48 Voltage Drops in Control Circuits

An example of voltage drops in a will illustrate the use of for troubleshooting.

Troubleshooting control schematic

Referring to the diagram above and assuming a line of 120V, if the circuit is operating normally we can determine what all the meters would be reading.

If someone were to press the start button, then only voltmeter A would read line voltage, as it is in with the two loads.

Voltmeter B will measure zero volts across the closed contacts of the stop button.

If an occurs and the open, voltmeter C will read line voltage while the start button is being pressed, and nothing when it is released.

Assuming the motor is not running, only voltmeter D would give a reading of line voltage. If the contactor is not engaged, then the will remain closed and the red will be on.


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Basic Motor Control by Aaron Lee and Chad Flinn is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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