Problem Solution 1. Its efficiency will poor due to hysteresis and eddy current losses 1. Use – Compensating W

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Problem Solution 1. Its efficiency will poor due to hysteresis and eddy current losses 1. Use – Compensating Winding 2. Power factor of the motor is low due to inductive reactance of the field and armature windings 2. Use – Laminated Core Structure 3. The sparking at the brushes will be excessive due to the effect of armature reactance 3. Use – Reduced number of turns & Increase it Thickness

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Option C: Power factor of the motor is low due to inductive reactance of the field and armature windings 2. Use – Laminated Core Structure

Explanation & Concepts:
In an AC series motor (single-phase series motor/universal motor), operating on AC supply introduces significant problems compared to DC operation. The high inductive reactance of the armature and field windings causes a severe drop in the operational power factor and limits the motor current.

Working Principle: When AC voltage is applied to a series motor, the alternating current produces an alternating magnetic flux. The field and armature windings exhibit high inductive reactance ($X_L = 2\pi f L$), which causes the total impedance angle $\phi = 	an^{-1}(X_T / R)$ to be very large, drastically reducing the power factor ($\cos\phi$). Additionally, alternating flux causes hysteresis and eddy current losses in the core, and transformer EMF induced in short-circuited armature coils causes severe brush sparking.

Key Formulas: $\cos\phi = \frac{R}{\sqrt{R^2 + (X_a + X_f)^2}}$ — Power factor of AC series motor; $P_{eddy} \propto f^2 B_m^2 t^2$ — Eddy current loss equation

Key Points: Hysteresis and eddy current losses are minimized by laminating both the field yoke/poles and the armature core.; Inductive reactance of field turns ($X_f$) is reduced by reducing field turns, while armature reactance ($X_a$) is neutralized using compensating windings.; To compensate for reduced flux due to lower field turns, a high permeability magnetic material with smaller air gap is used.; Brushes with high contact resistance and interpoles are used to reduce sparking caused by transformer voltage in commutating coils.

Advantages: Compensating windings neutralize armature reaction reactance and improve power factor.; Laminated cores reduce iron losses ($P_e$ and $P_h$) significantly when operating on AC.

Disadvantages: Addition of compensating winding increases motor cost, weight, and complexity.; Higher maintenance requirements due to brush and commutator wear.

Applications: Universal motors in domestic appliances (mixers, vacuum cleaners, portable drills); AC traction motors

Additional Info: Correct Matches: 1 (Hysteresis & Eddy current losses) -> 2 (Laminated Core Structure); 2 (Low Power Factor due to inductive reactance) -> 3 (Reduced number of turns & Increase thickness / Compensating Winding); 3 (Sparking at brushes) -> 1 (Compensating Winding / Interpoles).. Statement 2 describes the primary cause of low power factor, which is addressed by reducing inductive reactance.

Answer: C is correct — Power factor of an AC series motor is inherently low due to high inductive reactance ($X_a + X_f$) of the windings.

Exam Tip: Always remember: To improve power factor in AC series motors, reduce $X_f$ (fewer field turns) and neutralize $X_a$ (use compensating winding). To reduce iron losses, laminate the yoke.

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Option A: Problem Solution

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Option B: Its efficiency will poor due to hysteresis and eddy current losses 1. Use – Compensating Winding

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Option D: The sparking at the brushes will be excessive due to the effect of armature reactance 3. Use – Reduced number of turns & Increase it Thickness

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Option E: 1,   2-2,   3-3

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Option F: F) 2,   2-3,   3-3

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Option G: G) 2,   2-3,   3-1

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Option H: H) 3,   2-1,   3-2

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