Which type of Motor is best suited for the excavator?

Question

Examoogle · Accepted Answer

DC series Motor

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.

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 Formulas: $T \propto \Phi I_a$; $T \propto I_a^2$; $N \propto \frac{E_b}{\Phi}$

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

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

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

Applications: Excavators and cranes; Electric locomotives; Hoists and conveyor belts

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

Answer: 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.

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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