Motor Control Devices can be classified as Primary Control Devices and Pilot Control Devices. Primary Motor Control Devices is one which connects to or disconnects the motors or loads from the line. Pilot Control Devices control the primary control devices and do not control the load directly. In case of Direct ON Line (DOL) starters, main contactors and triple pole switch which connects the motor to the line will be classified as primary control devices.
Start or Stop push buttons which does not connect or disconnect the motor from the line but only energize or de-energize the operating coil is an example of pilot control devices. Pilot Control Devices in general will have sensing element to sense parameters as fluid pressure, temperature, liquid level or mechanical pressure applied. Pilot control devices covert the above information that they sense into the control of primary control devices. Among various pilot devices some of the devices such as push buttons, limit switches, float switches, contactors, pressure switches, plugging switches, ac and dc electromagnetic relays, master controllers and transfer switches.
Push buttons is the most simple of all the pilot control devices. It is a device that provides control of an equipment by pressing a button. Push buttons is most commonly used for starting or stopping electrical motors by remote control. Push buttons are of generally two types: The maintained contact type and momentary contact type. In maintained contact type, when the push button is pushed, the contacts close and remain close until stop button is pushed. In momentary contact type push buttons, contact remains closed for whatever period of the time that push button is held down. The contacts of the push buttons are usually double break type. Two types of contacts are usually provided namely normally open (NO) and normally closed contacts (NC). When the push button is pressed its NO contacts closes and NC contacts opens. Since the equipment is not to be used in power circuits but is to be used in control circuits, current rating of the push button circuits is usually small. The contacts are made silver tipped to withstand numerous of switching operations. By pressing the push button, contactor control is energized. This closes the main contactors in the circuit. The operation of the contactors is not affected by the length of the wires leading from the contactor to the remote control push button stations.
Prime function of a limit switch is to open or close a contact in a circuit when a certain limit has been reached. These switches are actuated by the travel of a driven machinery. Limit switches are required to perform the functions such as slow down, stop or reverse the drive at the end of the travel. Limit switches can also be used to provide control impulses to intiate the control sequence as a function of the position of the driven machinery. Limit switches are available with different kinds of actuators, cams, levers and push rods. Lever type actuators may be either of normal roller or angular roller type. Limit switches are either stationary or actuated by some moving members or it moves with the machine and actuated by a stationary member. Limit switches have usually one NO and one NC contacts. Limit switches may have either snap action contacts or delayed action contacts. In the snap action type limit switches, the contact opening speed is independent of the speed with which the switch is tripped mechanically. When an actuating rod or level is released, the switch returns to its original position by means of spring. Limit switches are generally air break type. Limit switches find numerous applications such as in opening and closing of curtains, lifts and elevators, water level control of tanks etc
Float switches is one of the example of the use of a limit switch. Here the control circuit switches are operated by float the position of which is determined by the liquid level
The float actuates the switch through a chain, one end of which is connected to the float and the other end to the counter weight.
Contactors are mechanical switching devices capable of making and breaking over load conditions, the speed of make and break is independent of the operator.
Electromagnetic Contactor: In this type of contactor moving elements leave the position of the rest when the electromagnet is energized and acts directly on the mechanism of the contactor
Pneumatic Contactor: In this type of contactor the moving element leaves the position of rest when compressed air is fed without the use of electrical means, to a device which acts on the mechanism of the contactor
Electro pneumatic Contactor: This is similar to pneumatic contactor except that the compressed air is fed by means of electrical controlled device