Pelton wheel efficiency vs. power at constant head For an impulse (Pelton) turbine operating under a fixed supply head, describe how overall efficiency changes as the developed power is increased (e.g., by varying the jet size or number of jets). Choose the correct trend statement.

Concept/Approach

• A Pelton wheel is an impulse turbine with efficiency strongly governed by speed ratio (runner speed to jet speed), jet deflection, and hydraulic/mechanical losses.
• Under a constant head, jet velocity is essentially fixed; increasing power is achieved mainly by increasing the jet diameter and/or using multiple jets while maintaining the optimum speed ratio.

ReasoningWhen the runner is kept near its design speed ratio and the buckets provide nearly the same jet deflection, the hydraulic efficiency remains nearly unchanged as load (power) increases.Thus, overall efficiency remains practically constant over a broad power range at the same head, provided operation stays near the best-efficiency point.

Common pitfallsAssuming higher power always means higher losses and lower efficiency; in Pelton wheels, efficiency can remain stable if speed ratio and jet interaction are well maintained.

Final AnswerEfficiency remains practically constant as power increases at constant head.