What is an electromagnetic relay that typically closes the circuit to the compressor called?

The correct answer is a contactor. In HVAC systems, the contactor is an electromechanical switch that is used to control the power flow to the compressor and other components. When the control circuit sends a signal, the contactor closes the circuit, allowing electrical power to flow to the compressor and enabling it to operate.

Contactors are specifically designed to handle the high voltages and currents associated with HVAC systems, making them essential for reliable compressor operation. They serve as the intermediary between the control system and the compressor, effectively allowing low-voltage control signals to operate high-voltage equipment safely.

The other options refer to different devices that do not serve the same function. A controlling relay may serve to manage various operations within a system but is not typically used specifically for energizing the compressor like a contactor. Transformers are used to change voltage levels within an electrical system, and switch relays may control various circuits but do not specifically refer to the task of energizing the compressor in air conditioning systems.