Integrated DC link inductor and common mode current sensor winding

An integrated DC link inductor and current sensor winding having a core that includes at least two primary legs and at least one secondary leg, two direct current (DC) link windings each wound around one of the two primary legs, and a common mode current sensor winding wound around the secondary leg. Resistors coupled to the common mode current sensor winding may damp the common mode current oscillations.

BACKGROUND

The invention relates generally to filtering and sensing within a power converter. Particularly, this invention relates to a DC link inductor that is integrated with a sensor winding.

A DC link or bus in a power converter conducts DC power that the power converter converts into AC power at a desired frequency, which may be applied to drive an electric motor, among other things. A DC link inductor often includes a variety of windings or coils for performing various sensing and control functions. For example, a DC link may include a pair of inductors on each primary current path, and one or more windings to detect ground faults. The inductors on the primary current paths filter and smooth the primary DC currents, and the windings for detecting ground faults typically sense the relative magnitude of currents on the primary current paths. To identify a ground fault, the currents from each path are compared, and a determination is made whether the same amount of current is flowing into a load connected to the DC link as is flowing out of the load. If substantially more current flows into a load than out of a load, a ground fault may have occurred.

Unfortunately, the large number of discrete windings in traditional DC links can have undesirable effects. For example, the inductors often include large, heavy cores that can increase the size and weight of a DC link. Having several discrete inductors in a DC link may amplify the impact of these devices on the total size and weight of the DC link, thereby potentially increasing material, shipping, and storage costs.

There is a current need in the art, therefore, for an improved DC link.

BRIEF DESCRIPTION

Certain embodiments of the present technique provide for a DC link that addresses some of the abovementioned needs in the art. Some of the subsequently discussed embodiments are applicable to a wide variety of applications, such as an AC motor drive system or a power generation system. As explained below, certain embodiments may include an integrated DC link inductor and current sensor coil that share a common core. Such embodiments may tend to reduce the weight and size of an AC motor drive and, in some instances, reduce electromagnetic emissions arising from common mode currents relative to conventional systems.

DETAILED DESCRIPTION

As discussed above, in electric motor drives, a motor is connected to a drive, which provides electrical power to the motor in a controlled manner. The motor drive systems are commonly employed to provide speed control or torque control. For AC motors, electrical power is converted in the drive from AC power supply, typically from utility, into DC voltage (and current). The DC power is then converted, using an inverter, into AC power in which the frequency and amplitude can be controlled, thereby controlling the motor speed and torque. Often, control is achieved by using pulse-width modulation (PWM) techniques. In many AC motor drive systems, there are three main subsystems: a rectifier (e.g., an AC to DC converter), a DC link filter, and an inverter (e.g., DC to AC converter). The AC motor drives are mainly in two general forms; current source inverter (CSI), and voltage source inverter (VSI). The main difference between the power circuits of these two types of drives is in their DC link filters. The inductance value of the dc link choke in the CSI drives is relatively large in comparison with the VSI drives. While the DC link inductor regulates the DC link current in the CSI drives, the DC link inductor in the VSI drives (in conjunction with the DC link capacitors) filter the DC bus voltage.

In VSI-based drives, the DC link inductor is typically divided into two individual windings located in the positive and negative links that connect the drive's rectifier to its inverter. The main reason is to so that the common mode current (or the drive leakage current) flows through an inductive element that limits the current step changes. A DC link or bus for a motor drive provides DC power that the drive converts into AC power at a desired frequency that is applied to drive an electric motor. A DC link for an AC motor drive may include a variety of windings or coils for performing various sensing and control functions. For example, a DC link may include a pair of inductors on each primary current path, and one or more windings to detect ground faults. The inductors on the primary current paths filter and smooth the primary DC currents, and the windings for detecting ground faults typically sense the relative magnitude of currents on the primary current paths. If the positive and negative paths of the DC link do not carry the same amount of current at any instance, the difference in these currents leaks into the ground. If substantially more current flows into a load (through the positive path) than out of a load (through the negative path), a ground fault may have occurred. To identify a ground fault, the currents from each path are compared, and a determination is made whether the same amount of current is flowing into a load (through the positive path) connected to the DC link as is flowing out of the load (through the positive path). In other words, if the common mode current exceeds a predetermined value, a fault alarm is activated. The ground fault detection can be performed by adding a third winding to the DC link, positive and negative current paths, in a manner that the third winding senses the common mode currents.

In pulse-width modulation (PWM) based drives; a common mode voltage with relatively large step changes is produced by the drive's PWM inverter. This fast switching voltage may result in a non-negligible amount of high frequency leakage (common mode) current that flows through stray (parasitic) capacitors between motor stator windings and motor frame and also between the power lines of the motor feeder cable and its ground wire and/or cable shield. The peak value of the leakage current may even reach the rated current in the worst case. It may have an undesirable influence on the motor current control. Moreover, the leakage current may cause electromagnetic interference (EMI) to electronic equipments which are close to the motor drive system.

A common mode inductor (choke) may be used to reduce the undesirable leakage current, which is connected in series between the output terminals of the drive and those of the motor. The connection of conventional common mode choke is not effective to reduce the rms (root-mean-square) value of the leakage current, but effectively reduces the peak current values. However, the third winding in this invention (integrated DC link inductor) is loaded by a resistor to damp the oscillation of the leakage (or common mode) current, which consequently tends to reduce the electromagnetic interference (EMI) or noise caused by high frequency oscillations on the common mode (or leakage) current.

As discussed below, certain embodiments of the invention may provide an integrated DC link inductor (choke) for AC motor drive systems or power conversion systems. This integrated DC link may consist of three windings (coils) which are wound around two primary (or outer) legs and a secondary (or inner) leg. The two outer legs' windings may be used to suppress both the differential mode and common mode currents, while the inner leg's winding may be electrically loaded by two resistors in series. While the aggregate value of these resistors may tend to improve the damping impact for the common mode current, the voltage drop across one of the resistors may, in certain embodiments, be used for sensing the level of common model current as well as detecting a ground fault. In certain applications, the sensed common mode current may be employed for diagnosing (and/or predicting) a failure in an AC motor drive system.

Turning to the drawings,FIG. 1depicts an exemplary AC drive system10that includes an integrated DC link inductor and current sensor winding12(hereinafter an “integrated ICSW”). As explained below, some embodiments of the integrated ICSW12may provide a variety of benefits, such as reducing the weight and size of the AC drive system10relative to a traditional system, and dampening electromagnetic emissions from common mode currents (or leakage currents) by increasing a damping factor for such currents. Prior to addressing the integrated ICSW12in detail, features of the surrounding AC drive system10will be discussed.

The AC drive system (or more generally, a power conversion system)10may be employed in a factory automation system, a traction system, an AC motor drive, a power generation system, or a power supply. The presently discussed AC drive system10includes a power source14, a rectifier16, an inverter18, a load20(such as an electric motor), a current sensor22, and a controller24. As explained below, the AC drive system regulates power delivered to the load20.

The illustrated power source14is a three-phase, alternating current (AC) power source. In some embodiments, the power source14may include or couple to grid power or a generator configured to deliver three phases of AC power. In other embodiments, the power source may output direct current power or a different number of phases of AC power, e.g., one or two-phase AC power. Three phase paths26,28, and30electrically couple (i.e., connect, either directly or indirectly in such a manner that electrons may flow therebetween) the power source14to the rectifier16. The power source14may include contactors to stop currents flowing through one or more of these phase paths26,28,30.

The illustrated rectifier16is a three-phase, full-wave rectifier. However, in other embodiments, the rectifier16may include a half-wave rectifier or, in some embodiments (such as those having a DC power source14), the rectifier16may be omitted, which is not to suggest that any other feature discussed herein may not also be omitted.

In various embodiments, the integrated ICSW12may be alternatively referred to as a combination low pass filter and current sensor, an inductor/ground fault detector, or an integrated common mode current sensor and DC link inductor, for instance. The illustrated integrated ICSW12has two inductors32and34and a current sensor winding36. As explained further below in reference toFIG. 2, the current sensor winding36of the illustrated embodiment shares a magnetic core with the inductors32and34, thereby potentially reducing the size and weight of the integrated ICSW relative to a DC link having discrete cores. The inductors32and34may be coupled to the rectifier16and the inverter18by a pair of current paths38and40, with an inductor32or34serially disposed on each current path38and40between the rectifier16and the inverter18. The current paths38and40may be referred to as positive and negative current paths. Additionally, a capacitor C connects to both current paths38and40to form a second order low pass filter with the inductors32and34. Ripples in the currents on current paths38and40may be reduced by the combined transient effects of the inductors32and34and the capacitor C. The inductance value of the DC link inductor (differential mode) may be selected such that the cut off frequency of the low pass filter is less than 120 Hz. This is because a 120 Hz voltage component can be generated by an unbalance power source (utility).

The inverter18may be configured to receive DC power and output AC power having an appropriate frequency and waveform. For example, the inverter18may include various solid state switching devices, such as integrated gate bipolar junction transistors, solid state relays, transistors, solenoids, and/or various types of electromechanical inverter switches, and the like. In some embodiments, the inverter18may include circuitry adapted to pulse width modulate an output AC waveform. The inverter18may be configured to output single-phase AC power, two-phase AC power, three-phase AC power, or other types of AC power. Alternatively, the inverter18, like many of the other features discussed herein, may be omitted, and DC power may flow directly through the load20. The inverter18of the illustrated embodiment couples to the load20via three phase paths42,44, and46.

The illustrated load20is a synchronous AC motor with permanent magnets. However, other embodiments may include other types of motors, such as a stepper motor, an induction motor, a linear motor, or the like. The motor may couple to a variety of machines, such as a conveyor belt, a pump, a fan, a drive shaft, and/or a wheel, for instance. Alternatively, or additionally, the load20may include other electrical devices, such as a heating element, an inductive heating winding, an amplifier, an electrolysis bath, an energy storage device, a battery, and/or a capacitor, for example. In some embodiments, a plurality of loads may receive power from the inverter18.

In the illustrated embodiment, a common mode current sensor22(hereinafter “CMCS”) monitors aspects of the currents on the current paths38and40, i.e., iXand iYinFIG. 1. Specifically, as is explained further below, the CMCS22senses a difference in magnitude of currents on the current paths38and40, such as might occur during a ground fault. The CMCS12of the presently discussed embodiment includes the current sensor winding36, a rectifier48, two resistors in series (RL+RD), and a voltmeter (e.g., a voltage sensor)50. The resistor RDmay be referred to as a damping resistor.

The rectifier48may include single-phase, half-wave or full-wave rectifier and, in some embodiments, a low pass filter to smooth output currents. Inputs52and54of the rectifier50couple to opposing sides of the current sensor winding36, which may be electromagnetically coupled to the current paths38and40in the manner described below in reference toFIG. 2. Both the resistor RLand the voltmeter50may be coupled in parallel to outputs56and58of the rectifier48.

Finally, in the currently discussed embodiment, the AC drive system10includes a controller24. The controller24may include a variety of digital and/or analogue circuits configured to control the load20in response to, inter alia, signals from the CMCS22. For example, the controller24may include a microprocessor, a central processing unit, an application specific integrated circuit, a digital signal processor, a microcontroller, or the like. A signal path60may carry signals indicative of common mode currents to the controller24, and a signal path62may enable the controller24to communicate with the inverter18. The controller24may also communicate with circuit breakers27on the phase paths26,28, and30.

In certain embodiments, the controller24may be configured to perform a variety of functions. For instance, the controller24may be adapted to control the operation of the load20by modulating various aspects of the power delivered to the load20, as described further below. Additionally, or alternatively, the controller20may be configured to regulate power delivered to the load20in response to a common mode current sensed by the CMCS22. For instance, in some embodiments, the controller24may be configured to terminate the delivery of power to the load20in response to on a signal indicative of a ground fault from the CMCS22by signaling the circuit breaker27to open one or more phase paths27,28, and/or30. To these ends, the controller24will typically include memory storing code or instructions for performing one or more of these functions among others.

In operation, the presently discussed AC drive system10delivers power to the load20in a controlled manner. For instance, in some embodiments, the amplitude, frequency, voltage, waveform shape, number of phases, and/or other parameter associated with the current that reaches the load20may be controlled by the AC drive system10. As described below, these parameters may be modulated to control operation of the load20.

In the present embodiment, the power supply14outputs three phases of AC power on phase paths26,28,30. The AC power from the power supply14may be grid power of a standard frequency, such as 50 or 60 Hz. In certain embodiments, the power supply14may deliver a portion of each cycle of grid power to regulate the power delivered, for instance with a silicon controlled rectifier (SCR) or insulated gate bipolar transistor (IGBT).

The rectifier16rectifies and combines the currents on the three phase paths26,28,30to produce a DC current iXon current path38and a return DC current iYon current path40. The currents iXand iYare generally equal under certain operating conditions, but they may be different if, for example, the load20and other components are leaking current to ground (e.g., during a ground fault). In other words, in certain systems, substantially all of the current flowing into the inverter18and load20(i.e., ix) flows back out as current iYabsent a current leak. Herein, the currents iXand iY, which have different directions, are referred to as differential mode currents. A difference between the magnitude of iXand iY, which may indicate a net loss or gain of current on the two current paths38and40, is referred to herein as a common mode current. A common mode current may occur during a ground fault, as some current iXflowing into the load20leaks to ground rather than flowing back out of the load as iY.

In the present embodiment, the integrated ICSW12performs two functions: reducing ripples in the primary currents iXand iYand outputting a current iZindicative of a change in the difference between iXand iY, as might occur during the onset of a ground fault. To perform the first function, the inductors32and34may combine with the capacitor C to form a low-pass filter, thereby potentially suppressing ripples or other rapid changes in the currents iXand iY. The second function, sensing a change in the difference between iXand iY, may be performed by the current sensor winding36, which is electromagnetically coupled to the inductors32and34by a common core. As explained below in reference toFIG. 2, magnetic flux from each of the inductors32and34may cancel out in the vicinity of the current sensor winding36when the currents32and34are equal. When a difference between the magnitude of iXand iY(i.e., a common mode current) arises, a resultant flux may occur within the current sensor winding36, and the change in the resultant flux may induce a current iZ. The current iZmay be monitored to detect a ground fault or, more generally, sense a common mode current. The dual functions of the presently discussed integrated ICSW12are described in more detail below in reference to an exemplary integrated ICSW12depicted byFIG. 2.

The inverter18of the present embodiment receives the rectified and filtered DC currents iXand iYand forms three phases of AC current on phase paths42,44, and46. The inverter18may form AC power by, for instance, selectively temporarily closing circuit paths coupling various combinations of the phase paths42,44, and46with the DC current paths38and40. In some embodiments, the inverter18may shape the resulting waveforms by, for instance, pulse width modulation or other techniques.

FIG. 2illustrates the exemplary integrated ICSW12in more detail. The integrated ICSW12ofFIG. 2includes an integrated or monolithic core64, the two inductors32and34, and the CMCS winding36. The illustrated core64has three legs66,68, and70and two cross bars72and74, which collectively define two apertures76and78. The outer legs66and70may be referred to as primary legs, and the inner leg68may be referred to as a secondary leg. The core64has a length L, a perimeter width w, an inner leg width kw (where k<1 in the present embodiment), a depth d, and a height H. Appropriate values for these dimensions may be selected in light of the application and some of the considerations discussed below. Gaps80and82are disposed on the outer legs66and70where a distal end of each of the legs66and70approaches the bottom cross bar74. In the present embodiment, the gaps80and82both have a generally uniform width g, and the outer legs66and70have a generally uniform cross-sectional area A in a plane parallel to the gaps g.

The illustrated inductors/windings32,34, and36are wrapped around the legs66,68, and70, respectively. In the present embodiment, the windings32and36are a single layer of windings, which may tend to reduce parasitic capacitance between layers of windings. The illustrated windings32and36are wound in opposite directions, e.g., clockwise and counter-clockwise, respectively.

The operation of the integrated ICSW12is explained below with two approaches: first, by an explanation of the governing equations with reference toFIG. 2, and, later, by describing the flux in the core64with reference toFIG. 4.

As can be seen inFIG. 2, the windings32and36are connected in the integrated ICSW12of the AC drive system10in such a manner that their magneto motive forces, namely N1iXand N1iY(where N1is the number of turns in the windings), produce flux in the same direction. In the present embodiment, both windings32and36have the same number of turns N1. If currents in these windings are equal in magnitude, that is iX=iY, then generally no flux penetrates into the inner leg68. Accordingly, the total differential-mode inductance is approximately obtained by (2N1)2/(2 g/(μ.A))=(2N12μ.A)/g, assuming the core is not saturated (where μ. is the permeability of free space). To avoid saturation, in some embodiments, the air-gap length g and the core cross-section area A are chosen such that, for a rated dc-link current iXand iYand a given magnetic core material, the core flux density (B) is below the saturation value of flux density in the B-H curve (where H is the magnetic field strength).

If the current flowing through winding32iXis not equal to the current flowing through winding34iY(as might happen during a ground fault), then the resulting common-mode magneto motive force, namely N1(iX−iY), drops across one gap80or82, and the resultant flux flows mainly through the winding36. If the core64is not saturated due to the common-mode current, i.e., (iX−iY), and the winding36of the inner leg is open circuited (i.e., RL=∞) then the common mode inductance is approximately equal to (N12μ.A)/g. To prevent core saturation due to normal levels of common-mode current, e.g., not during a ground fault, the current sensor winding36is loaded by the resistor RLthrough rectifier48, as shown inFIG. 1. Accordingly, in some embodiments, the number of windings on the inner winding36(N2) and RLare selected such that N2iZ−N1|iX−iY| is small enough to prevent core saturation due to a normal level of common-mode currents. With the current sensor winding36loaded RL, the resultant common-mode inductance is approximately equal to ((N12μ.A/g) in parallel with (N1/N2)2ZL, where ZLis the effective load impedance of the combination of the rectifier48and (RL+Rd).

The operation of the integrated ICSW12is depicted in greater detail byFIGS. 3 and 4.FIG. 3is a flow chart of an exemplary filtering and sensing processes84, andFIG. 4depicts magnetic flux in the core64during the exemplary filtering and sensing process84.

Turning toFIG. 3, the exemplary process84begins with receiving magnetic flux induced by differential mode currents in a core, as depicted by block86. For instance, inFIG. 4, differential mode currents iXand iYthrough the oppositely wound windings32and34induce such a magnetic flux in the core64.FIG. 4depicts a magnetic flux88induced by the current iXand a magnetic flux90induced by the current iY. The illustrated fluxes90and88have the same rotational direction, i.e., clockwise from the perspective ofFIG. 4.

The exemplary process84further includes receiving magnetic flux induced by a common mode current in the core, as depicted by block92ofFIG. 3. InFIG. 4, a resultant flux94in the middle leg68arises due to a difference between the flux88from the winding38and the flux90from the winding34. If the number of winding revolutions N1is equal, the resultant flux94may correspond to the difference in magnitude of iXand iYInFIG. 4, the magnitude of iXis larger than the magnitude of iY, and the resultant flux94flows in the same rotational direction as the flux88passing through the winding38, which carries the larger current iX. In other situations, iYmay be larger than iX, and the resultant flux94may flow in the opposite direction, i.e., in the same rotational direction as the flux90passing through the winding34that carries the current iY. The magnitude of the resultant flux94may be proportional to the difference between the magnitude of currents iXand iY. In some applications, iXand iYmay be generally equal, e.g., in the absence of a ground fault, in which case the resultant flux94may be generally zero.

Returning to the process84ofFIG. 3, a change in the resultant flux94may induce a current iZin a current sensor winding36, as depicted by block96. As the resultant flux94increases or decreases, a current iZis induced in the winding36ofFIG. 4. The direction and magnitude of the current iZmay correspond with the direction and rate of change of the resultant flux94. For example, during the onset of a ground fault, the current iYreturning from the load20may drop, thereby increasing the resultant flux94and inducing a current iZin the winding36. This current, iZ, may be rectified, as depicted by block98ofFIG. 3, and the voltage of the rectified current may be sensed, e.g., by the voltmeter50ofFIG. 1.

In the present embodiment, the product of steps86,92,96, and98is a signal indicative of the magnitude and direction of a common mode current on current paths38and40. This signal has a variety of uses, as illustrated by the branches of the process84. For example, a parameter of the load20may be detected, as depicted by block100, and the parameter may be stored in memory or displayed, as depicted by block102. The parameter might be an imbalance in a motor, e.g., extra windings, a short in a motor terminal, or other parameter that corresponds with a common mode current. In another example, a ground fault may be detected based on the magnitude of the rectified current iZ, as depicted by block104. In response, the controller24may stop the delivery of power to the load20, as depicted by block106. In yet another example, the rectified current iZmay be integrated, e.g., with a capacitor, to measure current to ground, as depicted by block108. The magnitude of the measured current may be stored in memory or it may be displayed, as depicted by block110.

While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. For example, the inductor can also be used in a DC/DC application or any application that can benefit from the integration of a power inductor (common and differential mode inductor) and a common mode sensor. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.