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JUL-AUG 2018

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INTECH JULY/AUGUST 2018 41 Certification Review | association news CAP question "Slip" in an AC induction motor is defined as: A. synchronous speed minus no load speed B. difference between speed of stator field and rotor speed C. rated speed plus synchronous speed D. speed at which motor develops torque CAP answer The correct answer is B, "d ifference between speed of stator field and rotor speed. " Slip is usually expressed as a percentage, and varies by motor, from nominally 0.5 percent for very large motors to about 5 percent for small, specialized motors. If n s is the stator electrical speed and n r is the rotor's mechanical speed, the slip, S, is defined by: S = (n s – n r ) / n s Motor rotation is developed in an AC induction motor through the effects of a moving magnetic field. As the speed of the rotor drops below the stator speed, or synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the rotor's windings and creating more torque. Slip is required to produce torque. Under load, the rotor speed drops, and the slip increases enough to create sufficient additional torque to turn the load. A very efficient way to control slip is to use a variable frequency drive. References: Trevathan, Vernon L., A Guide to the Auto- mation Body of Knowledge, Second Edition, ISA, 2006. Dixon, P. (2016, September). "The veloc- ity of PID." Control Engineering. 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 CCST question An analog multimeter is a very useful calibration device, but it will not determine which of the following parameters? A. current B. voltage C. resistance D. frequency CCST answer The correct answer is D, "frequency." An analog voltmeter works by passing a current through a coil that is suspended between two permanent magnets. By selecting the different functions with the selector switch (ohms, volts, amps), different resistors are placed into the circuit, and the quantity to be measured is determined from Ohm's Law. Frequency cannot be measured with an analog meter multimeter, because the mov- ing coil construction of the analog multimeter meter is suited only for determining quantities by application of Ohm's Law. A more specialized piece of equipment, such as an oscilloscope or digital multimeter is required to determine frequency in an electrical AC circuit. Reference: Goettsche, L. D. (Editor), Mainte- nance of Instruments and Systems, Second Edition, ISA, 2005.

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