How can the small single phase motor reduce the harmonics still much further?

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Option C: grading the winding

Explanation & Concepts:
Grading the winding refers to the technique of distributing conductors in a winding such that the magneto-motive force (MMF) waveform closely approximates a sine wave. By varying the number of turns per slot, the space harmonics in the air-gap flux are significantly minimized.

Working Principle: In a uniform winding, the MMF distribution is a stepped square wave containing significant odd harmonics (such as 3rd, 5th, 7th). Grading (or short-pitching/distribution) modifies the winding distribution factor $K_d$ and pitch factor $K_p$. By properly selecting the number of turns in each slot to create a quasi-sinusoidal MMF profile, the amplitude of high-frequency space harmonics is suppressed, leading to smoother torque and reduced electromagnetic noise.

Key Formulas: $F_n = \frac{4}{\pi n} I N K_w \sin(n	heta)$ — MMF harmonic magnitude for order n; $K_w = K_p 	imes K_d$ — Winding factor definition

Key Points: Grading effectively acts as a spatial filter for air-gap flux density.; Higher order harmonics increase iron losses and torque pulsations.; Single-phase induction motors rely on distributed windings to approach a rotating field effect.

Advantages: Reduced torque pulsations and vibration; Improved efficiency by reducing harmonic copper and iron losses; Lower acoustic noise emission

Disadvantages: Increased manufacturing complexity; Higher cost due to precision winding patterns

Applications: High-efficiency household appliances; Precision small-scale motor drives

Additional Info: Option D (Shading) is used for starting torque in shaded-pole motors, not for harmonic reduction of the main running field.. Harmonics in single-phase motors cause 'crawling' or 'cogging' if not properly mitigated.

Answer: C is correct — Grading the winding involves varying the number of turns in different slots to shape the MMF waveform towards a sine wave, effectively suppressing space harmonics.

Exam Tip: Always remember that in electrical machines, space harmonics are reduced by distribution/pitching, while time harmonics are addressed through design and filtering of the input waveform.

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Option A: removing the winding

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Option B: insulating the winding

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Option D: shading the winding

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