Which of the following statements is true about the voltage control of DC motors?

Question

Examoogle · Accepted Answer

The correct answer is Option B: The application of this method is restricted to separately excited DC motors.. Voltage control of DC motors, commonly referred to as the Ward-Leonard system or armature voltage control, involves varying the armature terminal voltage while keeping the field flux constant. This method is strictly restricted to separately excited DC motors because the armature and field circuits must be electrically isolated to allow independent control of terminal voltage without affecting the excitation flux.

Step-by-Step Solution & Concepts:
Voltage control of DC motors, commonly referred to as the Ward-Leonard system or armature voltage control, involves varying the armature terminal voltage while keeping the field flux constant. This method is strictly restricted to separately excited DC motors because the armature and field circuits must be electrically isolated to allow independent control of terminal voltage without affecting the excitation flux.

Working Principle: In a DC motor, speed is governed by the relation $N \propto \frac{E_b}{\Phi}$. Since $E_b = V - I_a R_a$, keeping $\Phi$ constant and varying $V$ directly changes the back EMF $E_b$, thereby adjusting the speed. In self-excited machines, the field supply is derived from the armature terminal voltage; hence, changing $V$ would inadvertently change $\Phi$, making independent speed control impossible via this method.

Key Formulas: $N = K \cdot \frac{V - I_a R_a}{\Phi}$ — Represents speed as a function of terminal voltage, back EMF, and flux; $E_b = V - I_a R_a$ — Back EMF equation highlighting the dependency of speed on terminal voltage $V$

Key Points: Armature voltage control provides smooth speed control below the rated speed.; This method is a constant torque drive method.; The field circuit must be separately excited to maintain constant flux while the armature voltage varies.; Self-excited motors cannot use this method because their field voltage is linked to the armature supply.

Advantages: Very wide speed control range; Excellent speed regulation; Smooth and stepless speed control

Disadvantages: High initial cost due to the need for a separate DC source; Low overall efficiency at low speeds

Applications: Steel rolling mills; Electric excavators; Colliery winders

Additional Info: Option A describes armature voltage control but incorrectly ignores the specific motor type limitation.. Option C describes field flux control (field weakening), which is used for speeds above base speed.. Option D is incorrect as self-excited motors lack the electrical isolation between armature and field necessary for this method.

Answer: B is correct — The application of this method is restricted to separately excited DC motors.

Exam Tip: Remember: Armature voltage control is for speeds below base speed (constant torque), while field flux control is for speeds above base speed (constant power).

Examoogle · Answer

Option A: The field current is kept constant and the terminal voltage is varied to obtain speed control.

Examoogle · Answer

Option C: The terminal voltage is kept constant and the field current is varied so as to obtain speed control.

Examoogle · Answer

Option D: The application of this method is restricted to self-excited DC motors.

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