Troubleshooting a 120-Volt Simple Circuit with a Fuse, Switch, and Shaded Pole Motor
In a recent classroom assignment, a student was asked to troubleshoot a simple 120-volt circuit containing a fuse, a switch, and a shaded pole motor. The homework problem presented a scenario where a technician observed 118 volts across the motor but found the motor was not running. The intriguing question at the end asked, 'The technician concludes that ______ is defective.' Unfortunately, the student did not put any thought into the question and left it blank. Let's explore this scenario and the potential causes for the motor's failure.
Understanding the Circuit and the Problem
The circuit in question includes a fuse, a switch, and a shaded pole motor. A fuse is designed to protect the circuit by interrupting the flow of electricity in case the current exceeds the fuse's rating. A switch serves to control the flow of current to the motor. The shaded pole motor is a type of single-phase induction motor that is primarily used in small appliances and fans due to its simple construction and low cost.
The problem states that the technician measured 118 volts across the motor, which is close to the nominal 120 volts. However, the motor is not running. This difference might not be significant, as the volt meter might not perfectly match the voltage supply. Nonetheless, the motor's failure to operate raises several possibilities for further investigation.
Potential Causes and Troubleshooting Steps
Before concluding that any specific component is defective, let's explore the potential causes for the motor's failure and the appropriate troubleshooting steps:
Fuse
Although the fuse might have blown, it's unlikely to be the cause since the technician observed a reading of 118 volts. A blown fuse would likely result in no voltage readings at all, not a nearly nominal 118 volts.
Switch
The switch could be the issue. If the switch is not making a proper connection, it might pass some voltage but not enough to start the motor. This is one possible reason for the motor not running when 118 volts are present.
Motor
The shaded pole motor itself could be defective. There are several possible defects, such as a burnt coil, a shorted winding, or a seized bearing. If the bearings are frozen, the motor will draw current but not spin, which is consistent with the observed symptoms.
Further Examination and Tests
To accurately diagnose the problem, the technician should proceed with a series of tests and examinations:
Visual Inspection
Begin with a visual inspection of the motor. Check for any obvious signs of damage, such as burnt coils, dried oil, or physical damage to the motor casing. Also, check the switch and fuse for any visible signs of overheating or damage.
Testing the Switch
Use a multimeter to test the switch. Set the multimeter to the resistance mode (ohms) and check if the switch is making a proper connection when closed. It should show a low resistance value.
Testing the Motor
Use a multimeter to test the motor windings for continuity and resistance. A blown coil or a shorted winding will show a high resistance or no continuity. The motor should also be tested for run current using a clamp meter to see if it draws the expected amount of current.
Conclusion
The student's blank answer is a missed opportunity to demonstrate understanding of electrical circuits and motors. By taking the time to analyze the possible causes and conducting the appropriate tests, the technician can identify the issue and correct it. In this scenario, the switch is a plausible answer, but the motor itself, particularly its bearings, could also be the cause. A combination of these investigations should lead to a conclusive diagnosis and effective resolution of the problem.
By incorporating these steps, the classroom can enhance the learning experience and prepare students to better understand the intricacies of circuit design and fault diagnosis.