Heating, Ventilation, and Air Conditioning (HVAC ) Electrical Practice Test

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In testing the windings of a single-phase motor, which terminals show the lowest resistance?

Start to common
Run to common
Common to run terminals
Run to start

The correct answer is: Common to run terminals

When testing the windings of a single-phase motor, the terminals that typically show the lowest resistance are the common to run terminals. This is because the run winding is usually designed to have a lower resistance compared to the start winding. In a typical single-phase motor setup, the start winding is used to get the motor up to speed and has a higher resistance to allow for a higher starting torque. Once the motor reaches a certain speed, the start winding is often taken out of the circuit, leaving primarily the run winding. As a result, the resistance measured between the common terminal and the run terminal is lower than that of the other combinations, reflecting the design characteristics of the motor's windings. Understanding the relationship between the windings and their resistances is crucial in diagnosing motors, as it helps determine the health and functionality of the motor's electrical components.