Question 48 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is C **Why option C ("Check the operation of the unloaders.") is correct:** Squealing drive belts (V-belts) indicate that the motor is having difficulty turning the compressor pump, causing the belts to slip against the pulley sheave walls. A compressor motor needs to start the pump under minimal load to prevent excessive current draw and belt slippage. The purpose of the unloaders (often referred to as head unloaders or intake valve unloaders) is specifically to release the pressure trapped on top of the pistons or in the cylinder head space when the compressor stops, allowing the motor to start against zero pressure. If the unloaders fail to open or operate correctly upon shutdown, the compressor tries to start while the cylinder head is pressurized (a "loaded start"). This significantly increases the required starting torque, leading to belt slippage and the resulting squealing noise. **Why the other options are incorrect:** * **A) Check for a receiver outlet valve which may be partially closed:** The condition of the downstream outlet valve (whether fully open, partially closed, or fully closed) affects the *demand* for air and the *rate* at which the receiver pressure builds, but it does not change the physical pressure burden placed on the compressor pump during the critical startup sequence. The pump is protected from starting against receiver pressure by the check valve and the unloaders, regardless of the outlet valve position. * **B) Check the air filter:** A clogged air filter restricts airflow into the compressor, potentially reducing volumetric efficiency or causing overheating, but it does not create the massive initial mechanical resistance required to cause the drive belts to slip violently during start-up. * **D) Check for a defective high-pressure cut-out switch:** The high-pressure cut-out switch is a safety device that stops the motor if the receiver pressure exceeds a safe limit. While a defect could cause the compressor to run too long, it is a downstream pressure control issue, not a mechanism that creates excessive starting torque or belt squeal during the motor’s initial rotation.