Relay - A relay is an electrically operated switch. Power applied to its coil generates a magnetic field that operates the switch. The operating voltage of its coil may be different from the load voltage.

Contactor - It is essentially a large capacity relay specifically designed to control the flow of electrical power to large electrical loads, such as motors, heaters and lights. It generally has a multi pole, single throw, normally open configuration.

NVC-No Volt Coil - The coil of a relay or a contactor is often referred to as NVC. It often operates at a lower voltage and draw very little current as compared to that of load.

Potential Free Contact - Another term often used is ‘Potential Free Contact’ which simply means that the contact is not energized, or in other words, none of its pole is connected to internal electrical circuit.

Sensor - A Sensor is a device that converts a physical property such as temperature, pressure, relative humidity, flow etc. into an electrically or mechanically measurable signal.

Transducer - A transducer is a device that converts one form of energy to another form of energy such as pneumatic to electric or vice versa.

Transmitter -Output from a sensor or a transducer may not be strong enough to be transmitted over long distances. Transmitter is a device used to amplify the signal.

Controller - It is a device that compares the signal from a Sensor, Transducer or Transmitter with the desired value and generates a signal based on error.

Actuator - It is an electric, pneumatic or a mechanical device which generally converts a low value signal to a signal with sufficient force to overcome the forces acting on control device and to actuate it.

Controlled Device - It receives signal from controller and varies the flow of control agent like water, air. It may be a damper or a valve connected to an actuator.

Stat - Combination of sensor and controller functioning in one device is often called a ‘stat’ like thermostat, humidistat, etc.

Set-Point -It is desired value of the controlled variable.

Process & Controlled Variable - In an air-conditioning system, parameter being controlled such as temperature, RH, pressure etc. is controlled variable and the changes in these parameters is due to influencing factors in process.

Control Medium -It is the air, steam or water the flow regulation of which influences the process to get the desired value of controlled variable.