Transport refrigeration system and method of operation

A refrigeration system for a transport refrigeration unit for cooling a temperature controlled cargo space, said refrigeration system comprising:

• a primary refrigerant circuit including a refrigerant compression device, a refrigerant heat rejection heat exchanger downstream of said compression device, a refrigerant heat absorption heat exchanger downstream of said refrigerant heat rejection heat exchanger, and a primary expansion device disposed downstream of said refrigerant heat rejection heat exchanger and upstream of said refrigerant heat absorption heat exchanger;
• a motor operatively associated with said compression device for driving said compression device;
• a variable speed drive operatively associated with said motor for varying the speed of operation of said compression device;
• and a controller operatively associated with said variable speed drive and said compression device for controlling a cooling capacity of the refrigeration system by selectively controlling the speed of said compression device wherein the controller is configured to operate the variable speed drive in modes including:
• a pull down mode in which the temperature controlled cargo space is at a temperature higher than a desired temperature and the controller is configured to cool the temperature controlled cargo space rapidly using maximum cooling capacity of the system by operating the compression device in a continuous run mode;
• a capacity mode of operation in which the controller is operable to maintain the temperature in the temperature controlled cargo space within a temperature range by operating the compression device in a power on/off cycling mode at a fixed speed;
• a part load mode of operation, in which the controller is operable to operate the compression device at a cooling capacity that is less than a maximum cooling capacity by reducing the speed of the compression device to a speed less than the maximum seed of the compression device, the controller being configured to operate the compression device in a continuous run, variable speed mode in a first portion of the part load mode and a power on/off cycling run, fixed speed mode in a second portion of the part load mode.