and specification according to NEMA (National Electrical
Manufacturers Association).
SWITCH
OBJECTIVES
General objective : To understand the concept and basic application of a switch.
Specific objectives : At the end of the unit you should be able to:
· Identify the basic function of switch .
· Identify the type of switches.
· Describe the application of switch in industrial.
· Draw the symbol of a switch.
2.1 INTRODUCTION OF SWITCH.
The switch is a mechanical, electrical or electronic device that opens or close a circuit. Switching may also be called making or breaking the circuit.
2.2 BASIC FUNCTION OF SWITCH AND SWITCH DIAGRAM.
The switches function as a device that opens or close a circuit. The closing of a switch is called making the circuit. The opening of switch is called breaking the circuit. The output of system depends on the switching pattern of the converter switches and the input voltage (or current). Similar to the linear system, the output quantities of a converter can be expressed in terms of input quantities, by spectrum multiplication. The arrangement of single –phase diagram of converter and simple hold circuit are shown in figure 2.2(a) and 2.2(b) below.
2.3 TYPE OF STRUCTURE AND CONSTRUCTION OF SWITCH
The structure and construction of power electronics switch can divided into four parts:
i. Design of power circuits.
ii. Protection of power devices.
iii. Determination of the control strategy.
iv. Design of logic and gating circuits
In the analysis, the power devices are assumed to be ideal switches unless stated otherwise; and effects of circuit stray inductance, circuit resistances, and source inductance are neglected. The practical power devices and circuit are also affected. However, in the early stage of design, the simplified analysis of circuit is very useful to understand the operation of the circuit and to establish the characteristics and control strategy.
Before a prototype is built, the designer should investigate the effects of the circuit parameters (and devices imperfections) and should modify the design if necessary. Only after the prototype is built and tested, the designer can be confident about validity of the design and can estimate more accurately some of the circuit parameters (e.g., stray inductance).
2.4 TYPES OF SWITCH
There are many different type of switches. Some switch types are push buttons, limit switch, slide switch, rocker switch, precision switch and toggle switch.
2.4.1 PUSH BUTTONS SWITCH.
A pushbutton is a switch activated by finger pressure. Two or more contacts open or close when the button is depressed. Pushbuttons are usually spring loaded so as to return to their normal position when pressure is removed. Figure 2.4(a), show that the mechanical-interlocked pushbutton with NO (normally open) and NC (normally close) contacts, rated to interrupt an ac current of 6A one million times.
2.4.2 LIMIT SWITCH
2.4.3 SLIDE SWITCH.
A slide switch is a switch activated by sliding of two or more contacts open or close when the button is depressed. Pushbutton are usually spring loaded so as to return to their normal position when pressure is removed. Figure 2.4(c), show that the mechanical-interlocked sliding plate/rod with NO (normally open) and NC (Normally close) contacts.
2.4.4 ROCKER SWITCH.
A rocker switch is a switch activated by 3 finger which includes 2 finger push button for opening and closing of 2 circuits. Two contacts open or close for 2 circuits when the button are pressed and depressed. Pushbutton are usually spring loaded in the bottom to return to their normal position when pressure is removed. Figure 2.4(d), show that the mechanical-interlocked pushbutton with ON (normally open) and NC (Normally close) contacts through 2 circuit involved.
2.4.5 PRECISION SWITCH.
A precision switch is a switch activated by roller loaded on the top of pushbutton to open or close a circuit. Only one contact open or close when the button is depressed by load moving on top of the roller. Pushbutton are usually spring loaded so as to return to their normal position when load on top of roller is removed. Figure 2.4(e), show that the mechanical-interlocked pushbutton with ON (normally open) and NC (Normally close) contacts, rated to interrupt an ac current of mechanical effect to the top of roller device.
2.4.6 TOGGLE SWITCH.
A toggle switch is a switch activated by ball bearing moving for open or close a circuit. The movement of roller makes a contact to open or close a circuit when the toggle is depressed. A toggle usually parallel with the force loaded so as to open or close. Figure 2.4(f), show that the mechanical-interlocked toggle pushbutton with NO (normally open) and NC (Normally close) contacts.
2.5 PRINCIPLE OPERATION OF SWITCH
Switches should follow the international system of units (SI) which defines the ampere (the fundamental unit of electric current) as the constant current which, if maintained in two straight parallel contactors of infinite length and negligible circular cross section place in a mechanism part that will produce a force between two parallel contactors. By international agreement, the value of the international ampere for switches was based on the electrolytic deposition of silver form a silver nitrate solution. The resistance standard of switches is absolute measurement of the ohm and is carried out by the international standards laboratories, which preserve a group of primary resistant standard. For the voltage standard the saturated cell has a temperature dependence, and the out put voltage change about -40 m V/0C from the nominal 1.0185 V. The output of a converter depends on the switching pattern of the converter switches and the input voltage (on current). Similar to linear system, the output quantities of a converter can be expressed in terms of the output quantities, by spectrum multiplication.
