Sample On-the-Job Assignment and Solution

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You are working on a large machine with several differ ent AC and DC motors to make the various parts of the machine operate. You need to set up the machine and make a few modifications for a new product that the machine will make. The machine has an AC three-phase motor to drive the main mechanism and a DC motor that is used to drive an outfeed conveyor. A series DC motor is permanently mounted on this machine to lift large rollers into place for maintenance. The machine also has several small AC three-phase pump motors, and several small fractional horsepower AC single-phase motors.

Your job is to check the large AC motor and ensure that it's wired as a wye-connected motor, and that it's turning in the proper direction. Make changes as necessary. You will also need to locate the brushes and commutator on each of the DC motors and check them to ensure they are in good working order. Also check the rotation of each DC motor and ensure that it's rotating in the proper direction and make changes if necessary.

One of the fractional horsepower single-phase motors is having trouble starting so you will need to check the centrifugal switch and make the necessary repairs or replace the motor. You will need to take data from all of the motors on this machine for your maintenance records and determine what types of spare motors you should keep on hand when a motor needs to be changed out.

Solution

Your solution to the job assignment should include reference to the cut-away diagram of a three-phase AC motor in this figure so that you can review all of the basic parts. You should include the diagram from this figure for the wiring diagram of a wye-connected motor, and the diagram from this figure to show how you would reverse the direction of rotation if needed. Refer to this Section through this Section for additional information.

Your solution for the DC series motor should include a reference to cut-away diagrams of the DC motor i to be able to identify the location of the brushes and motor terminals. You should include the diagrams and how you would reverse the direction of rotation for the DC motor and to explain how you would change the direction of rotation.

Your solution for checking the single-phase AC motor should include the cut-away diagrams in this figure and this figure and you should refer to the diagrams of the centrifugal switch in this figure to explain how you would test the switch to ensure it's operating correctly. Refer to this Section through this Section for additional information.



Your solution should include a typical data plate shown in this figure and you should refer to this Section to explain which data make the single-phase AC motors unique. Be sure to explain which motors can be exchanged for other motors. For instance, if you stock a split-phase motor, you can add a capacitor to it to replace a capacitor start, induction run motor.

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