Class 4 (4th Engineer) MET 📅 Sep 2022

Exam Question

(a) describe the normal criteria used for setting thermal protection, release and their advantage compare to magnetic types (6)

(b) A series motor runs at 600 r/min when taking 110A from a 230V supply. The resistance of the armature circuit is 0.12Ω and that of series winding is 0.03Ω. The useful flux per pole for 110A is 0.024 wb and that for 50A is 0.0155wb. Calculate the speed when the current has fallen to 50A. (10)

Reference Answer

### (a) Thermal Protection: Criteria, Release, and Advantages (6 Marks)
#### 1. Principle of Operation & Setting Criteria
Thermal protection relays, commonly used for motor overload protection, operate on the principle of differential thermal expansion of a bimetallic strip.
* **Construction:** The core component is a bimetallic strip, which consists of two dissimilar metals (e.g., steel and brass or copper) with different coefficients of thermal expansion, bonded together. This strip is connected in series with the motor circuit, either directly or via a heater element through which the motor current flows.

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