What is used to control capacity on commercial ac compressors?

• A. Unloaders
• B. Unloader Valves
• C. Variable Frequency Drive
• D. Capacitors

Answer: (A)

Capacity control for large air-conditioning compressors is achieved by unloading some of the compressor’s internal capacity. Unloaders reduce the effective displacement of the compressor during part-load operation, allowing the system to run at lower capacity instead of cycling the compressor on and off. This smooths operation and saves energy while maintaining enough cooling when demand is lower. When more cooling is needed, the unloaders are re-engaged to bring the compressor back to full capacity.

A variable frequency drive changes motor speed to alter capacity, which is a different method from unloading, and capacitors are used for starting or power factor/running torque rather than capacity modulation. Unloader valves can perform a similar idea in some systems, but the common term used for this capacity-reduction mechanism in commercial compressors is unloaders.