A frog-leg winding is used in a dc machine to eliminate

(a) the use of equalizers in lap winding.
(b) the use of compensating winding.
(c) the use of compensating winding.
(d) reactance voltage produced due to commutation.

Answer: (a) the use of equalizers in lap winding.

Explanation:

1. CONCEPT: Frog-Leg Winding

A frog-leg winding is a special type of armature winding in DC machines that is a combination of lap and wave windings on the same armature.

It is mainly used in large DC machines.

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Purpose of Frog-Leg Winding

The main purpose is to:

✔ Provide a fully equalized winding
✔ Ensure balanced current distribution
✔ Improve commutation
✔ Eliminate the need for separate equalizer rings

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Why Frog-Leg Winding Is Used

In large lap-wound machines:

• Many parallel paths exist

• Large circulating currents flow

• Equalizer rings become bulky

Frog-leg winding internally provides equalization.

So:

👉 No separate equalizer rings are required.

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Features of Frog-Leg Winding

• Combination of lap + wave winding

• Self-equalizing winding

• Used in heavy-duty machines

• Suitable for high current and large ratings

• Better stability under load variation

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2. CONCEPT: Armature Reaction

When current flows in the armature, it produces its own magnetic field.

This field distorts the main field and is called armature reaction.

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Types of Armature Reaction

1. Demagnetizing effect

2. Cross-magnetizing effect (most important)

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Cross-Magnetizing Effect

• Distorts the main flux

• Shifts the Magnetic Neutral Axis (MNA)

• Causes poor commutation

• Leads to sparking

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3. Problem Due to Armature Reaction

In machines with varying loads:

• Armature current changes

• Armature reaction changes

• MNA shifts suddenly

This causes:

✔ Poor commutation
✔ Induced emf in shorted coils
✔ Sparking at brushes
✔ Possible flash-over on commutator

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Flash-Over

Flash-over means:

• Heavy arcing across commutator segments

• Damage to commutator

• Machine failure

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4. CONCEPT: Compensating Winding

Compensating winding is used to neutralize the cross-magnetizing effect of armature reaction.

It is also called:

👉 Pole Face Winding

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5. Location of Compensating Winding

Compensating winding is embedded in:

✔ Slots cut on pole faces

So, it lies very close to armature conductors.

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6. Connection of Compensating Winding

Compensating winding is:

✔ Connected in series with armature

So:

$${\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">I</span></span>}}_{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">c</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">o</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">m</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">p</span></span>}}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">=</span></span>{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">I</span></span>}}_{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">a</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">r</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">m</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">a</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">t</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">u</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">r</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">e</span></span>}}$$

Their MMFs are proportional.

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7. Direction of Current in Compensating Winding

For proper compensation:

👉 Current in compensating winding must be opposite to armature current under the pole face.

This cancels armature flux.

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8. Function of Compensating Winding

Main Functions:

✔ Neutralizes cross-magnetizing effect
✔ Prevents MNA shifting
✔ Improves commutation
✔ Reduces sparking
✔ Prevents flash-over
✔ Maintains uniform flux

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9. Why Compensating Winding Is Used Only in Large Machines

Because:

• Increases cost

• Increases construction complexity

• Requires extra copper

So used only when necessary.

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Used In:

✔ Large motors
✔ Large generators
✔ Machines with frequent load changes
✔ Traction motors
✔ Rolling mills

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10. Frog-Leg Winding vs Compensating Winding

Feature	Frog-Leg Winding	Compensating Winding
Purpose	Current balancing	Armature reaction compensation
Used in	Armature	Pole faces
Function	Eliminates equalizer rings	Eliminates flux distortion
Connection	Part of winding	Series with armature
Main effect	Better current distribution	Better commutation

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11. Important Exam Points ⭐

One-Line Facts

✔ Frog-leg winding = Lap + Wave combination
✔ Used in large DC machines
✔ Eliminates equalizer rings
✔ Compensating winding = Pole face winding
✔ Used to neutralize cross-magnetizing effect
✔ Connected in series with armature
✔ Current direction is opposite to armature
✔ Used only in big machines

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12. Typical MCQ Answers

Q1. Frog-leg winding is used to:
👉 Eliminate equalizer rings

Q2. Compensating winding is used to:
👉 Neutralize armature reaction

Q3. Compensating winding is placed on:
👉 Pole faces

Q4. Compensating winding is connected:
👉 In series with armature

Q5. Pole face winding is also called:
👉 Compensating winding

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13. Final Exam-Oriented Conclusion ⭐

Frog-leg winding is a combination of lap and wave windings used in large DC machines to provide automatic current equalization and eliminate equalizer rings. Compensating winding, also called pole face winding, is embedded in pole faces and connected in series with the armature to neutralize the cross-magnetizing effect of armature reaction, prevent MNA shifting, and improve commutation. It is used mainly in large machines due to high cost.