Common mode & differential mode filter for variable speed drive

Systems and methods for improved Variable Speed Drives having active inverters include an input filter for filtering common mode and differential mode currents. A three-phase inductor has three windings, each winding of the three-phase inductor having a center tap dividing each winding into a pair of inductor sections; and a three-phase input capacitor bank connected in a wye configuration to the three center taps at one end, and to a common point at the opposite end. The three-phase input capacitor bank provides a short circuit for frequencies above a predetermined fundamental frequency for shunting such frequencies through the three phase capacitor bank, while passing the predetermined fundamental frequency to an input AC power source.

BACKGROUND

The present application relates generally to variable speed drives. The application relates more specifically to a common mode & differential mode filter for a variable speed drive incorporating an active converter.

A variable speed drive (VSD) for heating, ventilation, air-conditioning and refrigeration (HVAC&R) applications typically includes a rectifier or converter, a DC link, and an inverter. VSDs that incorporate active converter technology to provide power factor correction and reduced input current harmonics also generate a significantly higher level of common mode RMS and peak to peak voltage to the motor stator windings as compared to conventional VSDs. This common mode voltage can cause motor and compressor bearing fluting, and these common mode voltages which result in currents flowing through the machine bearings may cause premature bearing failures in the motor and/or compressor.

Proper operation of the active converter control methodology, using the synchronous d-q reference frame requires knowledge of the instantaneous phase angle of the input line-to-line voltage. If the reference frame angle is incorrect or unknown, then the input power factor and the harmonic distortion of the input current to the Variable Speed Drive (VSD) with active converter cannot be controlled properly. If the VSD is required to ride-through an extended loss of the input line-to-line voltage and re-synchronize to the input mains when the power is restored, a means to retain the expected d-q reference frame angle during the loss of mains is needed. In addition, a means to quickly lock back onto the input mains line-to-line voltage and generate the actual phase angle of the line-to-line voltage is required. What is needed is a system and/or method that satisfy one or more of these needs or provides other advantageous features. While the present invention is directed specifically to VSDs that incorporate an Active Converter type AC to DC converter topology, the invention is also effective for VSDs utilizing conventional AC to DC rectifier converters.

Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments that fall within the scope of the claims, regardless of whether they accomplish one or more of the aforementioned needs.

SUMMARY

The present invention is directed to a circuit for application on three-phase Pulse Width Modulated (PWM) Variable Speed Drives (VSDs), and preferably for application on PWM VSDs having Active Converter topologies.

In one embodiment, a variable speed drive system is configured to receive an input AC power at a fixed AC input voltage magnitude and frequency and provide an output AC power at a variable voltage and variable frequency. The variable speed drive includes a converter stage connected to an AC power source providing the input AC voltage. The converter stage is configured to convert the input AC voltage to a boosted DC voltage. A DC link is connected to the converter stage, the DC link configured to filter and store the boosted DC voltage from the converter stage. An inverter stage is connected to the DC link, the inverter stage configured to convert the boosted DC voltage from the DC link into the output AC power having the variable voltage and the variable frequency. Finally, an input filter is connected to the VSD at the input to the converter stage for filtering a common mode component and a differential mode component induced by conducted electromagnetic interference or radio frequency interference present at the AC power source.

Another embodiment relates to an input filter for filtering common mode and differential mode currents. The input filter includes a three-phase inductor having three windings. Each winding of the three-phase inductor includes a center tap dividing each winding into a pair of inductor sections. A three-phase input capacitor bank of three capacitors is connected in a wye configuration to the three center taps at one end, and to a common point at the opposite end. The three-phase input capacitor bank is configured to substantially provide a short circuit for frequencies above a predetermined fundamental frequency for shunting frequencies above a predetermined fundamental frequency through the three phase capacitor bank, while passing the predetermined fundamental frequency to the mains.

A further embodiment relates to an output filter for filtering common mode and differential mode currents associated with a variable speed drive. The output filter includes a first output capacitor bank of three capacitors. Each capacitor of the first output capacitor bank is connected in a wye configuration to an output phase of the inverter stage. The three capacitors of the first output capacitor bank are each connected in common at a common capacitor connection at an end opposite the output phase connection. The common capacitor connection is also connected to earth.

Still another embodiment, is directed to a chiller system. The chiller system includes a refrigerant circuit comprising compressor, a condenser, and an evaporator connected in a closed refrigerant loop. A motor is connected to the compressor to power the compressor. A variable speed drive is connected to the motor. The variable speed drive is configured to receive an input AC power at a fixed AC input voltage magnitude and frequency and provide an output AC power at a variable voltage and variable frequency. The variable speed drive includes a converter stage connected to an AC power source providing the input AC voltage. The converter stage is configured to convert the input AC voltage to a boosted DC voltage. A DC link is connected to the converter stage, with the DC link configured to filter and store the boosted DC voltage from the converter stage. An inverter stage is connected to the DC link, with the inverter stage configured to convert the boosted DC voltage from the DC link into the output AC power having the variable voltage and the variable frequency. Finally, an input filter for filtering common mode and differential mode currents is connected to the variable speed drive at an input to the converter stage. The input filter includes a three-phase inductor having three windings, wherein each winding of the three-phase inductor has a center tap dividing each winding into a pair of inductor sections, and a three-phase input capacitor bank having three capacitors connected in a wye configuration to the three center taps at one end, and to a common point at the opposite end. The three-phase input capacitor bank is configured to substantially provide a short circuit for frequencies above a predetermined fundamental frequency for shunting frequencies above a predetermined fundamental frequency through the three phase capacitor bank, while passing the predetermined fundamental frequency to the converter stage.

One advantage is to reduce the common mode and differential mode currents associated with conducted electromagnetic interference and radio frequency interference present at the AC power source as a result of the operation of the VSD.

A second advantage is to reduce the common mode voltage stress presented to the motor stator in both RMS and peak terms, thereby alleviating issues associated with premature machine bearing failure and premature insulation to earth ground failure.

Another advantage is to reduce the differential mode voltage stress presented to the motor stator in both RMS and peak terms, thereby alleviating issues associated with premature machine turn-to-turn stator winding failure.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIGS. 1A and 1Billustrate generally system configurations. An AC power source102supplies a variable speed drive (VSD)104, which powers a motor106(seeFIG. 1A) or motors106(seeFIG. 1B). The motor(s)106is preferably used to drive a corresponding compressor of a refrigeration or chiller system (see generally,FIG. 3). The AC power source102provides single phase or multi-phase (e.g., three phase), fixed voltage, and fixed frequency AC power to the VSD104from an AC power grid or distribution system that is present at a site. The AC power source102preferably can supply an AC voltage or line voltage of 200 V, 230 V, 380 V, 460 V, or 600 V, at a line frequency of 50 Hz or 60 Hz, to the VSD104depending on the corresponding AC power grid.

The VSD104receives AC power having a particular fixed line voltage and fixed line frequency from the AC power source102and provides AC power to the motor(s)106at a desired voltage and desired frequency, both of which can be varied to satisfy particular requirements. Preferably, the VSD104can provide AC power to the motor(s)106having higher voltages and frequencies and lower voltages and frequencies than the rated voltage and frequency of the motor(s)106. In another embodiment, the VSD104may again provide higher and lower frequencies but only the same or lower voltages than the rated voltage and frequency of the motor(s)106. The motor(s)106is preferably an induction motor, but can include any type of motor that is capable of being operated at variable speeds. The induction motor can have any suitable pole arrangement including two poles, four poles or six poles.

FIGS. 2A and 2Billustrate different embodiments of the VSD104. The VSD104can have three stages: a converter stage202, a DC link stage204and an output stage having one inverter206(seeFIG. 2A) or a plurality of inverters206(seeFIG. 2B). The converter202converts the fixed line frequency, fixed line voltage AC power from the AC power source102into DC power. The DC link204filters the DC power from the converter202and provides energy storage components. The DC link204can be composed of capacitors, inductors, or a combination thereof, which are passive devices that exhibit high reliability rates and very low failure rates. Finally, in the embodiment ofFIG. 2A, the inverter206converts the DC power from the DC link204into variable frequency, variable voltage AC power for the motor106and, in the embodiment ofFIG. 2B, the inverters206are connected in parallel on the DC link204and each inverter206converts the DC power from the DC link204into a variable frequency, variable voltage AC power for a corresponding motor106. The inverter(s)206can be a power module that can include power transistors, insulated gate bipolar transistor (IGBT) power switches and inverse diodes interconnected with wire bond technology. Furthermore, it is to be understood that the DC link204and the inverter(s)206of the VSD104can incorporate different components from those discussed above so long as the DC link204and inverter(s)206of the VSD104can provide the motors106with appropriate output voltages and frequencies.

With regard toFIGS. 1B and 2B, the inverters206are jointly controlled by a control system such that each inverter206provides AC power at the same desired voltage and frequency to corresponding motors based on a common control signal or control instruction provided to each of the inverters206. In another embodiment, the inverters206are individually controlled by a control system to permit each inverter206to provide AC power at different desired voltages and frequencies to corresponding motors106based on separate control signals or control instructions provided to each inverter206. This capability permits the inverters206of the VSD104to more effectively satisfy motor106and system demands and loads independent of the requirements of other motors106and systems connected to other inverters206. For example, one inverter206can be providing full power to a motor106, while another inverter206is providing half power to another motor106. The control of the inverters206in either embodiment can be by a control panel or other suitable control device.

For each motor106to be powered by the VSD104, there is a corresponding inverter206in the output stage of the VSD104. The number of motors106that can be powered by the VSD104is dependent upon the number of inverters206that are incorporated into the VSD104. In one embodiment, there can be either 2 or 3 inverters206incorporated in the VSD104that are connected in parallel to the DC link204and used for powering a corresponding motor106. While the VSD104can have between 2 and 3 inverters206, it is to be understood that more than 3 inverters206can be used so long as the DC link204can provide and maintain the appropriate DC voltage to each of the inverters206.

FIG. 3illustrates generally one embodiment of a refrigeration or chiller system using the system configuration and VSD104ofFIGS. 1A and 2A. As shown inFIG. 3, the HVAC, refrigeration or liquid chiller system300includes a compressor302, a condenser arrangement304, a liquid chiller or evaporator arrangement306and the control panel308. The compressor302is driven by motor106that is powered by VSD104. The VSD104receives AC power having a particular fixed line voltage and fixed line frequency from AC power source102and provides AC power to the motor106at desired voltages and desired frequencies, both of which can be varied to satisfy particular requirements. The control panel308can include a variety of different components such as an analog to digital (A/D) converter, a microprocessor, a non-volatile memory, and an interface board, to control operation of the refrigeration system300. The control panel308can also be used to control the operation of the VSD104, and the motor106.

Compressor302compresses a refrigerant vapor and delivers the vapor to the condenser304through a discharge line. The compressor302can be any suitable type of compressor, e.g., screw compressor, centrifugal compressor, reciprocating compressor, scroll compressor, etc. The refrigerant vapor delivered by the compressor302to the condenser304enters into a heat exchange relationship with a fluid, e.g., air or water, and undergoes a phase change to a refrigerant liquid as a result of the heat exchange relationship with the fluid. The condensed liquid refrigerant from condenser304flows through an expansion device (not shown) to the evaporator306.

The evaporator306can include connections for a supply line and a return line of a cooling load. A secondary liquid, e.g., water, ethylene, calcium chloride brine or sodium chloride brine, travels into the evaporator306via return line and exits the evaporator306via supply line. The liquid refrigerant in the evaporator306enters into a heat exchange relationship with the secondary liquid to lower the temperature of the secondary liquid. The refrigerant liquid in the evaporator306undergoes a phase change to a refrigerant vapor as a result of the heat exchange relationship with the secondary liquid. The vapor refrigerant in the evaporator306exits the evaporator306and returns to the compressor302by a suction line to complete the cycle. It is to be understood that any suitable configuration of condenser304and evaporator306can be used in the system300, provided that the appropriate phase change of the refrigerant in the condenser304and evaporator306is obtained.

The HVAC, refrigeration or liquid chiller system300can include many other features that are not shown inFIG. 3. These features have been purposely omitted to simplify the drawing for ease of illustration. Furthermore, whileFIG. 3illustrates the HVAC, refrigeration or liquid chiller system300as having one compressor connected in a single refrigerant circuit, it is to be understood that the system300can have multiple compressors, powered by a single VSD as shown inFIGS. 1B and 2Bor multiple VSDs, see generally, the embodiment shown inFIGS. 1A and 2A, connected into each of one or more refrigerant circuits.

Referring next toFIG. 4, there is a schematic diagram of elements of an input filter10shown. EMI/RFI sources generated by the Active Converter202are filtered ahead of the converter202by splitting a three-phase AC input inductor16into a line-side inductor26and load-side inductor28per phase. The line-side inductors26and load-side inductors28are connected by inductor tap portions18. A capacitive three-phase filter element20is wye-connected between the inductor tap portions18. An optional earth connection22may be connected to a common point21of the wye-connected filter element20. The earth connection22may alternately include a grounding capacitor23. The line- and load-side inductors26and28, respectively, and the capacitive filter element20are designed with inductance and capacitance values to provide a roll off of the EMI/RFI sources—i.e., high frequency switching components of the input current conducted by the converter202. The input filter provides a high impedance via the differential mode inductive components of inductances26and28and a low impedance via the three-phase wye connected capacitance20to the EMI/RFI sources, while passing the fundamental component of the power current, e.g., 60 Hz, through the network with minimal impedance. By utilizing a four- or five-legged (4/5) input inductor16, a common mode inductive component is formed via inductances26and28and together with the optional earth connection22or the grounding capacitor23, increases to the capacity of the filter10acts to prevent common mode current generated by the converter202from flowing into the mains power source102. The wye-connection point21of the input filter10may be directly earthed or alternately earthed through a separate capacitor23to provide greater shunting of high-frequency currents to earth. In one embodiment, the inductor16may be provided with low inter-winding capacitance.

Line-side inductors26provide impedance at a predetermined switching frequency of the VSD104between the wye-connected capacitors20and the AC power source102. The impedance of the line-side inductors26is designed to allow the wye-connected capacitors20to be more effective than a system with no significant impedance between the input AC mains102and the VSD104. Inductors26also provide high-frequency impedance in the reverse direction, to restrict the flow of high-frequency current from the converter202to the AC power source102. Thus the inductors26restrict or limit high frequency emissions from reflecting back to the AC power source102.

Inductors28provide impedance between the capacitors20and the input to the VSD104. Inductors28provide high impedance between the AC power source102and the active converter202portion of the VSD104. Alternately, if the VSD104is a conventional VSD with a passive rectifier converter, the impedance of inductor28isolates the VSD104from the input AC mains102and reduces high frequency emissions conducted to the mains102from the VSD104.

The wye-connected capacitor bank20provides low impedance between phase conductors A, B & C for at least one switching frequency of the VSD104, and provides low impedance for differential mode current flow. The wye-connected capacitor bank20also provides a low impedance path for flow of at least one switching frequency to an earth ground connection22, assuming that an earth ground connection is provided, for reducing common mode current flow.

Referring next toFIGS. 5 and 6, in one embodiment the common mode input filter10may be implemented using either a four-legged AC inductor516′ (see, e.g.,FIG. 8, the four-legged inductor embodiment designated by a prime symbol) or five-legged AC inductor516(collectively referred to as 4/5 inductor) applied to the input of the VSD104with Active Converter technology. Conventional filters employ three-legged inductors to provide power factor and harmonic input current control. The 4/5 inductor516provides both common mode and differential mode inductance.FIGS. 5 and 6illustrate a five-legged inductor516, which provides more geometric symmetry in a three-phase power system. The common mode inductance is generated by providing a magnetic flux path504, indicated by arrow502. The flux path504, in magnetic communication with three core legs510,512and514, each of which are connected to one of the phases in the three phase input power102. The flux path is a continuous, magnetically permeable magnetic loop that surrounds the inner three core legs510,512and514. Each of the core legs510,512and514is has a coil winding or conductor26(see, e.g.,FIG. 4) wrapped around substantially the entire surface area of the respective core leg510,512and514. The direction of the magnetic flux in the flux path are dependent upon the direction and magnitude of the currents in coil windings, and are therefore shown as flowing in either direction, although in practice, the magnetic flux may flow in one direction or another about the about the periphery of the inductor516. The common mode magnetic flux is induced by electrical currents that are common to all three inductor coils16. This common flux path504can only be excited by common mode current components flowing through the inductor coils. A picture of the cross section of such an inductor is shown inFIG. 5. This inductor516has a liquid-cooled core to improved heat dissipation and increase the power capacity of the inductor516.

Referring toFIG. 6, an elevational cross-section of the five-legged inductor516illustrates air gaps520that are inserted in the legs510,512and514, to prevent core saturation and increase the working flux density range of the inductor516. In the inductor516, an air gap520is arranged between the horizontal sections of the flux path504. Two air gaps520are also inserted intermediately in each of the core legs510,512and514, to break up each core leg510,512and514, into three discrete segments. Other air gap configurations may be used to achieve the same result.

Referring next toFIG. 7, an embodiment of a variable speed drive having an output filter with common mode/differential mode input filter circuit is illustrated. The EMI/RFI input filter as described with respect toFIG. 4, above, is connected at the input of the converter202, and performs the same filtering functions as described above. The addition of the input filter with an inductor16at the input to the VSD104effectively provides a high-impedance circuit between the AC power mains102and the VSD104. To provide a low impedance path for common mode current flow, a three-phase wye connected capacitor bank30including three common mode capacitors32are connected between the VSD's motor connection terminal38, and earth ground22. The capacitor bank30is equivalent to a short circuit—i.e., low impedance—at high frequency, effectively earthing the destructive high frequency AC components present on the three VSD output terminals34and shunting the destructive AC components from reaching the motor or other type of load connected to the VSD, thus filtering out currents resulting from common mode voltages. The capacitor bank30allows high-frequency AC components to bypass the parasitic capacitive earthing elements of the motor and eliminates bearing damage caused by common mode voltages and currents.

The inverter output terminals34feed a second filter arrangement that includes a three phase inductor36connected in series with the output terminals38, which are connected to the system load, e.g., a motor106. A second three-phase capacitor bank42is wye-connected to the output power phases, L1, L2and L3, between the load side of the three phase inductor36, providing a low impedance path for the differential mode current to flow among the capacitor bank42. The combination of the second three-phase capacitor bank wye-connected at the load side of the three phase inductor36provides an L-C differential mode output filter. By combining the common mode filter capacitor bank30, with the L-C differential mode inductor36and capacitor bank42, both of the destructive conditions, i.e., common mode and differential mode currents, are prevented from reaching a load that is powered by the VSD104.

While the exemplary embodiments illustrated in the figures and described herein are presently preferred, it should be understood that these embodiments are offered by way of example only. Accordingly, the present application is not limited to a particular embodiment, but extends to various modifications that nevertheless fall within the scope of the appended claims. The order or sequence of any processes or method steps may be varied or re-sequenced according to alternative embodiments.

It is important to note that the construction and arrangement of the common mode and differential mode filter for variable speed drives, as shown in the various exemplary embodiments is illustrative only. Although only a few embodiments have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. For example, elements shown as integrally formed may be constructed of multiple parts or elements, the position of elements may be reversed or otherwise varied, and the nature or number of discrete elements or positions may be altered or varied. Accordingly, all such modifications are intended to be included within the scope of the present application. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. In the claims, any means-plus-function clause is intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Other substitutions, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the exemplary embodiments without departing from the scope of the present application.