1 October 2005
ISA Certified Automation Professional (CAP) Program
These questions are from the CAP study guide, Performance Domain III, System Design.
Definition: Design, specify, and procure the hardware/software used in the system.
1. An application includes a variable frequency drive (VFD) that implements a V/Hz method of torque control while maintaining a fixed V/Hz ratio. The motor has a nameplate rating of 460 volts and 60Hz. If the drive is operating at 30Hz, what percent of rated torque will the motor MOST likely develop?
2. To minimize electrical interference when AC power and DC signal wiring meet in a control panel, it is BEST to:
A. Use a different size wire.
B. Cross the wires at 90 degrees.
C. Run the wires parallel to each other.
D. Twist the AC wires around the DC wires.
1. The correct answer is C, 100%. The volts per hertz ratio (V/Hz) controls the torque of a motor. As long as this ratio is in proportion with the nameplate rating, the motor will develop rated torque. The only answer that represents rated torque is 100% rated torque. Reference: Polka, Motors and Drives - A Practical Technology Guide. ISA Press, 2003.
2. B, crossing the wires at 90 degrees is the correct answer. This minimizes the exposure of the wires to each other, and therefore minimizes the effects of interface. Using a different size wire will have no effect on the electrical interference. Running the wires parallel to each other will increase the effects of interference. Twisting the AC wires around the DC wires will also increase the effects of interference. Reference: Hughes, Programmable Controllers. ISA Press, 2001.
Nicholas Sheble edits the Certification department. For information about the CAP program, go to www.isa.org/cap.