Circuits
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
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.
In motor control terminology, a two-wire circuit utilizes a manual motor starter with a maintained contact. A two-wire circuit provides low-voltage release.
In contrast to the Power Circuit, the Control Circuit consists of inputs, in the form of switches, pushbuttons or pilot devices, which when activated, can either directly, or through a magnetic motor starter, energize a load. The Control Circuit often operates at a lower voltage than the Power Circuit for safety and ease of installation.
A device for making or breaking the connection in an electric circuit.
An auxilary device that provides indication or control of a process to an operator. Pilot devices include automatic switches such as float and pressure switches, as well as indicating lights.
Circuits with low-voltage release are designed to re-energize automatically when voltage is restored after a power outage. Examples include lights or the kitchen fridge.
In electrical terms, refers to a connection where current has only one path to flow.
Loads connected in series will have the the same value of current flowing through them, and share the total voltage between them. Switches and overcurrent equipment is connected in series with equipment to control and protect it.
A device that controls the flow of electrical power to a motor. It is designed to safely start and stop a motor, and provide overload protection.
A heater element paired with normally-closed contacts that open once the heater gets too hot. Two types of relays are the bimetallic strip and the melting solder pot.
A diagram that shows how a circuit works logically and electrically. It uses symbols to identify components and interconnecting lines to display the electrical continuity of a circuit. It is often used for troubleshooting purposes. Also known as a ladder diagram.
Used to convert between wiring and schematic diagrams, the numbering system is a method of identifying and labelling each electrically common point in a circuit. Two wires are considered electrically common if they share an electrical connection with no switches or loads between them, and so would be assigned the same number in the diagram.
