Patent Publication Number: US-9890811-B2

Title: Multiple-axis magnetic bearing and control of the magnetic bearing with active switch topologies

Description:
FIELD OF INVENTION 
     Embodiments relate generally to refrigerant vapor compression systems for residential or light commercial heating and refrigeration applications and, more particularly, to one or more multiple-axis active magnetic bearings (AMB) and control of the AMBs that are connected to a rotor shaft of the refrigerant vapor compression system. 
     DESCRIPTION OF RELATED ART 
     Traditional centrifugal compressors in the heating ventilation and air conditioning (HVAC) industry use roller bearings and hydrodynamic bearings, both of which consume power, require oil, and a lubrication system. Ceramic roller bearings, which avoid issues related to oil and power consumption, have been introduced in these HVAC applications. Ceramic roller bearings are not suitable for compressor speeds above 30,000 revolutions per minute (rpm), as these bearings generate excessive heat at high rpms. Gears operate at high rpms and are typically used in lieu of ceramic roller bearings. While gears are a proven technology, they require more lubrication, create excessive noise and vibration, and consume more power. 
     AMBs have been used more widely in industrial applications as a replacement for conventional lubricated bearings. These industrial applications of the AMB enable machines to spin at very high speeds due to minimal friction. However, the high costs of these AMBs including their controls have limited the wide scale implementation of magnetic bearings, particularly within the HVAC industry. Typically, power amplifiers with varying topologies are utilized to provide energizing currents for controlling the AMBs. But, as more coils are used in these AMBs, additional power amplifiers and associated driver and sensor circuits are required to implement the control algorithms. The additional power amplifiers make implementing the AMB topology more expensive and complex. An improved topology with a reduced number of switches for controlling the AMB would be well received in the art. 
     BRIEF SUMMARY 
     According to one aspect of the invention, a refrigerant vapor compression system including a condenser, an expansion valve, an evaporator, and a compressor coupled to a multiple-axis magnetic bearing system, includes a motor operatively coupled to the compressor via a rotor shaft; the multiple-axis magnetic bearing system including a first active magnetic bearing (AMB) having a first group of electromagnetic actuators electrically coupled to a second AMB having a second group of electromagnetic actuators; and a controller including a three-phase controlling circuit having a plurality of active current switches for controlling each of the first AMB and the second AMB; wherein a first electromagnetic actuator of the first AMB is electrically coupled to a second electromagnetic actuator of the second AMB, each of the first and second electromagnetic actuators coupled to two phase legs of the controlling circuit; and wherein the controller is operable to receive information indicative of a position of the rotor shaft and supply an adjustment signal to the magnetic bearing system to adjust the position of the rotor shaft. 
     According to another aspect of the invention, a magnetic bearing system includes a first active magnetic bearing (AMB) including a first group of electromagnetic actuators coupled to a shaft; a second active magnetic bearing (AMB) including a second group of electromagnetic actuators coupled to the shaft; a controller including a three-phase controlling circuit having a plurality of active current switches for controlling each of the first AMB and the second AMB; wherein a first electromagnetic actuator of the first AMB is electrically coupled to a second electromagnetic actuator of the second AMB, each of the first and second electromagnetic actuators coupled to two phase legs of the controlling circuit; and wherein the controller is operable to receive information indicative of a position of the rotor shaft and supply an adjustment signal to the magnetic bearing system to adjust the position of the shaft. 
     According to another aspect of the invention, a method for controlling a rotor shaft that operatively connects a compressor to a motor in a vapor compression system, includes receiving information indicative of a first position of a first active magnetic bearing (AMB), the first AMB includes a first group of electromagnetic actuators being coupled to the rotor shaft at the first position; receiving information indicative of a second position of a second AMB, the second AMB includes a second group of electromagnetic actuators being coupled to the rotor shaft at the second position; and providing a controller in electrical communication with each of the first AMB and the second AMB; and the controller generating a signal to position the rotor shaft at least one of the first position or the second position; wherein the controller includes a three-phase controlling circuit having a plurality of active current switches for controlling each of the first AMB and the second AMB; and wherein a first electromagnetic actuator of the first AMB is electrically coupled to a second electromagnetic actuator of the second AMB, each of the first and second electromagnetic actuators coupled to two phase legs of the controlling circuit. 
     Other aspects, features, and techniques of the invention will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Referring now to the drawings wherein like elements are numbered alike in the figures: 
         FIG. 1  depicts a schematic view of a refrigerant vapor compression system including a magnetic bearing system according to an embodiment of the invention; 
         FIG. 2  depicts a schematic perspective of a rotor shaft coupled to the magnetic bearing system according to an embodiment of the invention; 
         FIG. 3  depicts a schematic view of a methodology to couple the AMBs to a rotor shaft according to an embodiment of the invention; 
         FIG. 4A  depicts a circuit topology for connecting current switches to the magnetic bearing system according to an embodiment of the invention; 
         FIG. 4B  depicts an alternate circuit topology for connecting current switches to the magnetic bearing system according to an embodiment of the invention; 
         FIG. 5  depicts a schematic diagram of a control algorithm for controlling the magnetic bearing system according to an embodiment of the invention; and 
         FIG. 6  depicts a two-level six-switch circuit topology according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments include a vapor compression-type HVAC system that includes a multiple-axis AMB for supporting a rotor shaft. The rotor shaft connects a compressor to a motor and is constrained by the AMB, which receives one or more energizing signals to adjust the radial position of the rotor shaft. In an exemplary embodiment, the AMB may include a reduced switch multi-phase circuit topology with unidirectional active current switches. This exemplary embodiment with the multi-phase circuit topology reduces the number of diodes and switches that are used to constrain the radial position of the rotor shaft. The HVAC system also includes a controller having a processor for implementing a control algorithm for controlling the bias current used to control the coils in the AMB in order to adjust the position of the rotor shaft. 
     Referring now to the drawings,  FIG. 1  illustrates an exemplary refrigerant vapor compression system  100  including a variable speed motor  118  having a rotor shaft  206  ( FIG. 2 ) that is coupled to a compressor  102  according to an embodiment of the invention. The rotor shaft is supported by a magnetic bearing system  120 , which includes a plurality of AMBs  202 ,  204  (shown in  FIG. 2 ). The compressor  102  includes an impeller/rotor that rotates and compresses liquid refrigerant to a superheated refrigerant vapor for delivery to a condenser  104 . In the condenser  104 , the refrigerant vapor is liquefied at high pressure and rejects heat to the outside air (e.g., via a condenser fan). The liquid refrigerant exiting condenser  104  is delivered to an evaporator  108  through an expansion valve  106 . The refrigerant passes through the expansion valve  106  where a pressure drop causes the high-pressure liquid refrigerant to achieve a lower pressure combination of liquid and vapor. In embodiments, the expansion valve  106  may be a thermostatic expansion valve or an electronic expansion valve for controlling superheat of the refrigerant. As the indoor air passes across the evaporator  108  (e.g., via an evaporator fan), the low-pressure liquid refrigerant evaporates, absorbing heat from the indoor air, thereby cooling the air and evaporating the refrigerant. The low-pressure refrigerant is again delivered to compressor  102  where it is compressed to a high-pressure, high temperature gas, and delivered to condenser  104  to start the refrigeration cycle again. It is to be appreciated that while a specific refrigeration system is shown in  FIG. 1 , the present teachings are applicable to any refrigeration system, including a heat pump, HVAC, and chiller systems. In a heat pump, during cooling mode, the process is identical to that as described hereinabove. In the heating mode, the cycle is reversed with the condenser and evaporator of the cooling mode acting as an evaporator and condenser, respectively. 
     Also shown in  FIG. 1 , vapor compression system  100  includes a compressor  102  driven by a variable speed motor  118  through an inverter drive  114 . In embodiments, the inverter drive  114  may be a variable frequency drive (VFD) or a brushless DC motor (BLDC) drive. Particularly, inverter drive  114  is operably coupled to the compressor  102 , and receives an alternating current (AC) electrical power (for example, a three-phase AC line power at 480V/60 Hz) from a power supply  110  and outputs electrical power on line  116  to a variable speed motor  118 . The variable speed motor  118  shares a common shaft with the compressor  102 . The variable speed motor  118  provides mechanical power to rotate the shaft and compress refrigerant inside the compressor  102 . The rotor shaft is constrained by a magnetic bearing system  120 , which includes a plurality of AMBs  202 ,  204  (shown in  FIG. 2 ) for holding the rotor shaft in position. The rotor shaft is radially constrained by a levitated magnetic cushion generated by the magnetic bearing system  120 . Since the rotor shaft levitates, there is no structure-borne vibration as the air buffer created by the magnetic bearing system  120  prevents the motor  118  from transmitting vibrations to the system  100 . In an embodiment, the magnetic bearing system  120  may include position sensors for detecting the radial position of the magnetically supported rotor shaft along a plurality of radial directions of the rotor shaft. The magnetic bearing system  120  includes a magnetic bearing controller  122  that receives information indicative of the position of the rotor shaft and rotor speed and supplies an adjustment signal to adjust its position via the AMBs  202 ,  204  ( FIG. 2 ). The magnetic bearing controller  122  includes a processor for implementing an algorithm for controlling one or more active current switches needed for adjusting the position of the rotor shaft. The switches selectively produce a magnetic bias current for excitation of the coils in each AMB  202 ,  204  of the magnetic bearing system  120 . The excitation of the coils adjusts the radial position of the rotor shaft in the x and y directions ( FIG. 2 ) based on an actual value of the position of the rotor shaft, detected by position sensors, in the x and y directions. It is to be appreciated that the magnetic bearing system  120  may also be utilized with other rotating machines utilizing a rotor shaft without departing from the scope of the invention. In an embodiment, the variable speed motor  118  may be integrated inside a housing containing the compressor  102 . 
     Inverter drive  114  includes solid-state electronics to modulate the frequency of electrical power on line  116 . In an embodiment, inverter drive  114  converts the AC electrical power, received from supply  110 , from AC to direct current (DC) using a rectifier, and then converts the electrical power from DC to a pulse width modulated (PWM) signal, using an inverter, at a desired PWM frequency in order to drive the motor  118  at a motor speed associated with the PWM DC frequency. For example, inverter drive  114  may directly rectify electrical power with a full-wave rectifier bridge, and may then chop the electrical power using insulated gate bipolar transistors (IGBT&#39;s) or thyristors to achieve the desired PWM frequency. In embodiments, other suitable electronic components may be used to modulate the frequency of electrical power from power supply  110 . Further, a control unit  112  includes a processor for executing an algorithm to control the PWM frequency that is delivered on line  116  to the motor  118 . By modulating the PWM frequency of the electrical power delivered on line  116  to the electric motor  118 , control unit  112  thereby controls the torque applied by motor  118  on centrifugal compressor  102  thereby controlling its speed, and consequently the capacity of compressor  102 . 
       FIGS. 2-3  depict an exemplary magnetic bearing system  120  having four axes for supporting a rotor shaft  206  ( FIG. 2 ) according to an embodiment of the invention. In other non-limiting embodiments, a multiple axis magnetic bearing system may also be used without departing from the scope of the invention. In the example shown in  FIGS. 2-3 , the rotor shaft  206  ( FIG. 2 ) passes through the stator of AMBs  202 ,  204 , which apply deflection forces across the x and y axes in order to adjust the position of the rotor shaft  206 . Also shown, one of two radial control axes, which are perpendicular to each other, is set to be an x axis and the other is set to be a y axis. Particularly shown in  FIG. 2 , the magnetic bearing system  120  includes a rotor shaft  206  being supported by a first active magnetic bearing  202  (hereinafter “AMB  202 ”) and a second active magnetic bearing  204  (hereinafter “AMB  204 ”). In an embodiment, each AMB  202 ,  204  is shown with four electromagnets that provides for a four-axis AMB. But, in another embodiment, a multiple-axis magnetic bearing system may be implemented with the architecture described below without departing from its scope. Each AMB  202 ,  204  includes coaxially aligned coils arranged on a stator. In the four-axis AMB system  120 , the magnetic bearings  202 ,  204  receive bias currents for energizing the coils in order to exert displacement forces on the rotor shaft  206  along the x and y axes of AMBs  202 ,  204  (i.e., along a radial direction). 
       FIG. 3  depicts an AMB  202  having four electromagnetic actuators  306 ,  310 ,  314 , and  318  that exert forces on rotor shaft  206  ( FIG. 2 ) in the x and y directions respectively, the x and y directions being mutually orthogonal to each other. Each of the actuators  306 ,  310 ,  314 ,  318  includes an electromagnet having two-coil windings on a pole shoe or yoke. The actuators  306 ,  310 ,  314 ,  318  are coupled to a plurality of active current switches for amplifying the current supplied to the coils in the electromagnets. Additionally, the active current switches apply forces to control the displacement of the rotor shaft  206  ( FIG. 2 ) in the x and y directions. Actuators  306 ,  310 ,  314 ,  318  couples two windings together for exerting these forces in the x and y directions. Specifically, actuator  306  is coupled to coil windings at x 1 + and x 2 +, actuator  310  is coupled to coil windings at y 1 + and y 2 +, actuator  314  is coupled to coil windings at x 1 − and x 2 −, and actuator  318  is coupled to coil windings at y 1 − and y 2 −. 
     Similarly, the second magnetic radial bearing  204  includes four actuators  308 ,  312 ,  316 , and  320  for exerting forces in the x and y directions respectively, the x and y directions being mutually orthogonal to each other. Each of the actuators  308 ,  312 ,  316 ,  320  includes an electromagnet having windings on a pole shoe or yoke, with actuators  308 ,  312 ,  316 ,  320  coupled to active current switches for exerting the displacement forces in the x and y directions. Specifically, actuator  308  is coupled to coil windings at x 3 + and x 4 +, actuator  312  is coupled to coil windings at y 3 + and y 4 +, actuator  316  is coupled to coil windings at x 3 − and x 4 −, and actuator  320  is coupled to coil windings at y 3 − and y 4 −. In an embodiment, the actuators  306 - 320  are reluctance-type actuators that receive a bias current from the active current switches for energizing the electromagnets in the actuators  306 - 320 , which cause a displacement in the x and y directions. Further, the coils in AMB  202  are coupled to opposite coils in AMB  204  through, in one non-limiting example, a reduced switch multi-phase phase circuit topology. Particularly, the coils in actuator  306  are coupled to coils in actuator  316 , coils in actuator  310  are coupled to coils in an actuator  320 , coils in actuator  314  are coupled to coils in actuator  308 , and coils in actuator  318  are coupled to coils in actuator  312 , as is shown and described with reference to  FIG. 4 . It is to be appreciated that the coupling of AMBs  202  with AMB  204  provides additional degrees of freedom to the control scheme required for the magnetic bearing system  120  whereby actuators coupled together are energized at the same time with a common control command and deenergized at the same time with a different but common control command. 
       FIG. 4A  depicts a reduced switch multi-phase circuit topology  400  used for controlling the coils in AMBs  202 ,  204  ( FIGS. 2-3 ) according to an embodiment of the invention. In an embodiment, the multi-phase circuit topology  400  may be implemented in the controller  122 . The circuit topology  400  includes active current switches  402 ,  404 ,  406  that couple coils in actuator  306  to coils in actuator  316 . Similarly, active current switches  428 ,  430 ,  432  couple coils in actuator  310  to coils in actuator  320 , switches  434 ,  436 ,  438  couple coils in actuator  314  to coils in actuator  308 , and active current switches  440 ,  442 ,  444  couple coils in actuator  312  to coils in actuator  318 . The active current switches  402 - 406 ,  428 - 432 ,  434 - 438 , and  440 - 444  create a reluctance-type actuator that supplies the bias currents to the magnetic bearing system  120 . In an embodiment, the active current switches  402 - 406 ,  428 - 432 ,  434 - 438 , and  440 - 444  are IGBTs, although MOSFETs, or other similar types of high-voltage power amplifiers may be utilized in other embodiments. In a non-limiting example shown implemented for phases  418 ,  420 ,  422 , the reduced switch topology  400  includes three high-voltage bi-directional active current switches  402 ,  404 , and  406  and three diodes  412 ,  414 , and  416  that are connected to one of three phases  418 ,  420 , and  422 . Specifically, an active current switch  402  and a diode  412  are connected in series in a first phase  418  to a positive DC voltage rail  424  and DC ground  426 . The diode  412  is inserted with its anode connected to DC ground  426  (i.e., diode  412  is in its conduction direction). Similarly, an active current switch  404  and a diode  414  are connected in series in a second phase  420  to the positive DC voltage rail  424  and DC ground  426  with the diode  414  inserted opposite to the direction of diode  412 , and an active current switch  406  and a diode  416  are connected in series in a third phase  422  to the positive DC voltage rail  424  and DC ground  426  with the diode  416  inserted in the same direction as diode  412  (i.e., in the direction of positive current). The design  400  also includes actuator  306  connected to phase  418  and phase  420 , and actuator  316  connected to phase  420  and phase  422 . In electrical terms, the actuators  306 ,  316  may be represented essentially by a large inductance. Also, the active current switches  402 ,  404 ,  406  are driven by means of modulation techniques like pulse-width modulation (PWM) in order to produce gating pulses to periodically turn ON and OFF active current switches  402 ,  404 ,  406  and control the time averaged current through each switch  420 ,  404 ,  406 . 
     Similarly, active current switches  428 ,  430 ,  432  couples coils in actuator  310  to coils in actuator  320  with diodes  452 ,  454 ,  456  along phases  446 ,  448 ,  450 ; switches  434 ,  436 ,  438  couples coils in actuator  314  to coils in actuator  308  with diodes  464 ,  466 ,  468  along phases  458 ,  460 ,  462 ; and active current switches  440 ,  442 ,  444  couples coils in actuator  312  to coils in actuator  318  with diodes  476 ,  478 ,  480  along phases  470 ,  474 ,  476 . 
     In an embodiment, controlling the magnetic bearing system  120  by turning ON the switches such as, for example active current switch  402  energizes the actuator  306  by causing a current to flow through the active current switch  402  and into the actuator  306 . It is to be appreciated that the control scheme for controlling active current switches  402 - 406 ,  428 - 432 ,  434 - 438 , and  440 - 444  may be employed through a PWM technique, which turn the active current switches  402 - 406 ,  428 - 432 ,  434 - 438 , and  440 - 444  ON and OFF at about 10 KHz to about 20 KHz, which is a smaller time constant than the mechanical time constant of the system  100 . 
     An exemplary operation, with reference to  FIG. 4A , for energizing and deenergizing actuators  306 ,  316  coupled to phases  418 ,  420 ,  422  in order to exert radial deflection forces in the x and y directions is described below. Initially, all three active current switches  402 ,  404 ,  406  are OFF. In a first cycle, coils connected to x 1 + and x 2 + (i.e., actuator  306 ) are energized in order to deflect the shaft  206  ( FIG. 2 ) in the x 1 + and x 2 + directions. In order to energize actuator  306 , active current switches  402 ,  404  are turned ON at the same time while active current switch  406  is OFF. In this state, current flows from positive DC voltage rail  424  into active current switch  402  of phase  418 , into actuator  306 , into active current switch  404  of phase  420 , and into DC ground  426 . The actuator  306  acts as a large inductance, i.e., current will gradually build up in the actuator  306 . Once the desired bias current is reached, active current switch  402  is turned OFF in order to deenergize the coils in actuator  306  and control the deflection in both x 1 + and x 2 + directions. The current now flows in a closed loop through the remaining closed active current switch  404 , actuator  306 , and diode  412 . The current may be increased by turning ON both active current switches  402 ,  404  or decreased by turning OFF active current switch  402  in order to regulate the current in the actuator  306  and exert a deflection force in x 1 + and x 2 +. Similarly, current in actuator  316  may also be controlled. To energize the coils connected to x 3 − and x 4 − in a first cycle, active current switches  406 ,  404  are turned ON, at the same time, while active current switch  402  is OFF. In this state, current flows from positive DC voltage rail  424  into active current switch  406  of phase  422 , into x 4 − and x 3 − (i.e., actuator  316 ), into active current switch  404  of phase  420 , and into DC ground  426 . The actuator  316  acts as a large inductance, i.e., current will gradually build up in the actuator  316 . Once the desired bias current is reached, active current switch  406  is turned OFF in order to deenergize the coils in actuator  316 . The current now flows in a closed loop through the remaining closed active switch  404 , diode  416 , and actuator  316 . The current may be increased by turning ON both active current switches  406 ,  404  or decreased by turning OFF active current switch  406  in order to regulate the current in the actuator  316 . Similarly, in other embodiments, actuators  310 ,  320  may be controlled using the methodology described above using active current switches  428 ,  430 ,  432  and diodes  452 ,  454 ,  456 ; actuators  314 ,  308  may be controlled using active current switches  434 ,  436 ,  438  and diodes  464 ,  466 ,  468 ; and actuators  312 ,  318  may be controlled using active current switches  440 ,  442 ,  444 , and diodes  476 ,  478 ,  480 . The turn ON and turn OFF events happen at switching frequencies of around 20 KHz to regulate the current in the coils. 
       FIG. 4B  depicts an alternate embodiment of a reduced switch multi-phase circuit topology  450  used for controlling the coils in AMBs  202 ,  204  ( FIGS. 2-3 ) while all other aspects remain substantially the same as those described with reference to  FIG. 4A . In one non-limiting described example for phases  452   a ,  454   a ,  456   a , the circuit topology  450  includes respective active current switches  458   a ,  460   a ,  462   a . The active current switches  458   a ,  460   a ,  462   a  couples coils in actuator  312  to coils in actuator  318 . Also, the reduced switch topology  450   a  includes three diodes  464   a ,  466   a ,  468   a  that are connected to one of three phases  452   a ,  454   a , and  456   a , respectively. As shown, the two active current switches  458   a ,  462   a  are electrically coupled directly to DC ground  426  and active current switch  460   a  is coupled directly to the positive DC voltage rail  424 . Similar to the embodiment shown as described with reference to  FIG. 4A , the active current switches  458   a ,  460   a ,  462   a  are driven by means of modulation techniques like pulse-width modulation (PWM) in order to produce gating pulses to periodically turn ON and OFF active current switches  458   a ,  460   a ,  462   a  and control the time averaged current through each switch  458   a ,  460   a ,  462   a.    
       FIG. 5  illustrates a schematic of a control algorithm  500  implemented by the magnetic bearing controller  122  for controlling the magnetic bearing system  120  ( FIG. 2 ) according to an embodiment of the invention. The magnetic bearing controller  122  implements an algorithm  500  that adaptively controls the excitation currents in order to energize the actuators  306 - 320  ( FIG. 3 ) in its active region and avoid “negative currents”. The controller  122  includes a preprogrammed microprocessor for executing instructions stored in a computer readable medium. In an embodiment, the computer readable medium may be a ROM, an EPROM or other suitable data storage device. The excitation current includes a bias current and a control current. For ease of explanation, the active current injection methodology will be described with reference to the reduced switch multi-phase topology  400  for controlling the energizing current in the actuators  306 ,  316  ( FIG. 4A ). It is to be appreciated that while aspects of the control algorithm is described with reference to a circuit, the algorithm  500  may be implement by software of a microprocessor to provide controlling signals for the magnetic bearing system  120 . 
     In an embodiment, output voltage signals are received on line  502  from position sensors (not shown) coupled to the AMBs  202 ,  204 . The voltage signals represent the real position of the rotor  206  along x 1 +, x 2 +, x 3 −, and x 4 − directions. The voltage signals are filtered by system  500  and sent to a comparator  504 . The comparator  506  compares the filtered voltage signal with a position reference value  504  to produce an error value to be applied to a control circuit. In one non-limiting embodiment, a proportional integral differential (PID) circuit  508  may be used, but other similar circuits may also be utilized. The output of the PID circuit  508  is a control current that is supplied to circuit paths  510 ,  512 . Circuit path  510  represents the path for controlling switch  402  in phase  418  ( FIG. 4A ) while circuit path represents the path for controlling switch  406  in phase  422  ( FIG. 4A ). The control current from PID  508  is added to a bias current value  514  in an adder circuit  516  while the control current from PID  508  is subtracted from the bias current value  514  in a subtracter circuit  518 . The output value from the adder  516  is compared with a feedback current  520  in comparator  522  while the output value from the subtracter circuit  518  is compared with the feedback current  524  in comparator circuit  526 . Feedback currents  520 ,  524  represent the total scaled current that passes through respective phases  418 ,  422  ( FIG. 4A ), which includes the bias current and the control current. The error signal from comparator  522  is sent to a PID  528  while the error signal from the comparator  526  is sent to a PID  530 . The PID&#39;s  528 ,  530  output control voltages that control the respective switches  402 ,  406  in respective phases  418 ,  422  ( FIG. 4A ) for exciting the actuators  306 ,  316  ( FIG. 4A ). Additionally, the output voltages from PID&#39;s  528 ,  530  are sent to a comparator circuit  532 , which determines a scaling factor α to be applied to the output voltages from PID&#39;s  528 ,  530 . The scaling factor α enables a full coupling between the actuators  306 ,  316  ( FIG. 4A ) and increases the bandwidth of the magnetic bearing system  120 . The comparator circuit  532  sends its output to a gain amplifier  534  and to a second attenuator  536 , which determines the current values in phases  418 ,  420 ,  422  ( FIG. 4A ) and a scaling factor α according to the expression (1):
 
 b =−α( S )*( a+c )  (1)
         where:   α(S)=scaling factor;   b=current in phase  420  ( FIG. 4A );   a=current in phase  418  ( FIG. 4A );   c=current in phase  422  ( FIG. 4A ); and   0&lt;α(S)&lt;1.       

     Upon receiving the output voltages from PID&#39;s  528 ,  530  and the scaling factor α(S) from attenuator  536 , the three-phase current controller  538  controls the corresponding currents in phases  418 ,  420 ,  422  by outputting control voltages  540 ,  542  having transfer functions v 1 (s), v 2 (s), respectively. The control voltages  540 ,  542  control the respective actuators  306 ,  316  ( FIG. 4A ) according to the expressions (2) and (3) for the transfer functions v 1 (s), v 2 (s): 
     
       
         
           
             
               
                 
                   
                     
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       FIG. 6  depicts an alternate embodiment of a reduced switch multi-phase circuit topology  600  used for controlling the coils in AMBs  202 ,  204  with the algorithm  500  that was discussed with reference to  FIG. 5 . In one non-limiting described example for phases  602 , the circuit topology  600  includes respective active current switches  608 ,  610  connected in series to positive DC rail  622  and ground  624 . Similarly, switches  612 ,  614  are connected in series to phase  604 ; and switches  616 ,  618  are connected in series to phase  606 . The active current switches  608 ,  610 ,  612 ,  614  are coupled to actuator  202  while active current switches  612 ,  614 ,  616 ,  618  are coupled to actuator  204 . Similar to the embodiment shown as described with reference to  FIG. 4A , the active current switches  608 - 618  are driven by means of modulation techniques like pulse-width modulation (PWM) in order to produce gating pulses to periodically turn ON and OFF active current switches  608 - 618  and control the time averaged current through each switch  608 - 618 . It is also to be appreciated that the reduced switch multi-phase circuit topology  600  requires a reduced number of active switches over the H-bridge circuit topologies in order to control the magnetic bearing system  120  ( FIG. 2 ) while also optimizing bearing stiffness, and preventing negative currents in the magnetic bearing system  120 . 
     The technical effects and benefits of exemplary embodiments include a vapor compression-type HVAC system that utilizes a multiple-axis magnetic bearing system for supporting a shaft connecting a compressor with a motor. The multiple-axis magnetic bearing system may include a reduced switch multi-phase circuit design with unidirectional active current switches and diodes in order to reduce the number of diodes and switches being used. Also, a control algorithm is provided for implementation by a controller for controlling the bias current used to control the coils in actuators for the magnetic bearing system. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. While the description of the present invention has been presented for purposes of illustration and description, it is not intended to be exhaustive or limited to the embodiments disclosed. Exemplary embodiments are described with reference to a compressor and a vapor compression system, but embodiments of the invention should not be considered as limited to such. Many modifications, variations, alterations, substitutions, or equivalent arrangement not hereto described will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. Additionally, while the various embodiments of the invention have been described with reference to a compressor and a vapor compression system, it is to be understood that aspects of the invention should not be considered to be limited to such a reference. Accordingly, the invention is not to be seen as defined by the foregoing description, but is set forth by the scope of the appended claims.