Question 21 1AE01 - First Assistant Engineer

The Correct Answer is A **Why option A is correct:** The term "isochronous" (meaning "equal time") applied to a speed control governor means the governor is designed to bring the prime mover (engine or turbine) back to exactly the original set speed after a load change, resulting in zero steady-state speed error or zero permanent speed droop. To achieve this stable, constant speed across varying loads, the governor must temporarily introduce a control signal that mimics speed droop (a corrective action proportional to speed change) to dampen oscillations and prevent instability (hunting). This stabilizing feedback, which often involves a component like a compensating dashpot or a hydraulic relay (in mechanical-hydraulic governors) or integral action (in electronic governors), is referred to as **temporary speed droop** or compensation. Once the transient disturbance has passed, the governor eliminates this temporary droop and resets the speed precisely to the set point. Therefore, isochronous governors maintain constant prime mover speed regardless of load by utilizing temporary speed droop for stability during transients. **Why the other options are incorrect:** * **Option B is incorrect:** This option incorrectly states that the governor maintains constant prime mover *load* regardless of speed. Governors are primarily speed controllers, not load controllers (though they control speed by adjusting load-carrying capability). The goal of an isochronous governor is constant *speed*. * **Option C is incorrect:** This option describes an accurate outcome (maintaining constant speed regardless of load) but attributes it to the wrong mechanism. **Permanent speed droop** is the characteristic that defines non-isochronous (droop) governors, where the speed *must* decrease proportionally as the load increases. Isochronous operation is defined by the *absence* of permanent speed droop. * **Option D is incorrect:** This option incorrectly identifies the control objective (constant load) and incorrectly identifies the mechanism (permanent speed droop), which is contradictory to isochronous operation.