Discussion
Arrange these in increasing order of the speed they give a dc motor at fixed field.
1. 120 V armature 2. 240 V armature 3. 240 V armature with field weakened to 80 per cent 4. 60 V armature
Answer: A
Correct answer: 4, 1, 2, 3
At fixed field the speed follows the armature voltage, so 60 V gives the lowest speed, then 120 V, then 240 V. The fourth condition applies the same 240 V but weakens the field to 80 per cent, and since \( N \propto V/\Phi \) that raises the speed by a further 25 per cent, making it the highest of the four. The sequence 4, 1, 2, 3 traces the whole control strategy: voltage control up to base speed, then field weakening above it.
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