28 Automatic Control Circuits

Also known as two-wire circuits, automatic control circuits can be controlled manually from simple two-wire ON/OFF toggle switches or any number of two-wire pilot devices such as temperature switches or float switches.

If there is a power failure while the switch is in the ON or closed position, the motor will automatically restart when power returns. This is an example of low-voltage release (LVR). Some examples include sump pump circuits and temperature circuits.

Two-Wire Temperature Circuit

Two-wire control circuit

The diagram below has two components: the normally open temperature switch and the magnetic contactor. The three normally closed contacts that represent the OLR protection are connected in series with the motor coil and so can be treated as a single device. In practice, a motor starter often only has two external connection points and all internal overload contacts connected in series. This arrangement allows for the control of the magnetic contactor by the opening and closing of the temperature switch.

The switch can be housed in a separate enclosure from the one housing the motor contactor, which is typically close to the motor it controls or in a motor-control centre (MCC).  Separate housing of the pilot device that controls the load is useful for remotely monitoring situations. When wiring these pilot devices, it is useful to refer to the schematic diagrams and the numbering system.

This circuit takes two wires: One constant hot from the source and one switch leg to control the load, hence two-wire control.

If the ambient temperature causes the switch contacts to close while there is a power failure, the load will automatically re-energize when power returns, which is an example of low-voltage release (LVR).

In practicality, the temperature switch could be replaced with any pilot device and the essential nature of the circuit wouldn’t change. The main limitation of using only a single-pilot device to control the load is the range of sensitivity of that single pilot device.

Consider the next circuit which uses two float switches to control a pump motor.



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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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