🔍 Key Concepts

• Synchronous speed of an AC induction motor depends on frequency and number of poles ((N_s = 120 f / P)). • A squirrel‑cage rotor has fixed bars and is not designed for adding external resistance. • A multi‑speed motor is usually built to change speed in steps (two‑speed, four‑speed, etc.), not by smoothly varying supply conditions.

💭 Think About

• Which of the listed methods matches how a ship’s power system normally operates (constant frequency or variable frequency)? • For a squirrel‑cage motor, which part of the motor is practical to reconfigure from outside: the rotor circuit or the stator windings? • Which option explains how you can get distinct speed ‘steps’ (low, medium, high) within the same motor frame?

✅ Before You Answer

• Check which option uses the basic synchronous speed formula: (N_s = 120 f / P). • Verify which choice would not apply to a squirrel‑cage rotor because you cannot conveniently add or change rotor resistance. • Confirm which method is commonly used in multi‑speed shipboard motors without changing the ship’s main power frequency.