Integrated Model, MATLAB/SIMULINK, Compressor, Multi-physics, Dynamics & Controls, Valves
The performance of a refrigerator is largely guided by the efficiency of its drive unit, namely, the compressor & understanding the compressor behavior requires a detailed study of its dynamics, flow-thermals, electrical and controls aspects. Having a simulation model which captures these physics reasonably well is a critical part of the design and performance prediction of a compressor. The current paper describes a systematic approach of making a system level simulation framework by first developing individual models and then integrating them into a single framework to capture the multi-physics interaction of the different sub-components. This framework, developed in MATLAB/SIMULINK contains five modules, namely, Dynamics, Thermal, Motor, Controls and Post-processor. Dynamics is modeled as a spring-mass system with adjustable static equilibrium and head-crash prevention algorithm. The thermodynamics model essentially captures the valve physics. The valves in a reciprocating compressor contribute to pressure losses (pressure profile deviation from ideal suction & discharge processes, valve dynamics, leakages, pressure pulsations) and thermal losses (refrigerant back-flow caused by incorrect valve timing). The starting point of simulating these details is considering the gas dynamics and coupling it to the valve motion. The prediction of this model is validated against test data of a baseline compressor. As part of the integrated design framework, a permanent magnet motor is simulated as a resistance-inductance network with a series, velocity dependent voltage. To impress the desired operating conditions (capacity, stroke, clearance etc.) upon the integrated system model, a set of controllers were designed to control the motor.