Question 61 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is B **Explanation of Option B (Correct):** Option B is correct because the "Hand" (or Manual) mode on a typical industrial air compressor control station is designed to bypass the automatic pressure sensing and cycling controls (the pressure switch). When the selector is placed in "Hand" mode, the motor runs continuously until the operator physically moves the switch back to "Off" or "Auto." To prevent the pressure from building up indefinitely and damaging the system, the compressor mechanism itself switches to an **unloading** state once the high-pressure setpoint is reached. In the unloading state, the motor is running, but the intake valves are held open or the inlet throttle is closed, meaning the compressor is running but not creating pressurized air (or is venting the created air), thus allowing continuous running without over-pressurization. **Why the Other Options Are Incorrect:** * **A) In the off mode, the compressor will cycle on and off.** * **Incorrect.** In the "Off" mode, the power circuit to the motor starter is intentionally interrupted. The compressor motor will not run or cycle under any circumstance in this position. * **C) In the auto mode, the compressor will run continuously with unloading.** * **Incorrect.** In the "Auto" (Automatic) mode, the compressor operates based on system demand monitored by the pressure switch. It will **cycle** (start and stop) to maintain pressure between the low-pressure setpoint (cut-in) and the high-pressure setpoint (cut-out). Continuous running with unloading is characteristic of the "Hand" mode, or certain specialized variable speed drive (VSD) compressors, but not standard "Auto" operation for fixed-speed compressors. * **D) In the hand mode, the compressor will cycle on and off.** * **Incorrect.** As explained above, the defining characteristic of "Hand" mode is continuous running (to bypass the pressure switch controls). Cycling on and off based on pressure is the function of the "Auto" mode.