29 Sump-Pump Circuit

A three-position selector switch may be used to provide either manual or automatic control. When the switch is moved to the HAND position, the magnetic starter is energized. This is useful for testing the condition of the motor and verifying the direction of rotation. When the selector switch is moved to the AUTO position, the magnetic starter is controlled by the opening or closing of the float-switch contacts.

The diagram below shows a sump-pump circuit, while the figure represents the tank that it empties.

Sump-control circuit

If the water level is low, both float switches are normally open and the motor does not run. If the water level rises, FS 2 will close first, but the motor will not start. Current cannot get past FS 1 nor the normally open auxiliary contacts. If the water level rises to fill the tank and FS 1 closes, the motor will energize and start pumping water out of the tank and very quickly FS 1 will open again.

This is where having two pilot devices allows us a greater range in sensitivity. Even though FS 1 opens almost immediately, the auxiliary contact keeps the motor running until the tank is fully emptied and FS 2 opens. With only a single pilot device the motor would only drain a small portion of the tank and would be subjected to multiple starts and stops. Using two pilot devices to control the motor allows for more efficient operation of the motor.

This circuit shows the arrangement for a tank-drain function. A simple rearrangement of contacts can provide a tank-fill function as well.



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Basic Motor Control Copyright © 2020 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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