Vfd control for multiple circuit refrigeration system

Abstract

A refrigeration system includes a first refrigerant circuit having a first compressor, and a second refrigerant circuit having a second compressor, the second refrigerant circuit separate from the first refrigeration circuit. A variable frequency drive (VFD) is in signal communication with the first compressor and the second compressor and is configured to enable a variable speed operation of the first and second compressors. A controller is in signal communication with the VFD. The controller is programmed to selectively switch the first and second compressors between a variable speed operation using the VFD and a fixed speed operation such that in a first mode the first compressor is operated in the fixed speed operation and the second compressor is operated in the variable speed operation, and in a second mode the second compressor is operated in the fixed speed operation and the first compressor is operated in the variable speed operation.

1 . A refrigeration system comprising:
 a first refrigerant circuit having a first compressor;   a second refrigerant circuit having a second compressor, the second refrigerant circuit separate from the first refrigeration circuit;   a variable frequency drive (VFD) in signal communication with the first compressor and the second compressor, the VFD configured to enable a variable speed operation of the first and second compressors; and   a controller in signal communication with the VFD, the controller programmed to selectively switch the first and second compressors between a variable speed operation using the VFD and a fixed speed operation such that in a first mode the first compressor is operated in the fixed speed operation and the second compressor is operated in the variable speed operation, and in a second mode the second compressor is operated in the fixed speed operation and the first compressor is operated in the variable speed operation.   
     
     
         2 . The refrigeration system of  claim 1 , wherein the refrigeration system only comprises a single VFD. 
     
     
         3 . The refrigeration system of  claim 2 , wherein the first compressor and the second compressor are the only compressors in signal communication with the single VFD. 
     
     
         4 . The refrigeration system of  claim 1 , further comprising a transport container comprising a first cargo compartment and a second cargo compartment, wherein the first refrigerant circuit is thermally coupled to the first cargo compartment and the second refrigerant circuit is thermally coupled to the second cargo compartment. 
     
     
         5 . The refrigeration system of  claim 4 , wherein the first cargo compartment is a frozen cargo compartment and the second cargo compartment is a perishable cargo compartment. 
     
     
         6 . The refrigeration system of  claim 4 , wherein the controller is programmed to switch the first and second compressors between the variable speed operation and the fixed speed operation when a predetermined condition occurs. 
     
     
         7 . The refrigeration system of  claim 6 , wherein the predetermined condition is when a temperature in at least one of the first and second cargo compartments reaches a predetermined setpoint temperature. 
     
     
         8 . The refrigeration system of  claim 6 , wherein the predetermined condition is when an ambient temperature reaches a predetermined temperature. 
     
     
         9 . The refrigeration system of  claim 4 , further comprising a first sensor disposed in the first cargo compartment and a second sensor disposed in the second cargo compartment, the first and second sensors configured to send respective signals to the controller indicative of at least one condition in the respective first and second cargo compartments, wherein the controller is programmed to switch the first and second compressors between the variable speed operation and the fixed speed operation based on the signals. 
     
     
         10 . A method of controlling a temperature in a first compartment and a second compartment with a refrigeration system comprising a first refrigerant circuit having a first compressor, a second refrigerant circuit having a second compressor, a variable frequency drive (VFD) in signal communication with the first compressor and the second compressor, and a controller in signal communication with the VFD, the method comprising:
 operating, with the controller and the VFD, the refrigeration system in a first mode such that the first compressor is operated at a variable speed and the second compressor is operated at a fixed speed;   monitoring the first and second compartments for the occurrence of a predetermined condition; and   switching, with the controller and the VFD, upon the occurrence of the predetermined condition, the refrigeration system to operate in a second mode such that the second compressor is operated at a variable speed and the first compressor is operated at a fixed speed.   
     
     
         11 . The method of  claim 10 , further comprising operating only one of the first and second compressors at the variable speed during each of the first and second modes. 
     
     
         12 . The method of  claim 10 , further comprising switching between the first and second modes with only a single VFD. 
     
     
         13 . The method of  claim 10 , wherein the step of monitoring the first and second compartments comprises monitoring a first sensor and a second sensor respectively disposed in the first compartment and the second compartment for the occurrence of a predetermined condition. 
     
     
         14 . The method of  claim 13 , wherein the predetermined condition is when a monitored temperature in at least one of the first and second cargo compartments reaches a predetermined setpoint temperature. 
     
     
         15 . The method of  claim 13 , wherein the predetermined condition is when an ambient temperature reaches a predetermined temperature.