Ward-Leonard controlled D.C. drives are generally used for _______________ excavators

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Examoogle · Accepted Answer

Heavy duty

Explanation: Ward-Leonard controlled D.C. drives are used for heavy-duty excavators because they provide precise, wide-range speed control and high starting torque capabilities. This system allows for smooth acceleration, deceleration, and frequent reversals, which are essential operational requirements for large-scale earthmoving machinery.

Working Principle: The system employs a motor-generator (M-G) set where a constant-speed AC motor drives a DC generator. The generator's output voltage is controlled by varying its field excitation, which is then fed to the armature of the DC motor driving the load. By controlling the generator field current, the motor speed can be varied smoothly from zero to rated speed in both directions without using large power-wasting resistors.

Key Formulas: $E_g = k_g \Phi_g \omega_g$ — Generated EMF proportional to field flux and speed; $T = k_m \Phi_m I_a$ — Electromagnetic torque developed by the motor

Key Points: Provides extremely smooth speed control over a wide range.; Enables regenerative braking capabilities, allowing energy to be returned to the supply.; Highly suitable for applications requiring frequent starting, stopping, and reversing.; Essential for high-inertia loads like excavators, rolling mills, and hoists.

Advantages: Precise speed control in both directions.; High torque capacity at low speeds.; Smooth and fast acceleration/deceleration.; Easy and inherent regenerative braking.

Disadvantages: High initial cost due to M-G set (three machines required).; Low overall efficiency compared to modern solid-state drives.; Requires significant floor space and maintenance for rotating components.; High noise and vibration levels.

Applications: Heavy-duty excavators; Mine hoists; Rolling mills; Large printing presses

Additional Info: Modern excavators have largely shifted to AC drives or solid-state PWM-controlled DC drives, but the Ward-Leonard system remains the classic benchmark for high-torque industrial control.. Option A and B are incorrect as the high cost and maintenance of the M-G set are not justified for light or medium-duty applications where simpler controllers suffice.

Answer: C is correct — The Ward-Leonard system provides the high torque, precise control, and robust performance required for the heavy-duty cycles of excavators.

Exam Tip: Always relate the drive selection to the nature of the load; high-inertia loads (like excavators) with frequent speed changes demand systems with inherent regenerative capabilities.

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