In a paper mill where constant speed is required

Question

Examoogle · Accepted Answer

individual drive is preferred

Explanation: In a paper mill, multiple rollers must operate at precisely synchronized speeds to maintain the integrity of the paper web. Individual drive systems allow for independent control of speed and torque for each section, preventing web breakage or wrinkling.

Working Principle: In an individual drive system, each section of the paper mill (e.g., press roll, drying cylinder, calendar) is equipped with its own dedicated electric motor and speed controller. By employing sophisticated closed-loop control, such as Ward-Leonard systems or modern Variable Frequency Drives (VFDs), the motors are synchronized to maintain a constant draw, ensuring the tension in the paper sheet remains within limits even if load conditions change across sections.

Key Formulas: $N = \frac{120f}{P}$ — relationship between synchronous speed, frequency, and poles for AC motors; $E_b = V - I_a R_a$ — back EMF equation for DC motor speed control

Key Points: Individual drives eliminate the complex and high-maintenance mechanical coupling seen in group drives.; Sectional synchronization is critical to preventing paper sheet rupture.; Individual drive systems improve overall process reliability and facilitate precise tension control.

Advantages: Independent speed adjustment for each section; High operational flexibility; Reduced downtime due to modularity

Disadvantages: Higher initial capital investment; Complexity in control circuitry compared to group drives

Applications: Paper manufacturing; Rolling mills; Textile finishing plants

Additional Info: Group drives were historically used due to lower costs, but they suffer from poor speed regulation and high mechanical losses due to transmission shafts.. In individual drive systems, the failure of one drive does not necessarily cause the entire line to stop immediately, which is a major advantage over group drives.

Answer: C is correct — individual drive is preferred because it provides precise, independent control over the speed of each section of the paper mill to maintain paper sheet integrity.

Exam Tip: Whenever a question mentions 'constant speed' with 'multiple interconnected processes', look for 'individual drive' solutions as they provide superior control over process variables.

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