I am doing project for refrigeration cycle . I notice that the suction pressure is regulated by speed control of the compressors which are driven by Gas Turbines . Why is that ??

What is the relation between condensing , evaporating temperature and suction pressure because I noticed from data that the speed control increases on afternoon(11 am -12 pm ) but at night or morning it decreases why is that ?

Also , the suction flow rate is higher in the afternoon that at night or morning !