September/October 2013
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.

CAP question

The fastest "electronic" method to bring a DC motor to a full stop is to use:

A. regenerative braking

B. mechanical braking

C. ramp to stop

D. coast to stop

CAP answer

The correct answer is A. In regenerative braking, all of the motor's energy is fed directly back into the AC power line. This is accomplished through the use of "reverse-connected" SCRs, which allow the drive to conduct current in the opposite direction and oppose the rotation of the motor, bringing it to a quick stop.

Answer B is incorrect, because mechanical braking is not an electric method for braking a DC motor. This is a mechanical method, which uses friction to stop the motor shaft once power has been removed.

Answer C, ramp to stop, is an electronic means of stopping a motor over a longer period of time. A high-wattage bank of resistors transforms the rotating energy into heat, thereby dissipating the voltage generated by the motor, slowly bringing the motor to a stop.

Answer D, coast to stop, is the slowest way to stop a motor, and is accomplished by simply removing the motor current and allowing inertia to bring the motor to a stop.

Reference: Trevathan, Vernon L., A Guide to the Automation Body of Knowledge, Second Edition, ISA, 2006.