For which of the following applications D.C. motors are still preferred?

Question

Examoogle · Accepted Answer

Variable speed drive

Explanation: DC motors are primarily preferred in industrial applications for their exceptional speed control capabilities. By adjusting the armature voltage or the field flux, the speed of a DC motor can be precisely controlled over a wide range, which is difficult to achieve with traditional AC induction motors without complex power electronics.

Working Principle: The speed of a DC motor is governed by the equation $N = K \frac{V_a - I_a R_a}{\Phi}$. By controlling the input voltage ($V_a$) or the field flux ($\Phi$), the back EMF and speed can be varied continuously. This inherent relationship allows for smooth speed regulation, making it ideal for applications like steel rolling mills, paper machines, and electric traction.

Key Formulas: $N = k \frac{V - I_a R_a}{\Phi}$ — Speed relationship showing dependence on voltage, current, and flux; $T_e = k_t \Phi I_a$ — Electromagnetic torque equation showing dependence on flux and current

Key Points: DC motors offer high starting torque, often utilized in traction systems.; Speed control in DC motors is simpler and cheaper compared to AC motors.; DC motors are widely used where constant torque is required over a wide speed range.; Modern power electronics (VFDs) have increased the utility of AC motors, but DC remains dominant in specific precision control niches.

Advantages: Excellent speed control characteristics; High starting torque; Linear speed-torque relationship

Disadvantages: Requires maintenance due to commutator and brushes; Higher cost compared to squirrel cage induction motors; Sparking issues at high speeds

Applications: Electric traction and locomotives; Rolling mills and paper machine drives; Machine tools and robotics; Battery-operated electric vehicles

Additional Info: While high starting torque is an advantage of DC series motors, variable speed control is a universal characteristic applicable to all DC motor types (shunt, series, compound).. Option D is a characteristic of DC series motors specifically, whereas Variable Speed Drive (Option C) is the broader fundamental reason for their continued industrial relevance.

Answer: C is correct — DC motors are preferred for variable speed drives because their speed can be easily and precisely controlled by varying the armature voltage or field current.

Exam Tip: In competitive exams, if 'Variable Speed' and 'Starting Torque' are both options, prioritize Variable Speed for general DC motor classification, and 'High Starting Torque' specifically for DC Series motors.

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