Start-stop control systems and methods for gas foil bearing machine
An HVAC system includes an unloading device, a centrifugal compressor, a gas foil bearing, a VFD and a controller. The controller is programmed to start the centrifugal compressor from a stopped condition by operating the unloading device to remove a load from the centrifugal compressor, accelerating the motor to a first speed above a liftoff speed of the gas foil bearing and below an operating speed of the centrifugal compressor, running the motor for a period of time, operating the unloading device to apply the load to the centrifugal compressor, and accelerating the motor to the operating speed.
1. An HVAC system comprising:
an unloading device; and
a centrifugal compressor comprising:
a compressor housing;
a motor having a driveshaft rotatably supported within the compressor housing;
an impeller connected to the driveshaft and operable to compress refrigerant gas upon rotation of the driveshaft;
a gas foil bearing supported by the compressor housing and supporting the driveshaft; and
a controller connected to the motor and the unloading device, the controller programmed to:
start the centrifugal compressor from a stopped condition by:
operating the unloading device to remove a load from the centrifugal compressor,
accelerating the motor to a first speed above a liftoff speed of the gas foil bearing and below an operating speed of the centrifugal compressor,
running the motor at the first speed for a period of time,
operating the unloading device to apply the load to the centrifugal compressor, and
accelerating the motor to the operating speed.
2. The HVAC system of claim 1 , wherein the controller is further programmed to stop the centrifugal compressor from an operating condition by:
operating the unloading device to remove a load from the centrifugal compressor,
decelerating the motor toward a minimum speed greater than zero, and
removing power from the motor when the speed of the motor reaches the minimum speed and allowing the motor to coast to a stop.
3. The HVAC system of claim 1 , wherein the period of time is a predetermined period of time.
4. The HVAC system of claim 1 , wherein the period of time is a variable period of time that begins when the motor reaches the first speed and ends when surging of the centrifugal compressor stops.
5. The HVAC system of claim 4 , further comprising a current sensor to sense a current of the motor, wherein the controller determines that surging of the centrifugal compressor has stopped when the sensed current of the motor is a substantially constant current.
6. The HVAC system of claim 1 , wherein the controller accelerates the motor to the first speed at a first acceleration and accelerates the motor to the operating speed at a second acceleration that is less than the first acceleration.
7. The HVAC system of claim 1 , wherein the minimum speed comprises an estimated surge speed below which surging of the centrifugal compressor may occur plus a margin.
8. The HVAC system of claim 7 , wherein the estimated surge speed is retrieved by the controller from a lookup table and varies with a pressure ratio of the HVAC system.
9. The HVAC system of claim 7 , wherein the minimum speed is retrieved by the controller from a lookup table and varies with a pressure ratio of the HVAC system.
10. The HVAC system of claim 1 , wherein the unloading device comprises a refrigerant bypass valve.
11. A controller for controlling a centrifugal compressor with a gas foil bearing supporting a shaft of an impeller driven by a motor, the controller comprising:
a VFD including a motor interface for connection to the motor;
an unloading interface for connection to an unloading device;
a processor; and
a memory containing instructions that, when executed by the processor, cause the controller to:
start the centrifugal compressor from a stopped condition by:
operating the unloading device to remove a load from the centrifugal compressor,
accelerating the motor to a first speed above a liftoff speed of the gas foil bearing and below an operating speed of the centrifugal compressor,
running the motor at the first speed for a period of time,
operating the unloading device to apply the load to the centrifugal compressor, and
accelerating the motor to the operating speed.
12. The controller of claim 11 , wherein the memory containing instructions that, when executed by the processor, further cause the controller to stop the centrifugal compressor from an operating condition by:
operating the unloading device to remove a load from the centrifugal compressor,
decelerating the motor toward a minimum speed greater than zero, and
removing power from the motor when the speed of the motor reaches the minimum speed and allowing the motor to coast to a stop.
13. The controller of claim 10 , wherein the period of time is a variable period of time that begins when the motor reaches the first speed and ends when surging of the centrifugal compressor stops.
14. The controller of claim 13 , further comprising a current sensor interface to receive a signal representing a current of the motor from a current sensor, wherein the controller determines that surging of the centrifugal compressor has stopped when the sensed current of the motor is a substantially constant current.
15. The controller of claim 1 , wherein the controller accelerates the motor to the first speed at a first acceleration and accelerates the motor to the operating speed at a second acceleration that is less than the first acceleration.
16. The controller of claim 1 , wherein the minimum speed comprises an estimated surge speed below which surging of the centrifugal compressor may occur plus a margin.
17. The controller of claim 16 , wherein the estimated surge speed or the minimum speed is retrieved by the controller from a lookup table and varies with a pressure ratio of the HVAC system.
18. A method of controlling a centrifugal compressor with a gas foil bearing supporting a shaft of an impeller driven by a motor, the method comprising:
starting the centrifugal compressor from a stopped condition by:
operating an unloading device to remove a load from the centrifugal compressor,
accelerating the motor to a first speed above a liftoff speed of the gas foil bearing and below an operating speed of the centrifugal compressor,
running the motor at the first speed for a period of time,
operating the unloading device to apply the load to the centrifugal compressor, and
accelerating the motor to the operating speed.
19. The method of claim 18 , further comprising stopping the centrifugal compressor from an operating condition by:
operating the unloading device to remove a load from the centrifugal compressor,
decelerating the motor toward a minimum speed greater than zero, and
removing power from the motor when the speed of the motor reaches the minimum speed and allowing the motor to coast to a stop.
20. The method of claim 18 , wherein the period of time is a variable period of time that begins when the motor reaches the first speed and ends when surging of the centrifugal compressor stops.
21. The method of claim 20 , further comprising:
receiving a signal representing a current of the motor from a current sensor; and
determining that surging of the centrifugal compressor has stopped when the sensed current of the motor is a substantially constant current.
22. The method of claim 18 , wherein accelerating the motor to a first speed comprises accelerating the motor to the first speed at a first acceleration, and accelerating the motor to the operating speed comprises accelerating the motor to the operating speed at a second acceleration that is less than the first acceleration.
23. The method of claim 18 , wherein the minimum speed comprises an estimated surge speed below which surging of the centrifugal compressor may occur plus a margin, and the method further comprises retrieving the estimated surge speed or the minimum speed from a lookup table.