US 7957166 B2
Systems and methods for improved Variable Speed Drives are provided. One embodiment relates to apparatus for common mode and differential mode filtering for motor or compressor bearing protection when operating with Variable Speed Drives, including conducted EMI/RFI input power mains mitigation. Another embodiment relates to a method to extend the synchronous operation of an Active Converter to the AC mains voltage during complete line dropout. Another embodiment relates to an Active Converter-based Variable Speed Drive system with Improved Full Speed Efficiency. Another embodiment relates to a liquid- or refrigerant-cooled inductor. The liquid- or refrigerant-cooled inductor may be used in any application where liquid or refrigerant cooling is available and a reduction in size and weight of a magnetic component is desired.
1. A variable speed drive system 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 comprising:
a converter stage connected to an AC power source providing the input AC voltage, the converter stage being configured to convert the input AC voltage to a boosted DC voltage;
a DC link connected to the converter stage, the DC link being configured to filter and store the boosted DC voltage from the converter stage;
an inverter stage connected to the DC link, the inverter stage being configured to convert the boosted DC voltage from the DC link into the output AC power having the variable voltage and the variable frequency; and an input filter; the input filter comprising:
a three-phase inductor having three windings, wherein 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 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;
wherein 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.
2. The variable speed drive system of
3. The variable speed drive system of
4. The variable speed drive system of
5. The variable speed drive system of
6. The variable speed drive system of
a core element having at least three leg portions, each leg portion being wound with a pair of electric current carrying coils, and a flux portion, the flux portion connecting the three leg portions in a continuous magnetic path, the flux portion having a pair of vertical legs connected at a top end by a top leg and at a bottom end by a bottom leg to form a substantially rectangular frame portion, the three leg portions being disposed within and in magnetic communication with the frame portion.
7. The variable speed drive system of
a core element having at least three leg portions, each leg portion being wound with a pair of electrical coils, and a flux portion, the flux portion connecting the three leg portions in a continuous magnetic path, the flux portion providing a common flux path, the flux path being excited by common mode current components flowing through the electrical coils.
8. The variable speed drive system of
a first output capacitor bank of three capacitors, each capacitor connected in a wye configuration to an output phase of the inductor;
each of the three capacitors of the first output capacitor bank being connected in common at an end opposite the output phase connection; the common capacitor connection also being connected to earth.
9. The variable speed drive system of
an output inductor having three output phase windings connected in series with the output phase of the inverter stage; and
a second output capacitor bank of three capacitors, each capacitor of the second output capacitor bank being connected in a wye configuration to a load side of the output inductor;
wherein currents induced by differential mode voltage components are substantially removed from the load side of the output inductor and prevented from flowing to a load connected to the output inductor.
The present application relates generally to variable speed drives. The application relates more specifically to systems and methods for improved efficiency in variable speed drives having active converters.
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. The rectifier or converter converts the fixed line frequency, fixed line voltage AC power from an AC power source into DC power. The DC link filters the DC power from the converter and typically contains a large amount of electrical capacitance. Finally, the inverter is connected in parallel with the DC link and converts the DC power from the DC link into a variable frequency, variable voltage AC power.
Variable Speed Drives 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 Variable Speed Drives. This common mode voltage can be coupled to the rotor of the motor via various stray machine capacitances, causing 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.
Typical bypass means provided by drive manufacturers are active only when the VSD is incapable of running, in emergency situations. The bypass typically incorporates a minimum of two sets of three-phase contactors, one in series with the output of the inverter section and another between the incoming mains and the motor. In some cases a third set of three-phase contactors is implemented between the power mains and the input mains connection to the VSD. These bypass means are typically actuated via operator intervention via the drive keypad. Some suppliers may implement control means to provide automatic switchover to the bypass mode if the VFD fails. Some suppliers also provide “catch the spinning load” also called “windmill start” control means to catch and control an un-energized motor that is spinning, and bring it back up to full speed. Transfer from VSD operation to mains operation usually results in locked rotor torque being presented to the mechanical load and very high motor inrush current as the motor is started across-the-line.
In the past, VSD power assembly designs were bulky and heavy. They utilized aluminum electrolytic capacitors which have an inherent wear-out mechanism and are physically heavy and difficult to mount due to their cylindrical nature. The heatsinks were composed of either copper or aluminum material. Aluminum raises corrosion concerns when used in a closed loop uninhibited cooling system where copper components are also in intimate contact with the cooling fluid.
Typically, Low Voltage (less than 600 VAC) Voltage Source type Variable Speed Drives utilize air-cooled inductors, as the losses dissipated by the inductors are not high relative the losses in the remainder of the drive system. Also air-cooling is a less expensive option as compared to liquid cooling based on a de-ionized liquid style cooling loop. In addition, liquid cooling is often not available in the end use application.
Ground fault protection within a VSD can be implemented in various ways, e.g., an external ground fault sensor (a single “zero sequence” current transformer and detection circuitry) that opens a set of relay contacts or a molded case circuit breaker with a trip unit that incorporates a ground fault detection circuit. The level at which the ground fault current trip can be sensed is greater, and the accuracy of the sensed ground fault current is reduced, as a result of the three phase sensors. Another example of prior art ground fault protection employs motor current sensing means to shut down the inverter section of the VSD. This method does not provide ground fault protection for a ground fault occurring internally in the VSD.
Existing low voltage (i.e., less than 600 VAC) voltage source type VSDs utilize air-cooled inductors since the losses dissipated by the inductors are not high relative to the losses in the remainder of the drive system. In addition, air-cooling is often less expensive than liquid cooling. However, liquid cooling is often not available for end use applications. As active converter style VSDs are more widely used, the inductor losses may become more problematic as the size and cost of the required inductors may grow considerably.
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.
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 a variable speed drive system. The variable speed drive system is configured to receive an input AC voltage at a fixed AC input voltage 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 is 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, a phase angle control circuit includes a squaring amplifier, a first phase-lock loop circuit associated and a second phase-lock loop circuit; configured to receive the AC power source and output a substantially rectangular output signal based on the AC power source. The first phase-lock loop circuit has a first lag-lead filter configured with a high filter cutoff frequency and a first capacitor to provides the converter stage with a phase angle parameter. The second phase-lock loop circuit has a second lag-lead filter configured with a low cutoff frequency and a second capacitor to provide the lag-lead filter the capability of storing an angle of the mains voltage in the feedback loop of the PLL during mains interruption.
Still another embodiment includes a plastic cooling system for cooling electronic components having a base, a cooling well formed in the top of the base and open at the top, a feed channel formed in the base for accepting a cooling fluid to be introduced to the cooling well, a drain channel formed in the base through which the cooling fluid is to be carried away from the cooling well, a cooling well inlet formed in the cooling well and in communication with the feed channel, and a cooling well outlet formed in the cooling well opposite the cooling well inlet and in communication with the drain channel. The feed channel is sufficiently large relative to the size and flow characteristics of the well and cooling well inlets and outlets such that when the cooling fluid flows through the cooling device, the pressure drop across the feed channel is substantially less than the pressure drop across the well.
Another embodiment relates to a variable speed drive system configured to receive an input AC voltage at a fixed AC input voltage 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 being configured to convert the input AC voltage to a boosted DC voltage; a DC link connected to the converter stage, the DC link being configured to filter and store the boosted DC voltage from the converter stage; and an inverter stage connected to the DC link, the inverter stage being configured to convert the boosted DC voltage from the DC link into the output AC power having the variable voltage and the variable frequency. An integral bypass contactor is connected in parallel with the VSD between the AC power source and the AC output power. The integral bypass connector is arranged to bypass the VSD when the VSD output frequency and voltage are approximately equal with the AC input voltage and frequency.
Yet another embodiment relates to a variable speed drive system configured to receive an input AC voltage at a fixed AC input voltage 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 being configured to convert the input AC voltage to a boosted DC voltage; a DC link connected to the converter stage, the DC link being configured to filter and store the boosted DC voltage from the converter stage; and an inverter stage connected to the DC link, the inverter stage being configured to convert the boosted DC voltage from the DC link into the output AC power having the variable voltage and the variable frequency. The variable speed drive also includes a ground fault protection system for interrupting fault current flowing to an input phase of the active converter, the ground fault protection system including at least one current sensor for sensing a ground fault on an input phase of the active converter, and a controller; wherein the active converter further includes at least two semiconductor switches for each power phase of the AC power source; each of the at least two semiconductor switches comprising a pair of reverse blocking IGBTs inversely connected in parallel, wherein each of the reverse blocking IGBTs is controllable by the controller to switch the RB IGBTs to a nonconductive state in response to a sensed ground fault current.
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 the integral bypass active converter configuration may be utilized for VSD controlled systems that operate at a maximum frequency & voltage equal to the power line mains frequency supplied to the VSD. Contactor bypass eliminates the losses associated with the VSD when the system is required to operate at maximum frequency.
Another advantage is a ground fault protection system in an active converter for instantaneously interrupting a ground fault at an input phase of the active converter, using reverse blocking IGBTs to controllably switch off fault current in response to a sensed fault.
Still another advantage is improved cooling and reduced size, weight and cost of the inductor.
Alternative exemplary embodiments relate to other features and combinations of features as may be generally recited in the claims.
The application will become more fully understood from the following detailed description, taken in conjunction with the accompanying figures, wherein like reference numerals refer to like elements, in which:
Before turning to the figures which illustrate the exemplary embodiments in detail, it should be understood that the application is not limited to the details or methodology set forth in the following description or illustrated in the figures. It should also be understood that the phraseology and terminology employed herein is for the purpose of description only and should not be regarded as limiting.
The VSD 104 receives AC power having a particular fixed line voltage and fixed line frequency from the AC power source 102 and provides AC power to the motor(s) 106 at a desired voltage and desired frequency, both of which can be varied to satisfy particular requirements. Preferably, the VSD 104 can provide AC power to the motor(s) 106 having higher voltages and frequencies and lower voltages and frequencies than the rated voltage and frequency of the motor(s) 106. In another embodiment, the VSD 104 may 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) 106 is 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.
With regard to
For each motor 106 to be powered by the VSD 104, there is a corresponding inverter 206 in the output stage of the VSD 104. The number of motors 106 that can be powered by the VSD 104 is dependent upon the number of inverters 206 that are incorporated into the VSD 104. In one embodiment, there can be either 2 or 3 inverters 206 incorporated in the VSD 104 that are connected in parallel to the DC link 204 and used for powering a corresponding motor 106. While the VSD 104 can have between 2 and 3 inverters 206, it is to be understood that more than 3 inverters 206 can be used so long as the DC link 204 can provide and maintain the appropriate DC voltage to each of the inverters 206.
Compressor 302 compresses a refrigerant vapor and delivers the vapor to the condenser 304 through a discharge line. The compressor 302 can be any suitable type of compressor, e.g., screw compressor, centrifugal compressor, reciprocating compressor, scroll compressor, etc. The refrigerant vapor delivered by the compressor 302 to the condenser 304 enters 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 condenser 304 flows through an expansion device (not shown) to the evaporator 306.
The evaporator 306 can 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 evaporator 306 via return line and exits the evaporator 306 via supply line. The liquid refrigerant in the evaporator 306 enters into a heat exchange relationship with the secondary liquid to lower the temperature of the secondary liquid. The refrigerant liquid in the evaporator 306 undergoes a phase change to a refrigerant vapor as a result of the heat exchange relationship with the secondary liquid. The vapor refrigerant in the evaporator 306 exits the evaporator 306 and returns to the compressor 302 by a suction line to complete the cycle. It is to be understood that any suitable configuration of condenser 304 and evaporator 306 can be used in the system 300, provided that the appropriate phase change of the refrigerant in the condenser 304 and evaporator 306 is obtained.
The HVAC, refrigeration or liquid chiller system 300 can include many other features that are not shown in
Referring next to
Line-side inductors 26 provide impedance at a predetermined switching frequency of the VSD 104 between the wye-connected capacitors 20 and the AC power source 102. The impedance of the line-side inductors 26 is designed to allow the wye-connected capacitors 20 to be more effective than a system with no significant impedance between the input AC mains 102 and the VSD 104. Inductors 26 also provide high-frequency impedance in the reverse direction, to restrict the flow of high-frequency current from the converter 202 to the AC power source 102. Thus the inductors 26 restrict or limit high frequency emissions from reflecting back to the AC power source 102.
Inductors 28 provide impedance between the capacitors 20 and the input to the VSD 104. Inductors 28 provide high impedance between the AC power source 102 and the active converter 202 portion of the VSD 104. Alternately, if the VSD 104 is a conventional VSD with a passive rectifier converter, the impedance of inductor 28 isolates the VSD 104 from the input AC mains 102 and reduces high frequency emissions conducted by to the mains 102 from the VSD 104.
The wye-connected capacitor bank 20 provides low impedance between phase conductors A, B & C for at least one switching frequency of the VSD 104, and provides low impedance for differential mode current flow. The wye-connected capacitor bank 20 also provides a low impedance path for flow of at least one switching frequency to an earth ground connection 22, assuming that an earth ground connection is provided, for reducing common mode current flow.
Referring next to
Referring next to
The inverter output terminals 34 feed a second filter arrangement that includes a three phase inductor 36 connected in series with the output terminals 38, which are connected to the system load, e.g., a motor 106. A second three-phase capacitor bank 42 is wye-connected to the output power phases, L1, L2 and L3, between the load side of the three phase inductor 36, providing a low impedance path for the differential mode current to flow among the capacitor bank 42. The combination of the second three-phase capacitor bank wye-connected at the load side of the three phase inductor 36 provides an L-C differential mode output filter. By combining the common mode filter capacitor bank 30, with the L-C differential mode inductor 36 and capacitor bank 42, both of the destructive conditions, i.e., common mode and differential mode currents, are prevented from reaching a load that is powered by the VSD 104.
Referring next to
The second PLL 904 circuit is similarly configured as PLL 902, described above, with phase detector 920 comparing the input reference signal SIG with a comparison signal COMP, and outputting an error signal to lag-lead filter 908. The lag-lead filter 908 has S&H circuit 914 controlled by 1-shot circuit 930 and analog switch 916. The lag-lead filter 908 has a low cutoff frequency, as more fully described below. The VCO 932 is input to a divide-by-N circuit 934, which generates the COMP signal input to the phase detector 920, and outputs a second signal indicating the mains voltage d-q axis digital angle output slow response 936.
The control system 900 may be used to retain synchronous operation of a VSD 104 with an active converter 202 to reduce current distortion and eliminate regeneration of energy upon reapplication of the AC input mains voltage 102—for providing extended ride-through capability in the VSD 104. Use of the two PLLs 902, 904 enables the control system 900 to maximize the ability of the active converter 202 to retain the best available knowledge of the line-to-line voltage phase angle at the AC input source 102 under all conditions. The first PLL 902 lag-lead filter 906 has a relatively high filter cutoff frequency and small value integrating capacitor C1. This filter 906 provides the active converter 202 the capability for fast and accurate phase angle tracking under normal converter operating conditions. The filter 906 components include resistor R1, resistor R2 and capacitor C1. In one embodiment the component value for resistor R1 may be 43K ohms, for resistor R2, 120K ohms, and for capacitor C1, 0.47 uF, although the lag-lead filter 906 components R1, R2 and C1 may be varied to adjust the desired cutoff frequency of the filter 906. The second PLL 904 lag-lead filter 908 has a low cutoff frequency, a large value integrating capacitor C2, and resistors R3 and R4. The low cutoff frequency provides the lag-lead filter 908 with the capability for storing the angle of the mains voltage in the feedback loop of the PLL during mains interruption. In one embodiment the typical component values for R3, R4 and C2 may be 510K ohms, 68K ohms and 2.2 uF, respectively. To increase the capability to retain mains phase angle information during a power interruption, each PLL feedback loop 906, 908 includes a sample and hold circuit (S&H) 910, 914 respectively, and analog switch integrated circuits 912, 916 respectively. The S&H circuits 910, 914 with analog switches 912, 916 hold the stored charge on the integrating capacitors C1, C2 within each lag-lead filter 906, 908, and prevent the discharge of the capacitors C1, C2 through leakage to the output of the phase detectors. The component sizing of the ratio R3/R4 is also selected to minimize step change in the voltage fed to the Voltage Controlled Oscillator 932 when the analog switch 916 is transitioned.
The position of each analog switch 912, 916 is controlled by the sensing of the total loss of the mains voltage 102 via the mains voltage detector (or mains present) circuitry. The sample and hold circuits 910, 914 are controlled by the out of lock detectors incorporated into each phase detector. The VCO outputs are fed to divide by n bit counters 926, 934, where n is chosen as a function of the resolution of the phase angle required in the specific application. The counter outputs are then fed back into the second input (denoted COMP) of each phase detector 918, 920 to form a closed loop. The counter outputs are also used to provide a digital word 928, 936 representative of the mains phase angle. The digital words 928, 936 then govern the d-q angle output during mains interruption. Selection of timing to transition the phase angle information is a function of the specific application but ordinarily one would use the mains voltage detector (mains present) circuitry. In one embodiment the PLLs 902, 904 may be implemented using a 74HC7046 integrated circuit manufactured by Phillips Semiconductor Corp. The 74HC7046 integrated circuit includes a state machine type phase detector with out of lock detector and a Voltage Controlled Oscillator. The circuit design allows a power interruption of up to one second in duration without incurring phase error beyond a specified angle under worst-case conditions.
Referring next to
The power loss of the VSD 104 at full-speed operation can be reduced or eliminated by bypassing the VSD 104. Normal losses associated with a conventional VSD typically range from 2 to 3%, and the losses may range as high as 4 to 5% for a VSD that employs an active converter 202. Application of the VSD 104 with the integral bypass contactor 400 for powering an HVAC chiller system provides a significant increase in the full load KW/TR rating of the chiller system. Thus the VSD 104 equipped with an integral bypass contactor 400 chiller provides an efficiency rating comparable with that of a chiller that is not equipped with a VSD 104, resulting in substantial energy savings. The energy savings and higher efficiency ratings are achievable even during periods when the full HVAC capacity is required. By utilizing a VSD 104 having an active converter 202 and integral bypass contactor 400, a contactor that may typically be used for pre-charge means in existing active converter VSDs may now applied to eliminate power losses associated with the VSD 104 during full speed operation. The system of
To provide a smooth transition from VSD to mains or vice versa, three conditions must be present, as follows: 1) the input RMS voltage VIN to the VSD 104 must equal the RMS output voltage to the motor; 2) the input frequency of VIN must match the frequency at the output of VSD 104; and 3) the voltage distortion present at the output of the VSD 104 must be within a predetermined minimum level. The voltage distortion requirement requires that an output L-C filter 404 must be integrated into the VSD, to remove a majority of the output voltage harmonics from the VSDs output. It is also necessary that the control scheme of the VSD 104 integrate two other features as follows: 1) output current-limiting control and 2) sensorless torque control. Output current-limiting control is configured to limit the available output current at sensor I VSD to a predetermined limit. Sensorless induction motor torque control is configured to control the motor torque using sensed parameters IMTR and VMTR. To enable the VSD 104 to lock the output voltage in both phase and frequency to the AC input source 102, the AC input source 102, or VIN, must be detected. Finally, the voltage VDC LINK at the DC link 204 is detected and controlled to a predetermined voltage level to enable the VSD 104 to adjust the RMS motor voltage to match the voltage VIN at the input to the VSD 104.
The system controls are usually implemented in the system control panel 308. When the control panel 308 requires the chiller system compressor/motor 302, 106 to operate within a prescribed range below the AC voltage source frequency, the transition from VSD operation to bypass contactor operation occurs. The range may be prescribed by the plotting the efficiency of the non-VSD equipped chiller against the VSD equipped chiller using the integrated part-load value (IPLV), which is a weighted average of efficiency measurements at various part-load conditions, as described in ARI Standard 550/590-98, and incorporated herein by reference. The frequency range is generally within 1.0 Hz of the maximum frequency. For example, for a 60 Hz line, when operating at 59.0 Hz or above, more efficient operation is obtained by operating directly from the AC voltage source 102. In one embodiment the transition to the bypass contactor 400 does not occur until the chiller system is operating in steady state, i.e. the actual leaving chilled water temperature is within a predetermined band, e.g., plus or minus about 0.2° F., about the leaving chilled water temperature set point.
The reverse operation is set forth in
At step 700, the compressor/motor frequency fMTR is compared with the frequency of the AC input source 102, fAC INPUT, and if the difference is greater than 1.0, at step 705, the control panel sends a command signal to initiate transition from full voltage of the AC input source, to VSD operation. The transition process commences at step 710 as follows. At step 710 the active converter 202 is enabled and VDC LINK is controlled to its nominal set point. At step 720 the output frequency is set to the sensed input frequency of the AC input source 102, and the phase of the output voltage VMTR is set to the phase of the of the AC input source 102. At step 730 the current limit of the VSD 104 is enabled and the current limit level is set to equal the current level of the parameter I MTR. At step 740 the inverter gates are enabled and the DC link voltage is finely tuned until the difference between the VSD current and the motor current, i.e., parameter (I VSD-IMTR)—is minimized. At step 750, the control system determines whether the parameter (I VSD-IMTR) is within a prescribed limit. If so, at step 760 the bypass contactor 400 is opened and the VSD 104 powers the motor load. Otherwise, the control system returns to monitor the difference (I VSD-IMTR).
Each of the upper and lower switches 450 a and 450 b is comprised of two RB IGBTs 454, 456. An RB IGBT is capable of blocking voltages in the reverse as well as the forward direction. A first RB IGBT 454 is connected to an inverse or anti-parallel IGBT 456. The anti-parallel IGBT 456 is also an RB-type IGBT. The anti-parallel IGBT 456 can be controlled, e.g., during a precharge operation of the DC link 204, to permit only small pulses of inrush current to reach the DC link 204. Further, the anti-parallel IGBT 456 can be controlled to conduct current in one direction at all times, similar to the anti-parallel diode 452. The RB IGBT 454 blocks a positive emitter-to-collector voltage that is approximately equal to the peak line-to-line voltage that appears across the IGBT 454. The positive emitter-to-collector voltage remains blocked for as long as the conduction of the anti-parallel IGBT 456 is delayed for the purpose of, precharge. Commonly assigned U.S. Pat. No. 7,005,829 and U.S. Published Pat. App. No. 20060208685, No. 20060196203 & No. 20050122752, disclose various means to implement an active converter module to allow for precharging the DC link of a VSD or a parallel active harmonic filter, and the same are hereby incorporated by reference herein.
When a ground fault current is sensed by the VSD 104, both of the RB IGBTs 454, 456, in each power switch 450 are immediately turned off to preventing any current from conducting to the ground fault. The rapid switching of the RB IGBTs 454, 456 extinguishes the ground fault current in microseconds. By contrast, prior art circuit breaker mechanisms take approximately 40 milliseconds to interrupt the ground fault current.
Referring next to
The active converter 202 ground fault protection eliminates the need for an input circuit breaker equipped with ground fault protection, or with other electro-mechanical means to process the input power. The active converter 460 configuration allows for the use of power fuses rather than more costly circuit breakers to feed the power to the input converter of a VSD, while retaining the ground fault protection feature. Fuses provide a significant reduction in the let-thru energy associated with a line-to-line fault that may occur within the VSD or filter, thereby reducing instances of the semi-conductor package rupture, or of other significant damage incurred in the case of a fault. By utilizing high speed fuses for the power feed, the arc-flash rating of the equipment (see, e.g., the National Fire and Protection Agency (NFPA) regulation 70E) can be significantly reduced. The high-speed fuses reduce the hazard associated with installing, maintaining and repairing the system. By replacing main circuit breakers with fuses at the input of the active inverter, the system can interrupt higher levels of fault current, thus enabling the use of fused-input equipment on much lower impedance mains supplies. The active converter 460 significantly reduces the energy associated with clearing the ground fault, because semiconductors and controls can detect and extinguish the ground current flow in several microseconds, as contrasted with several milliseconds for conventional topologies. The rapid response of the fuses minimizes ancillary damage associated with a ground fault. This advantage may be particularly apparent when used in HVAC&R applications where hermetic motors are employed. A ground fault occurring in the stator winding of a hermetic motor can cause significant and costly damage to the entire refrigeration circuit. Limiting the ground fault current that can flow in a stator limits collateral damage to other components of the HVAC&R system.
Referring now to
To facilitate full operation of the devices in the module, the coolers are capable of operating at a continuous use temperature of approximately 100 degrees centigrade and meet the Underwriters Laboratory approval of plastic material for flammability according to the appropriate standard (UL746A-E). The plastic material used for the coolers 10 has a low level of liquid absorption, is physically durable with a high tensile strength and may be injection molded or machined. Because the power assemblies in which the coolers 10 are mounted are cycled by both temperature and power, the plastic material must exhibit a low temperature coefficient of thermal expansion to avoid wire bond breakage within the semi-conductor module due to a mismatch of coefficient of thermal expansion between the plastic cooler and the copper laminated structures attached to the semi-conductor power device terminals. Also, the plastic cooler 10 acts as a fastener to allow for the attachment of multiple power devices together permitting a single laminated busbar structure to be used to for electrical connections, thereby allowing for a reduction in the size and weight of the overall power assembly. While the plastic material can be obtained from multiple sources, one source for the material is known under the trade name Noryl, Valox or Vespel.
One embodiment of the power assembly, shown in
The plastic cooler 10 is provided upon which an electronic component or module, preferably including several high-speed switches, may be mounted. The plastic cooler shown in
Machined or otherwise formed in plastic cooler 10 are two main fluid channels 12 and 13, whereby a cooling fluid may be introduced into the plate via feed channel 12 and may exit the plate via drain channel 13. In the illustrated embodiment, these channels are relatively large, cylindrical channels that extend along the length of the plastic cooler 10. The channels are sized and designed to have a relatively low pressure drop along their lengths.
At the top of the plastic cooler are found a series of concave wells 20. In the preferred embodiment, wells 20 are surrounded by an O-ring groove 31 into which an O-ring may be placed. The electronic devices to be cooled are then positioned in place over the wells and fastened via mounting holes 11, or other devices or means, whereby a watertight seal is created between the base of the device and the plastic cooler 10 via the o-ring. Preferably, there is an individual well for each individual electronic switch or device to be cooled, and the electronic device is preferably is positioned directly over the well, so that its bottom is placed in direct contact with the cooling fluid.
The wells preferably have a width and length, and shape, designed to match the width, length, and shape of the electronic component to be cooled. For example, in an HVAC application where the electronic components are switches, the wells have a width of approximately 1.5 inches and a length of 3 inches. Cooling fluid enters a well from feed channel 12 through an inlet port 21 formed in the well, flows through the well, and then exits out outlet port 22 and into outlet channel 13. These channels in turn are connected to a heat exchanger for cooling the cooling fluid that exits channel 13.
The plastic cooler 10 and its components are designed to provide optimum heat transfer between the cooling fluid and the electronic components, in an efficient and cost effective manner. Optimum results are achieved with wells having a depth within the range of 0.02 to 0.20 inches, coupled with a hydraulic diameter between 0.05 and 0.20, and with inlets that are 90° nozzles, applying the cooling fluid at an angle of approximately 90° against the surface of the electronic component placed over the well. The hydraulic diameter of the wells is thus defined generally by the following equation: Hydraulic Diameter=4×Cross-sectional area/(2×Well Depth+2×Well Width). The nozzles preferably are located at the end of a well, as shown in the Figures, so that the cooling fluid in effect bounces off both the surface of the electronic component and the walls of the well adjacent the nozzle.
The nozzles promote a high degree of turbulence due to the impingement if cooling fluid on the surface of the electronic component. This turbulence is sustained by the optimal selection of the well depth and hydraulic diameter. A shallower well depth or smaller hydraulic diameter would tend to re-laminarize the flow, thereby decreasing some of the enhancement in heat transfer. On the other hand, a deeper well depth or larger hydraulic diameter would tend to decrease the heat transfer enhancement due to a reduction in the velocity of the fluid adjacent to the surface.
The plastic cooler 10 and its components are also preferably designed such that the pressure drop across the length of the inlet channel 12 is substantially less than the pressure drop across the wells. This is achieved by increasing the size of at least the inlet channel, relative to the size, shape, and flow characteristics of the well and its inlets and outlets, to achieve this relative relationship. Preferably the pressure drop across the length of inlet channel is no greater than 1/10th of the pressure drop across the individual wells. Preferably, each of the wells has the same size, shape, and fluid flow characteristics.
The inlets and outlets of the wells are in the form of elongated slots. These inlet slots have a width, length, and a depth. The resultant slots are designed to serve as nozzles that direct cooling fluid against the bottom surface of the electronic components. Ports 21 and 22 are sufficiently small in comparison to channels 12 and 13 such that no appreciable pressure drop is measurable across the channel 13 as cooling liquid flows into each of the wells 20. As shown in Figure, another embodiment of the inlet and outlet ports is shown whereby the inlet and outlet are actually a plurality of openings 25 formed into either end of the well 20.
The channels 12 and 13 are designed to provide substantially equal pressure along the entire length of both channels, with the result that each well 20 “sees” the same inlet pressure and pressure differential and is capable of having an equal flow and thus an equal cooling capability. The use of channels having these desired characteristics minimizes, and preferably avoids, the problem of reduced flow in each subsequent well that occurs in prior art devices.
As an example, when the wells have a width of approximately 1.291 inches, a length of approximately 4.033 inches, and a depth of approximately 0.05 inches; and when the plastic cooler includes three wells, it has been found that channels 12 and 13 with a diameter of 0.563 inches provides the desired flow and pressure drop characteristics. In that example, the ports 21 and 22 preferably extend along substantially the entire width of the wells and the ports have a nozzle width of approximately 0.094 inches, a length of approximately 0.906 inches, and a depth of at least 0.125 inches.
The ports preferably are formed as elongated slots that extend from the bottom of the well downward to the channels 12 and 13. These slots preferably are perpendicular to the surface of the plastic cooler 10. This combination achieves a more turbulent flow that enhances the heat transfer without significantly impacting pressure drop. The uncomplicated shape of the wells, inlets and channels provides for much easier manufacturing than is associated with other related devices that have wells of varying depths or require the use of obstacles placed in the flow path to enhance the turbulent flow.
Also, by connecting each well 20 directly to the inlet 13 as opposed to having the cooling fluid flow in series from the first well to the last, each well is fed with fresh coolant which maximizes the cooling capability of all of the wells. Similar prior art devices utilize a single path for the coolant such that by the time the coolant reaches subsequent wells, each prior well has transferred heat into the coolant. By the time the coolant reaches the last well in a series such as this, the cooling capability of the coolant is greatly diminished.
The power assembly may operate as single phase for applications that require higher power output levels, or as three phases for applications requiring lower power output levels. Referring now to
It is also anticipated that additional electronic components could be affixed to the plastic cooler on the surface opposite the one with the open wells. Additional open wells are included on the opposite surface, and the heat from the additional power devices is removed by the liquid coolant in the plastic cooler that is in direct contact with the bottom of the device. The additional cooling wells provides cooling to those components in a fashion similar to prior art devices by transferring the heat through the component to the plastic cooler and then to the liquid, but adds the advantage of a very compact overall package.
Referring now to
Core losses are caused by hysteresis losses and eddy current losses. Core losses increase the operating temperature of the core 402 and reduce the efficiency of the inductor 400. The operating temperature of the core 402 has an influence on the other materials used in the inductor 400, such as insulating materials and varnishes. Each material has a maximum operating temperature, and the operating temperature of the core 402 determines the available options for insulating materials. As the operating temperature increases the number of available options for use as insulating materials is reduced, and the costs of the materials is increased. The useful life of the inductor may also be compromised as the operating temperature of the inductor is increased.
The coil 403 subassembly is composed of insulating materials and current carrying conductors. The conductors may be any suitable type of conductive material, e.g. copper and aluminum. Copper conductors have a lower resitivity but a higher cost and weight than aluminum conductors. The sheets of the conductors are typically interleaved with layers of insulating material. The insulating material may be any suitable insulating material e.g. Nomex, ceramic or woven glass fiber. Air ducts are provided between the coil layers to provide for the movement of air, either forced air or natural convection, which removes the heat generated by the losses associated with the coil. The operating temperature of the coil conductors and insulators is ultimately determined by the combination of losses and air movement.
To allow for heat conduction throughout the core 602 including the core gaps, a thermally conductive, non-ferromagnetic material 605 is used to provide a proper magnetic gap, while also allowing for heat transfer across that gap. A material such as a “Grade A Solid Boron Nitride” material manufactured by Saint Gobain Ceramics can be used, however one of ordinary skill in the art would know that any suitable type of material that is easily machinable and provides the necessary thermal conductivity may be used, e.g. aluminum nitride, ceramics manufactured by ANCeram and alumina ceramics manufactured by Astro Met, Inc.
The coil 604 is formed by tightly interleaving layers of aluminum or copper foil with layers of an electrically insulating and thermally conductive material in order to form a low thermal impedance coil subassembly. The heat generated at the coil subassembly is transferred by heat conduction from the coil 604 to the core and subsequently to the heatsink by where it is absorbed by the liquid flow. The electrically insulating but thermally conductive sheets of material are commonly available e.g. Cho-Therm, Therma-Gap, Therm-Attach and Therma-Flow materials manufactured by Chomerics Inc., Sil-Pad and Gap-Pad manufactured by the Berquist Company and similar products produced by Fujipoly Corp. One of ordinary skill in the art will appreciate that any suitable materials that are compatible with the standard insulating varnishes used in conventional inductor manufacturing processes, and that also exhibits tear-through capability with maximum continuous use operating temperatures approaching 200 degrees Celsius may be used. The coil layers are tightly wound around the core leg to provide a thermally conductive path to the core 602.
Conductive connectors are placed between the IGBT and the laminated copper busbar to eliminate concerns with wire bond failures.
Another embodiment includes an active converter module with an integral means to control the pre-charging of the DC link capacitors in the power assembly.
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.