Class 4 (4th Engineer) MET 📅 Dec 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 440 V and takes a armature of 50 A. The resistance of the armature circuit is 0.28 ohm. 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

### (a) Main Types of A.C. Motors and Marine Applications
There are several types of A.C. motors used on board ships, each selected for its specific characteristics that match the application's requirements. The three main types are:
**1. Three-Phase Squirrel Cage Induction Motor**
* **Principle of Operation:** This motor operates on the principle of electromagnetic induction. A rotating magnetic field is produced in the stator windings by the three-phase supply. This field induces a current in the rotor's conductor bars. The interaction between the stator's magnetic field and the rotor's induced magnetic field produces torque, causing the rotor to turn. The rotor always turns at a speed slightly less than the synchronous speed of the stator field, a difference known as 'slip'.

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