Correct Answer

Concept & PrincipleElectricalElectric Drives

Option C

DC series Motor

Quick Summary: The DC Series Motor is the ideal choice for an excavator because it provides high starting torque and its speed is inversely proportional to the load. This characteristic allows the motor to handle heavy mechanical loads and sudden shocks typical in construction machinery while protecting the power source.

💡 Explanation

The DC Series Motor is the ideal choice for an excavator because it provides high starting torque and its speed is inversely proportional to the load. This characteristic allows the motor to handle heavy mechanical loads and sudden shocks typical in construction machinery while protecting the power source.

🔢 Key Formulas

$T \propto \Phi I_a$

$T \propto I_a^2$

$N \propto \frac{E_b}{\Phi}$

⚙️ Working Principle

In a DC series motor, the armature and field windings are in series, meaning the same current flows through both. Since torque is proportional to the product of flux and armature current ( $T \propto \Phi I_a$ ) and flux is directly proportional to current ( $\Phi \propto I_a$ for unsaturated conditions), the torque is proportional to the square of the armature current ( $T \propto I_a^2$ ). This results in massive torque generation at low speeds during heavy load starts.

📌 Key Points

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High starting torque is essential for moving heavy excavator buckets.
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Variable speed characteristic: speed drops as load increases, preventing motor stall.
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The series field winding is made of thick wire to carry the full load current.
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Provides excellent performance under shock loads.

✅ Advantages

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High starting torque
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Self-protective under heavy loading
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Compact and robust construction

❌ Disadvantages / Limitations

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Cannot run at no-load (speed becomes dangerously high)
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Speed control is complex compared to shunt motors

🛠️ Applications / Uses

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Excavators and cranes
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Electric locomotives
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Hoists and conveyor belts

📄 Additional Information

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DC Shunt Motors are constant speed motors and lack the high starting torque required for heavy-duty lifting.
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Synchronous motors are constant speed and are primarily used for power factor correction or steady-load applications.
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Differential compound motors are unstable as the flux decreases with increasing load, potentially causing runaway.

📊 Diagram / Illustration

✅

C is correct — DC series motors are specifically designed to provide the high starting torque and variable speed behavior necessary for the heavy-duty operation of an excavator.

Core Concepts Used

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Torque-current relationship Load-speed characteristics Electric Drive selection

💡 EXAM TIP

Remember the mnemonic 'Series for Start, Shunt for Speed': Use series motors whenever you need to lift or move heavy static loads.

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