AC in DC Motor

 It was 1983, my first year of Civil Engineering at GND Engineering College, Bidar. Among our subjects was Elements of Electrical Engineering, and the dreaded viva voce in the electrical lab had turned the place into a tense arena. Confident while solving numericals on paper, we first-year students suddenly found ourselves uneasy, for the examiner’s questions were as unpredictable as they were few—and likely to be repeated.

A peculiar system of “intelligence gathering” emerged. As one student faced the examiner, the rest of us stood outside with sharpened ears, trying to catch every possible word. The goal was simple: decode the questions in advance and prepare answers through hurried discussions.

Soon, one question echoed through the corridor like a challenge: “What happens if Alternating Current (AC) is fed into a Direct Current (DC) motor?” It sounded straightforward, yet none of us could confidently explain it.

Speculation took over. One student, with a serious Punjabi flair, declared, “Oh kambegi (It will shiver).” Another countered dramatically, “Nahin oh chaalaa maregi (It will jump off its base!).” Not to be left behind, a third whispered ominously, “Oh da pataka vaj jaaoga (It will blast).” Each theory sounded more entertaining than the last, but none brought us closer to a proper answer.

Realizing we were going in circles, we turned to our lab assistant, Nirvail Singh—a quiet man known for his practical wisdom. Approaching him felt almost like seeking guidance from a guru. “Sir, please tell us… what happens if AC is given to a DC motor?” we asked, hoping for a crisp, textbook explanation.

He didn’t respond immediately. Instead, he leaned back, looked up at the ceiling, and closed his eyes. The silence that followed was intense. We stood there, convinced that he was mentally revisiting circuit diagrams and electrical principles. It felt as though he was preparing to reveal something profound.

Finally, he opened his eyes and, with calm certainty, delivered his verdict in Punjabi: “Oh chal te jayegi… par oh mazaa nahin aayega (It will run, but that fun will not be there).”

For a moment, we were speechless. Then the simplicity—and brilliance—of the answer sank in. There would be no explosions, no dramatic shaking or jumping. The motor would indeed run, but not efficiently or smoothly. It was a perfect, practical explanation without a trace of technical jargon.

Relief swept over us, followed by suppressed laughter. His words, though simple, conveyed more clarity than all our wild assumptions combined. We quickly framed the explanation in more formal terms for the examiner: a DC motor on AC supply would run with fluctuating torque, reduced efficiency, and excessive heating, as it was not designed for alternating current.

That day, Nirvail Singh became more than just a lab assistant—he was our savior. His earthy wisdom had not only answered our question but also eased our anxiety. We walked into the viva room with newfound confidence.

In the end, his line captured the essence better than any textbook ever could: the motor would run—but not the way it was meant to.