Sample Job Assignment

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You are assigned to make changes to the control circuit of a main conveyor system that removes finished parts from a small assembly line. The present assembly line has one small conveyor that feeds parts to a main conveyor, and the main conveyor removes the parts from the assembly area. A second assembly line is being added and its conveyor will also feed parts to the main conveyor. You will need to add controls to make the two lines sequence so that they won't overload the main conveyor. The present control circuit is a two-wire control circuit with a selector switch that is turned on when the small conveyor needs to run (the main conveyor runs all of the time).

Diagram of two assembly conveyors & the main conveyors, & electrical diagram for job assignment.

Your job assignment is to convert the control circuit for the small conveyor into a three-wire control circuit that has two separate sets of start and stop push buttons, and add a three-wire control circuit for the new conveyor that also has a set of start and stop push buttons at each end. The control circuits for the two small conveyors should also be sequenced so that the original conveyor is always started before the new conveyor can be started. Y ou will also need to select the correct type of fuse to allow the 15 A motor on the new conveyor to start safely. You also need to select an enclosure for the motor starter that is rated for water-tight, dust-tight conditions for indoor applications. You will need to provide the electrical diagram of your proposed changes and a parts list for the fuses and enclosure.

Solution to Job Assignment

The three-wire control circuit that a tech needs to use is shown here and described here. Since your application needs additional start and stop switches connected to the circuit, you should refer to this schematic and this page. Since the two conveyors need to be sequenced so one will always be started before the other one can be turned on, the technician will need to refer to this diagram and read this section that explains sequence control. Information regarding fuses is located in this section, and information about enclosures is here. Use this table to select the enclosure for the motor starters in your application. Use this section to provide information about conductor sizes and ratings, and this table to select the correct conductor size.

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