At the moment of compressor start-up, a significant pressure differential may exist between the low-pressure suction side and the high-pressure discharge side. Without any pressure equalization, the compressor motor must overcome this high initial load, which can cause electrical strain, high inrush current, and potential mechanical shock. To address this, many Semi-Hermetic Compressors are designed with an internal pressure equalization mechanism—such as a bypass channel, pressure equalization port, or integrated unloader valve. This system momentarily allows gas from the discharge chamber to flow into the suction chamber, effectively reducing the pressure differential. By minimizing the pressure gap, the motor encounters less resistance when beginning rotation, resulting in a smoother, more energy-efficient start with reduced wear on electrical and mechanical components.

Once the motor reaches its normal operating speed and internal pressures stabilize, the pressure equalization mechanism disengages or closes. At this point, the compressor operates under standard conditions, maintaining a full differential between the high-pressure and low-pressure sides to drive effective refrigerant compression. In designs that use unloader valves, these valves are re-engaged to fully compress the suction gas. This controlled transition ensures that the pressure relief function does not interfere with cooling performance or efficiency during steady-state operation. The mechanism is therefore transient, active only during critical startup and shutdown windows.

After the compressor shuts down, residual high pressure may remain in the discharge chamber while the suction side returns to low pressure. If left unbalanced, this pressure differential can cause "hard start" conditions in the next operational cycle. To mitigate this, the compressor may include an automatic pressure equalization port or delayed bleed valve that allows pressures on both sides of compressor to equalize gradually once the motor stops. This post-shutdown equalization protects against high restart torque, reducing mechanical wear and avoiding nuisance tripping of protective devices. It also ensures the compressor is ready for the next cycle without requiring excessive electrical load to overcome trapped pressure.

In more advanced Semi-Hermetic Compressor systems, the pressure equalization mechanism may be integrated with external or onboard control logic. For example, start relays or programmable logic controllers (PLCs) may activate a solenoid valve or time-delay relay that opens the equalization port for a predetermined duration before initiating full-speed motor operation. In tandem with soft starters or variable frequency drives (VFDs), this ensures pressure equalization is completed precisely and reliably. The result is a coordinated, low-impact start-up sequence that optimizes compressor longevity and operational stability.