_______________ is preferred for automatic drives.

Question

Examoogle · Accepted Answer

Ward-Leonard controlled D.C. motors

Explanation: Ward-Leonard controlled D.C. motors are preferred for automatic drives because they provide smooth, precise, and stepless speed control over a wide range in both directions. This system is essential for applications requiring rapid acceleration, deceleration, and frequent speed adjustments.

Working Principle: The system utilizes a motor-generator (M-G) set where the output voltage of a DC generator is varied by adjusting its field current. This variable voltage is fed to the armature of the main DC motor, allowing control of motor speed according to the relation $N \propto \frac{E_b}{\Phi}$. By controlling both the field and the armature voltage, the system achieves flexible four-quadrant operation.

Key Formulas: $N = \frac{V - I_aR_a}{k\Phi}$ — Speed equation of a DC motor; $E_b = k\Phi\omega$ — Back EMF relationship

Key Points: Provides precise speed control over a very wide range.; Supports regeneration and rapid braking.; Commonly used in elevators, hoists, and steel rolling mills.; Allows smooth acceleration from zero to full speed.

Advantages: Stepless speed control; Wide range of speed; Excellent speed regulation; Easy reversal of rotation

Disadvantages: Low overall efficiency due to M-G set; High capital cost; Large floor space requirement; High maintenance due to three rotating machines

Applications: Steel rolling mills; Colliery winders; Large electric excavators; Elevators

Additional Info: The system is historically significant for heavy industrial drives requiring precision.. Squirrel cage motors (Option A) are typically constant-speed machines.. Synchronous motors (Option B) are used for constant speed and power factor correction.

Answer: C is correct — Ward-Leonard controlled D.C. motors are specifically preferred for automatic drives due to their ability to provide smooth, precise, and wide-range speed control.

Exam Tip: When comparing drive types for automatic control, always look for the option that offers the highest range of speed flexibility and direction reversal capabilities.

Related Questions

Discussion (0)