Question

If the number of poles of an alternator is increased, the generated e.m.f.

• A. increases
• B. decreases
• C. remain the same
• D. none of the above

Hint:

In alternators, the number of poles affects synchronous speed but not the magnitude of generated emf when frequency and flux are constant.

Answer 1

The Correct Option is C

The emf equation of an alternator is \( E = 4.44 k_w \Phi f T \), and this value depends only on frequency, flux per pole, winding factor and turns per phase, not directly on the number of poles.

1. Increases: Would require a rise in \(f\), \(\Phi\), or \(T\), which does not happen from a pole change alone at fixed frequency.
2. Decreases: Would require a fall in one of these same quantities, which likewise does not occur.
3. Remain the same: Since the grid frequency stays fixed, the rotor speed automatically adjusts to \( N = \dfrac{120f}{P} \) as poles change, leaving \(f\), \(\Phi\) and \(T\) untouched, so E is unchanged.
4. None of the above: Ruled out since option 3 already fits.

So changing the pole count alone, with frequency held constant, leaves the generated emf unaffected.

Answer 2

Think of the emf equation from first principles: each pole produces one half-cycle of induced voltage in a conductor as it sweeps past, and the frequency of the resulting waveform is fixed by how many complete electrical cycles occur per second, which is fixed here because the alternator is tied to a fixed-frequency system. Since the physical flux per pole and the number of turns in the winding are unchanged by simply adding poles, the peak flux linkage per cycle stays the same too.

1. Increases: This would only happen if the flux swept per cycle became larger or the machine ran faster in a way that raised the frequency, neither of which occurs when only the pole count is changed at fixed system frequency.
2. Decreases: Similarly, nothing in the winding or the flux linkage per cycle is reduced simply by adding poles while the frequency is pinned by the grid.
3. Remain the same: Because the electrical frequency is fixed, the amount of flux linkage reversed in every cycle stays identical no matter how many poles share the job of producing it, so the induced rms voltage stays the same.
4. None of the above: Not applicable since the machine's emf is unaffected, matching option 3 directly.

Changing the number of poles simply redistributes how the fixed flux and frequency are produced mechanically; it does not change the induced voltage itself.

Therefore, the correct answer is remain the same.