throws the breaker handle120to the ON position), the voltage regulator circuit408and the microcontroller226receive a reset signal from the power supply circuit216and the rectifier302begins to charge the capacitors420and422. A start-up delay time including a hardware time delay and a fixed software time delay elapses. The hardware time delay is dependent on the time it takes the secondary current to charge the stored energy circuit304to a voltage sufficient to operate the voltage regulator circuit408. In this example, the voltage regulator circuit408needs a minimum of 5 volts (nominal) to operate. The fixed software time delay is the time required for stabilization of the regulated component voltage from the voltage regulator circuit408to drive the electronic components of the motor circuit protector100. The software delay time is regulated by an internal timer on the microcontroller226. The overall start-up delay time typically covers the first half-cycle of the current.

After the start-up delay time, the microcontroller226executes the control algorithm230, which is optionally stored in the internal memory of the microcontroller226, and enters a “Self Protection” measurement mode, which relies upon the internal comparator of the microcontroller226for rapid detection of fault currents. The microcontroller226turns on the burden resistor control FET412allowing measurement of the secondary current. The burden resistor control FET412is turned on for a fixed period of time regulated by the internal timer on the microcontroller226. The voltage regulation module260configures the microcontroller226to couple the scaled secondary current input240to an input to the internal comparator of the microcontroller226. The scaled secondary current input240reads the signal from the scaled peak current input circuit404, which measures the secondary current from the rectifier302and requires minimal initializing overhead. The peak current from the secondary current is predicted via the secondary current detected by the scaled current comparator input circuit404.

The internal comparator in the microcontroller226is a relatively fast device (compared to, for example, an A/D converter, which may be more accurate but operates more slowly) and thus can detect fault currents quickly while in this mode. If the peak current exceeds a threshold level, indicating a fault current, the burden resistor control FET412is turned off by a signal from the burden resistor control output252of the microcontroller226. The threshold level is set depending on the desired self-protection model of the range of currents protected by the particular type of motor circuit protector100. The disconnection of the FET412causes the fault current to rapidly charge the capacitors420and422of the stored energy circuit304and actuate the trip solenoid228to trip the trip mechanism of the motor circuit protector100, which is visually indicated by the breaker handle120.

After the initial measurement is taken, the control algorithm230enters into a charge only mode of operation in order to charge the capacitors420and422of the stored energy circuit304. The control algorithm230sends a signal to turn off the burden resistor control FET412, causing the capacitors420and422to be charged. The control algorithm230remains in the charge only mode until sufficient energy is stored in the stored energy circuit304to actuate the trip solenoid228in the event of a detected fault condition. In the charge only mode, the voltage regulation module260configures the microcontroller226to take a voltage input from the peak current input circuit402to the secondary current input238which is configured for an analog to digital converter. The signal from the secondary current input238analog to digital conversion is more accurate then the internal comparator but relatively slower. During the charge only mode, if a fault current occurs, the stored energy circuit304is charged quickly and the fault current actuates the trip solenoid228therefore providing self protection.

It should be noted that the control algorithm230can be programmed to multiplex current measurement for self-protection sensing and power-supply charging for minimum stored-energy tripping.

The voltage regulation module260also configures the internal comparator in the current measurement circuitry241to be connected to the stored energy capacitor voltage input circuit406via the capacitor voltage input232to detect voltage levels from the stored energy circuit304. The voltage regulation module260thus maintains real time monitoring over the regulated voltage output from the stored energy circuit304while performing other software tasks such as monitoring fault currents.

During the charge only mode, the control algorithm230charges the stored energy circuit304from the minimum voltage regulation level (5 volts in this example from the hardware startup period) to a voltage level (15 volts in this example) indicative of sufficient energy to actuate the trip solenoid228. The charging of the capacitors420and422is regulated by the voltage regulation module260, which keeps the burden resistor control FET412off via the burden resistor control output252causing the capacitors420and422to charge. The voltage regulation module260holds the stored energy circuit304in the charge mode until a start voltage threshold level (15 volts in this example) is reached for the supply voltage from the stored energy circuit304and is thus sensed through the stored energy capacitor voltage input circuit406. The timing of when the start voltage threshold level is reached depends on the secondary current from the rectifier302to the stored energy circuit304. The ability of the voltage regulation module260to hold the charge mode allows designers to avoid external stability hardware components. This process reduces peak overshoot during high instantaneous startup scenarios while charging the capacitors420and422to the start voltage threshold level more efficiently.

Once the minimum energy for actuating the trip solenoid228is stored, the control algorithm230proceeds to a steady state or run mode. In the run mode, the control algorithm230maintains control of the voltage from the stored energy circuit304with the voltage regulation module260after the sufficient energy has been stored for tripping purposes. The voltage regulation module260maintains a voltage above the stored energy trip voltage by monitoring the voltage from the stored energy circuit304from the stored energy capacitor voltage input circuit406to the stored energy capacitor input232. The stored energy capacitor input232is internally configured as an A/D converter input for more accurate voltage level sensing for the run mode.

The voltage regulation module260also regulates the stored energy circuit304and avoids unintended activation of the over-voltage trip circuit220. The power supply regulation task is serviced in the run mode on a periodic basis to maintain the necessary energy in the stored energy circuit304. The regulation task may be pre-empted to service higher priority tasks such as the trip modules262and264. In the run mode, the voltage regulation module260monitors the voltage from the stored energy circuit304. The voltage regulation module260maintains the voltage output from the stored energy circuit304above the backup trip set points, which include a high set point voltage and a low set point voltage. If the energy falls below a high set point voltage threshold (14.7 volts in this example), the voltage regulation module260initiates fixed width charge pulses, by sending control signals via the burden resistor control output252to the burden resistor control FET412to turn on and off until a high voltage set point for the power supply voltage is reached. The width of the pulse corresponds with the maximum allowable voltage ripple at the maximum charge rate of the stored energy circuit304. The number of fixed width charge pulses is dependent on the voltage level from the stored energy circuit304. If the energy is above the high set point voltage, the voltage regulation module260will not initiate fixed width charge pulse in order to avoid unintended activation of the over-voltage trip circuit220.

If the voltage signals detected from the stored energy capacitor voltage input circuit406are such that the microcontroller226cannot maintain regulation voltage on the stored energy circuit304, a threshold voltage low set point (13.5 volts in this example) for the stored energy circuit304is reached and the control algorithm230will charge the stored energy circuit304to reach a minimum voltage necessary for trip activation of the trip solenoid228. The microcontroller226will restart the charge mode to recharge the capacitors420and422in the stored energy circuit304. During the charging process, fault current measurement is disabled, however if a fault current of significant magnitude occurs, the fault current will rapidly charge the capacitors420and422of the measured stored energy circuit304and thus overall trip performance is not affected. The application will also restart when the watchdog timer in the microcontroller226resets.

In the run mode, the microcontroller226is in measurement mode by keeping the burden resistor control FET412on. The microcontroller226monitors the secondary current via the secondary current input238, which is configured as an analog-to-digital converter for more accurate measurements. The instantaneous trip module262sends an interrupt signal from the trip output250of the microcontroller226to cause the trip circuit218to activate the trip solenoid228for conditions such as a motor in-rush current or a locked motor rotor (trip conditions A and B), which cause a trip curve to be exceeded based on the secondary current. The internal comparator of the microcontroller226is configured to accept an input from the scaled secondary current input240, which is read by the self protection trip module264to determine whether the trip circuit218should be tripped for self protection of the motor circuit protector100in the case of high instantaneous current (trip condition C) detected from the faster measurement of the comparator. As explained above, the trip conditions for self protection are a function of the user settings from the dials114and116.

In case of a failure of the microcontroller226to send the appropriate trip signal250, the solenoid228is triggered by the over voltage trip circuit220(shown schematically inFIG. 4). The over voltage trip circuit220includes a voltage divider430, which steps down the voltage level. In this example, pull up transistors cause the over voltage trip circuit220to send a discrete trip signal280to the trip circuit218, causing the trip circuit218to actuate the trip solenoid228to trip the breaker handle120.

FIG. 5is an isometric view of a current transformer (“CT”)500according to certain embodiments of the present disclosure, and is suitable for use as the current transformers210,212, or214shown inFIG. 2. The current transformer500is enclosed within a housing510. Though the housing can be configured in any of a number of ways, the housing510can, for example, comprise two housing elements512,514formed to fit in manner that encloses the various current transformer components. In certain embodiments, the housing can be configured to partially enclose the current transformer components. A first lead pin522and a second lead pin524extend from the enclosed current transformer components through the housing510. In certain embodiments, the lead pins522,524from current transformer500are connected to power supply circuit216as illustrated, for example, inFIGS. 2-4. The housing can be constructed with nonconductive materials such as, for example, plastics or ceramics. The current transformer500is mounted to a printed wire board (PWB) (not shown).

FIG. 6is an exploded isometric view of the current transformer500inFIG. 5according to certain embodiments of the present disclosure. The current transformer500includes two housing elements512,514that enclose a number of current transformer components. Housing element514is configured with a joint edge516that overlaps joint edge518of the housing element512. The overlapping joint arrangement between housing elements512,514allows for dielectric integrity between current transformer500and uninsulated current path sections of the current transformer500in contact with current transformer housing510. Housing510can be configured to substantially enclose or to partially enclose the current transformer components630,632,642,644,650. In the implementation illustrated inFIG. 6, the components630,632,642,644,650of the current transformer are substantially enclosed upon the joining of housing elements512,514with the exception of a tunnel620through which first lead pin522and second lead pin524extend from the interior to the exterior of housing510.

Current transformer500includes a core630that includes gapped laminations632and solid laminations734(an exemplary solid lamination734is shown inFIG. 7) combined together to form a single core element. The solid laminations734in the current transformer core630provide secondary current output sufficient to power the electronic components of the system200and further sense relatively low currents (for example, current in the range of 10 amperes). The gapped laminations632in current transformer core630decrease the amount of remnant flux or saturation in the current transformer500, while providing secondary current sufficient to power the electronic components of the system200and sensing relatively high currents (for example, currents in the range of 3,000 amperes).

In other words, the combination of the solid and gapped laminations increases the range of primary currents that can be sensed by the current transformer500while also providing a sufficient amount of secondary current available for powering the electronic components of the system200, including in particular the trip solenoid228and the power supply circuit216. According to aspects of the present invention, it is not necessary to implement a transformer separate from the sensing transformer(s) for powering the power supply and other electronic components of the system200. Both power supply and current sensing are accomplished in a single current transformer that also senses current over a very wide range of currents, e.g., motor locked-rotor (“LRA”) currents (on the order of 10 A for a lower threshold) to motor in-rush currents to high instantaneous short-circuit currents (as high as 3000 A for an upper threshold for in-rush motor currents). Thus, the ratio of the upper current threshold to the lower current threshold exceeds 100:1 and can be as high as 300:1.

In certain embodiments, the gapped laminations632and the solid laminations734are combined in a single stacked core630having a central opening660. Some benefits of a single stack core include that a higher lamination factor is achieved and post-annealing stresses are minimized in the current transformer core630. Another benefit simplifies the manufacturing process, e.g., the gapped laminations632and the solid laminations734can be punched from the same die. A retractable insert can be used to punch out the gap636in the gapped lamination632. Because both laminations are made from the same die, the consistency between individual laminations is increased. The current transformer500can be assembled efficiently with the single stacked core630according to aspects of the present invention.

The stacked gapped laminations and solid laminations are staked together to form the single stacked core630. Bobbin halves642,644circumscribe the core630when the two halves642,644are joined together. In an implementation, the two bobbin halves642,644are held together by a layer of tape650after the two bobbin halves are joined. Bobbin halves642,644function as an insulator while holding the secondary windings in place. The number of gapped laminations632and solid laminations734in a current transformer core630can be adjusted depending upon the range of current values that need to be sensed by the motor circuit protector100.

As a result of the increased current range sensing of the current transformer according to the present invention, lower motor locked-rotor current values are detected along with higher motor in-rush current values as well as high instantaneous short-circuit current values. For example, in certain embodiments, the ratio of gapped-to-solid laminations of the current transformer500can be adjusted to sense currents ranging from 9 amperes to 3,000 amperes or any ranges in between. The particular range may depend upon the particular locked-rotor or in-rush current specifications provided by the motor manufacturer.

FIG. 8illustrates an isometric view of a current transformer core800according to certain embodiments of the present disclosure with a gapped-to-solid laminations ratio of approximately 1:3. In other implementations, the gapped-to-solid laminations ratio is 1:2. A current transformer, such as the one illustrated inFIG. 8includes solid laminations810for relatively low current sensing, for example, in the range of 9 or 10 amperes, for current under locked-rotor conditions. The gapped laminations812can then be utilized to decrease the amount of remnant flux so that the electronic components of the system200will be able to receive power from the current transformers210,212,214and while also accurately sensing high instantaneous fault currents.

According to another implementation of the present invention, the core800can be constructed with twenty-four laminations comprising eight solid laminations810and sixteen gapped laminations812. Alternately, the core800can be constructed with sixteen solid laminations and eight gapped laminations. The twenty-four laminations are stacked and staked together as shown for core800so that the lamination edges820are substantially aligned with each other. The core800is assembled with the gapped laminations812having a cumulative thickness ranging from 0.13 inches to 0.145 inches. The solid laminations810are stacked to the gapped laminations812to achieve a total core thickness ranging from 0.39 inches to 0.44 inches. The individual gapped laminations812and solid laminations810are approximately 0.016-0.019 inches thick at lamination edge820. In certain embodiments, no lamination materials extend beyond the surface (the lamination edge820) of the outermost and innermost laminations due to the staking process. The nominal solid lamination area is approximately 0.0607 in2, and the nominal gapped lamination area is approximately 0.0304 in2. The size of the gap in the gapped laminations is approximately 0.085 in.

FIG. 9illustrates an isometric view of a current transformer core900according to certain embodiments of the present disclosure. The core900includes seven-eighths gapped laminations and one-eighth solid laminations, representing a gapped-to-solid laminations ratio of 7:1. In an example, the core900includes twenty-four laminations as illustrated with three solid laminations910and twenty-one gapped laminations912. The twenty-four laminations are stacked and staked together as shown for the core900so that the lamination edges920are substantially aligned with each other. The core900has a total core thickness ranging from 0.39 inches to 0.44 inches. The individual solid laminations910and gapped laminations912are each approximately 0.016-0.019 inches thick at the lamination edge920. In certain embodiments, no lamination materials extend beyond the surface (formed by the lamination edge920) of the outermost and innermost laminations due to the staking process. The nominal solid lamination area is approximately 0.0114 in2, and the nominal gapped lamination area is approximately 0.0797 in2. The size of the gap in the gapped laminations is approximately 0.085 in.

The ratio of gapped laminations632,812,912to solid laminations734,810,910in the single stacked current transformer core630,800,900can be determined by balancing output level and remnant flux parameters. The power-up output levels are adjusted by the number of solid laminations, and as the number of solid laminations increases, the linear portion of the current transformer's operating range is extended, pushing the knee threshold of the core's transfer function higher (i.e., the core's saturation region begins at higher peak currents). Then, gapped laminations are added for higher fault current detection based on the remnant flux requirements. As each gapped lamination is added, the core's saturation region shifts to a higher peak current value.

In some embodiments, the gapped laminations812,912and the solid laminations810,910inFIGS. 8 and 9have similar dimensions. In an embodiment, the gap830,930in gapped laminations812,912is approximately 0.085 inches. The gap can comprise air. As previously stated, the thickness of the individual laminations can be around 0.016-0.019 inches each. The width of the individual laminations from the side inner edge835,935to the side outer edge845,945and the top inner edge850,950to the top outer edge855,955can be around 0.21-0.22 inches. The height of the lamination from the top outer edge855,955to the bottom outer edge865,965can be around 1.13-1.15 inches. The height of the space defined by the interior space of the laminations from top inner edge855,955to the bottom inner edge860,960can be around 0.70-0.72 inches. The width of the laminations can vary and the laminations can taper slightly from the upper portion to the lower portion of the laminations. For example, the lamination width from the left upper outer edge870,970to the right upper outer edge875,975can be around 0.90-0.94 inches. The lamination width from the left lower outer edge880,980to the right lower outer edge885,985can be around 0.86-0.90 inches. The lamination width for the interior space of the laminations can also vary and taper slightly from the upper portion to the lower portion. The lamination width from the left upper inner edge872,972to the right upper inner edge877,977can be around 0.47-0.51 inches. The lamination width from the left lower inner edge882,982to the right lower inner edge887,987can be around 0.44-0.48 inches.

In the embodiments illustrated, for example, inFIGS. 9 and 10, the gapped laminations812,912are generally shown as C-shaped or reverse C-shaped. Other embodiments of the present invention contemplate gapped laminations that are L-shaped or U-shaped, or variations thereof, where the gapped laminations are staked with some solid laminations at the front or back of the core. In certain other embodiments, the gapped laminations are partially gapped or notched instead of having a full gap.

Gapped laminations812,912and solid laminations810,910can be made of an iron alloy that, for example, comprises silicon, aluminum and iron, such as 26 gauge non-oriented Si-Al-Fe semi-processed cold rolled steel (ASTM 47S175). The laminations can further be heat treated for approximately one hour at a temperature of approximately 1,550° F. in a hydrogen/nitrogen atmosphere as set forth in the American Society of Testing Material (ASTM) Standard 683. In other embodiments, alternate metallic materials can be used including, but not limited to, steel, transformer iron, or nickel.

The laminations can be coated with a C4—AS antistick coating available from AK Steel Corp., or an equivalent coating. The coating is applied to the surface of the individual laminations in the current transformer's core prior to the punching and stacking operations. The coating provides an insulating barrier between the laminations that can withstand elevated temperatures during the annealing process. A primary function of the coating is to provide surface insulation between the layers of the stacked core, which prevents eddy currents from flowing from one lamination to the next. Eddy currents are undesirable, because they cause the resistive steel laminations to heat up. This heating reduces the current transformer's efficiency and requires a more expensive construction to withstand the additional heat rise. Application of a coating can also inhibit rusting to a certain extent.

FIG. 10is a perspective view of a current transformer1000without a housing according to certain embodiments of the present disclosure. The current transformer1000includes a core1030comprising both solid laminations1032and gapped laminations1035combined in a single stacked core. A bobbin1040can be secured around the stacked laminations. In certain embodiments, a first lead pin1022and a second lead pin1024can be secured to the bobbin1040such that the pins1022,1024extend vertically from the bobbin1040.

The bobbin1040is placed around the core1030, and a magnet wire1050is wrapped around the bobbin1040. In certain embodiments, the wire1050is first wrapped approximately six turns around first lead pin1022, then wound around the bobbin1040, and then finished with approximately six turns around the second lead pin1024. In certain embodiments, the wire1050can be wrapped around the bobbin1040for approximately 420 turns to achieve an approximate resistance of 12Ω. The magnet wire1050can be #32 AWG with heavy build polyurethane and a temperature requirement of 155° C.

In the exemplary embodiments illustrated inFIGS. 1-4, a circuit breaker, such as motor circuit protector100, has three current transformers210,212,214, each of which may correspond to any of the current transformers shown and described in connection withFIGS. 5-10. The current transformers can have iron cores and function to send current and to fault power the trip unit electronics. Each current transformer210,212,214senses different phase currents (traditionally labeled A, B, and C, each 120 degrees apart from one another) of the motor circuit protector100. The number of secondary turns of wire1050about the bobbin varies and in certain embodiments ranges from 400 to 420 turns.