Class 4 (4th Engineer) MET 📅 Mar 2023

Exam Question

(a) Name the three main types of a.c motor and explain the use to which they are put in marine engineering (6)

(b) A four pole motor is fed at 440V and takes an armature of 50A. The resistance of the armature circuit is 0.28 Ω. The armature winding is wave connected with 888 conductors and the useful flux per pole is 0.023 wb. calculate the speed (10)

Reference Answer

### Part (a): Main Types of A.C. Motor and Marine Applications
There are numerous types of AC motors used in marine engineering, each selected for specific characteristics such as starting torque, speed control, efficiency, and robustness. The three main categories are:
**1. Three-Phase Squirrel Cage Induction Motor**
* **Principle of Operation**: A rotating magnetic field (RMF) is produced in the stator windings by the three-phase AC supply. This RMF induces a current in the rotor's squirrel cage conductors. The interaction between the induced rotor magnetic field and the stator's RMF produces a torque, causing the rotor to turn in the same direction as the RMF, but at a slightly lower speed (this speed difference is known as 'slip').

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