Class 4 (4th Engineer) MET 📅 Mar 2023

Exam Question

(a) Describe the normal criteria used for setting thermal protection relays and their advantage compared 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 is 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

### Part (a): Thermal Protection Relays
#### Normal Criteria for Setting
Thermal overload relays are set to protect motor windings from damage due to sustained overcurrents, which cause excessive temperature rise. The setting criteria are based on the motor's Full Load Current (FLC) and its thermal withstand characteristics.
1. **Full Load Current (FLC):** The primary setting parameter is the motor's FLC, as specified on the nameplate. The relay's trip current is typically set to a value slightly above the FLC, commonly in the range of 115% to 125% of the FLC.

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