Electrical Fundamentals - Introduction to Circuit Control Devices
A. Bhatia, B.E.
Many electrical devices are used some of the time and not needed at other times. Circuit control devices allow you to turn the device on when it is needed and off when it is not needed. Broadly, these serve three basic purposes:
This course provides you with insight to what circuit control devices are, how they are used, and some of their characteristics.
This 3-hr course material is based entirely on Naval Education and Training Materials (NAVEDTRA 14175), Electricity and Electronic Training Series; Module-3 “Circuit Control Devices” and covers Chapter 3.
At the conclusion of this course, the student will be able to:
This course is aimed at students, professional electrical & electronics engineers, service technicians, energy auditors, operational & maintenance personnel, facility engineers and general audience.
Circuit control, in its simplest form, is the application and removal of power. This can also be expressed as turning a circuit on and off or opening and closing a circuit. Before you learn about the types of circuit control devices, you should know why circuit control is needed.
If a circuit develops problems that could damage the equipment or endanger personnel, it should be possible to remove the power from that circuit. The circuit protection devices will remove power automatically, if current or temperature increases enough to cause the circuit protection device to act. Even with this protection, a manual means of control is needed to allow you to remove power from the circuit before the protection device acts.
In essence, a circuit control device makes possible the selection of the particular circuit you wish to use.
In this course, you are required to study Naval Education and Training Materials (NAVEDTRA 14175), Electricity and Electronic Training Series; Module-3, Chapter 3 titled “Circuit Control Devices”:
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Circuit control devices are used to “turn on” and “turn off” current flow in an electrical circuit. Circuit control devices have many different shapes and sizes, but most circuit control devices are switches, solenoids or relays.
A switch is the most common circuit control device. Switches normally have two or more sets of contacts. Opening these contacts is called “break” or “open” the circuit, closing the circuit is called “make” or “completing” the circuit. Switches are described by number of poles and throws they have. Poles refer to the number of input circuit terminals while throws refers to the number of output circuit terminal. They are further referred to as SPST (single pole, single throw), SPDT (single pole, double throw) or MPMT (multi-pole, multi-throw).
A relay is a simple remote control switch, which uses small amount of current to control a large amount of current. Its construction contains an iron core, electromagnetic coil and an armature. The iron core intensifies the magnetic field, which attracts the upper contact arm and pulls it down; thus closing the contacts and allowing power from the power source to go to the load. An example would be a computer, which controls a relay, and the relay controls a higher current circuit.
A solenoid is an electro-magnetic switch that converts current flow into mechanical movement. As the current flows through the winding, a magnetic field is created, which pulls the moveable iron core into the centre of the winding. These find use in the starting system that engages starter with the flywheel.