This automation industry quiz question comes from the ISA Certified Automation Professional (CAP) certification program. ISA CAP certification provides a non-biased, third-party, objective assessment and confirmation of an automation professional's skills. The CAP exam is focused on direction, definition, design, development/application, deployment, documentation, and support of systems, software, and equipment used in control systems, manufacturing information systems, systems integration, and operational consulting. Click this link for more information about the CAP program.
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
e) none of the above
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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.
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.
Reference: Nicholas Sands, P.E., CAP and Ian Verhappen, P.Eng., CAP., A Guide to the Automation Body of Knowledge. To read a brief Q&A with the authors, plus download a free 116-page excerpt from the book, click this link.