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SEP-OCT 2018

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48 INTECH SEPTEMBER/OCTOBER 2018 WWW.ISA.ORG association news | Certification Review ISA Certified Automation Professional (CAP) program Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems. Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. ISA Certified Control Systems Technician (CCST) program CAP question Why must voltage be reduced along with frequency in a variable frequency speed controller? A. to let the motor cool off B. because of capacitive reactance C. to maintain the volt/hertz ratio D. to keep the motor from over speeding CAP answer The correct answer is C, "to maintain the volts/hertz ratio." To maintain a constant (rated) flux density, the applied voltage must also be changed in the same propor- tion as the frequency (per Faraday's law). Varying the frequency affects both the motor speed and the strength of the mag- netic field. When the frequency is lowered (to obtain a slower motor speed), the magnetic field increases, and excessive heat is generated. When the frequency is increased (to obtain a higher motor speed), the magnetic field decreases, and lower torque is produced. In order to keep the magnetic flux constant, the V/Hz ratio must remain constant. This keeps torque production stable, regardless of frequency. References: Trevathan, Vernon L., A Guide to the Au- tomation Body of Knowledge, Second Edition, ISA, 2006. CCST question The Reynolds Number for a flowing fluid is most affected by which of the following parameters? A. viscosity B. pressure C. friction factor D. temperature CCST answer The answer is A, "viscosity." The Reynolds Number for a flowing fluid can be gener- alized as the ratio of kinetic (or inertial) forces divided by viscous forces. Although friction factor and temperature do affect inertial forces and viscosity to some de- gree, and therefore, Reynolds number, viscosity has the most direct effect upon the value of the Reynolds Number. Reference: Goettsche, L. D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.

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