Electrical Control System Components Basics of Magnetic Control :

Transcription

1 Electrical Control System Components Basics of Magnetic Control : Dr.M.S.Narkhede, LEE, GP Mumbai 1

2 Contact Types : Contacts are classified into different ways as follows. According to applications contacts are classified in two types. Power contacts (Main contacts) & Auxillary contacts. According to position contacts are classified as NO & NC i.e. normally open contact and normally closed contact. According to the characteristics of the load to be switched or controlled contacts are classified in various categories. e.g. The making and breaking current under resistance load corresponds to the continuous operational current while for squirrel-cage induction motors draws a variable current In order to make the choice of devices easier, utilization categories are defined for contacts, contactors, disconnectors, circuit breakers and load switches as IEC , -2, -3, -4, -5, -6.(International Electrotechnical Commission ) Examples of Categories : AC-20A, AC-20B - Connecting and disconnecting under noload conditions AC-21A, AC-21-B- Switching of resistive loads, including moderate overloads AC-22A, AC-22B - Switching of mixed resistive and inductive loads, including moderate overloads AC-23A, AC-23B - Switching of motor loads or other highly inductive loads AC-1 - Non-inductive or slightly inductive loads, resistance furnaces AC-2 - Slip-ring motors: starting, switching off AC-3 - Squirrel-cage motors: starting, switching off motors during running AC-4 - Squirrel-cage motors: starting, plugging1), inching2 Dr.M.S.Narkhede, LEE, GP Mumbai 2

3 Solenoids : A solenoid is a three-dimensional coil. In physics, the term solenoid refers to a loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create controlled magnetic fields and can be used as electromagnets. In engineering, the term solenoid may also refer to a variety of transducer devices that convert energy into linear motion. The term is also often used to refer to a solenoid valve, which is an integrated device containing an electromechanical solenoid which actuates either a pneumatic or hydraulic valve. According to supply given solenoids are classified as DC or AC. Dr.M.S.Narkhede, LEE, GP Mumbai 3

4 Input Devices : Switches / Push Buttons: A switch is an electrical component that can break the circuit. Switches are classified as SPST, SPDT,DPST,DPDT and so on. Dr.M.S.Narkhede, LEE, GP Mumbai 4

5 PUSH BUTTON It is a pilot device which provides control of an equipment by pressing an actuator. A spring returned momentary contact push button is shown in figure. The contact block has two contacts one NO and another NC. When push button is pressed NC contact opens and NO contact closes. Foot Switch These switches are operated by foot. These are also pilot devices Generally they rest on ground. The applications are in Lathes, Agriculture etc. Dr.M.S.Narkhede, LEE, GP Mumbai 5

6 Selector switch Selector switches are actuated with a rotary knob or lever of some sort to select one of two or more positions. Like the toggle switch, selector switches can either rest in any of their positions or contain spring-return mechanisms for momentary operation. Dr.M.S.Narkhede, LEE, GP Mumbai 6

7 Proximity switch Proximity switches sense the approach of a metallic machine part either by a magnetic or highfrequency electromagnetic field. Simple proximity switches use a permanent magnet to actuate a mechanism whenever the machine part gets close. More complex proximity switches work like a metal detector, energizing a coil of wire with a high-frequency current, and electronically monitoring the magnitude of that current. If a metallic part (not necessarily magnetic) gets close enough to the coil, the current will increase, and trip the monitoring circuit. Photo Electric switch A photoelectric switch is a device used to detect the distance, absence, or presence of an object by using a light transmitter, often infrared, and a photoelectric receiver. They are used extensively in industrial manufacturing. There are three different functional types: opposed (through beam), retroreflective proximity-sensing Dr.M.S.Narkhede, LEE, GP Mumbai 7

8 Temperature actuated switch Normal operating range is 60 to F. Normally contact is closed. When temperature of probe is reached to set value contact opens Temperature switches or thermostats are used for maintaining a prescribed value of temperature during the process in industry. There are two types of temperature switches used in industry First one used bimetallic strip type which works in a similar way to that of an overload relay. The accuracy is not high. The second one uses a liquid or gas or vapour as the sensing element in a short length of pipe or thin tube called as capillary tube with bulb at one end. The expansion of the liquid, gas, vapor is utilized to actuate the switch contact. The temperature ranges are as follows Liquid filled 37 0 C to C Gas filled 37 0 C to C Vapour filled 10 0 C to C Dr.M.S.Narkhede, LEE, GP Mumbai 8

9 Level Control Switch Dr.M.S.Narkhede, LEE, GP Mumbai 9

10 The Float switch shown in the figure is used to maintain liquid levels within a certain range in a tank by energizing a pump when a liquid level falls to a certain lower pre set height and by de-energizing the pump when liquid level rises above a certain higher pre set height. A floating object can be used to actuate a switch mechanism when the liquid level in an tank rises past a certain point. If the liquid is electrically conductive, the liquid itself can be used as a conductor to bridge between two electrodes Level switches can also be designed to detect the level of solid materials such as wood chips, grain, coal, or animal feed in a storage silo, bin, or hopper. Dr.M.S.Narkhede, LEE, GP Mumbai 10

11 A common design uses a "tuning fork" shaped metal prong, inserted into the bin from the outside at the desired height. The fork is vibrated at its resonant frequency by an electronic circuit and magnet/ electromagnet coil assembly. When the bin fills to that height, the solid material dampens the vibration of the fork, the change in vibration amplitude and/or frequency detected by the electronic circuit. Pressure Switch A Pressure switch is a form of switch that makes electrical contact when a certain set pressure has been reached on its input. This is used to provide on/off switching from a pneumatic or hydraulic source. The switch may be designed to make contact either on pressure rise or on pressure fall. Dr.M.S.Narkhede, LEE, GP Mumbai 11

12 Pressure switches are used in control systems to sense pressure of gas, air or liquid and feed signal to electrical circuit. Pressure switch can have NO or NC or both contacts. It can use Bellow or diaphragm or tube of semicircular shape to sense the pressure. Applications : In Pneumatics : switching off an electrically driven compressor when a set pressure is achieved in the reservoir In Hydraulics : To switch on a warning light if engine oil pressure falls below a safe level To switch on brake lights automatically by detecting a rise in pressure in hydraulic brake pipes In dust control systems (bag filter), a pressure switch is mounted on the header which will raise an alarm when air pressure in the header is less than necessary to gain or decline energy beyond the set value Dr.M.S.Narkhede, LEE, GP Mumbai 12

13 Relays Control relays are similar in construction to contactor. They accept the information from some sensing device and feed it to control circuit. The sensing devices used in conjunction with the relays are known as pilot devices. Pilot devices sense the variables like current, voltage, overload, frequency, temperature, pressure and many other. Relays are also used to multiply the contacts available on the pilot devices like pressure switches, level switches etc. With the help of relays the interlocking wiring is possible which removes the complexity in the circuit Dr.M.S.Narkhede, LEE, GP Mumbai 13

14 Symbols Dr.M.S.Narkhede, LEE, GP Mumbai 14

15 Developing Control Circuit: Basics & Thumb rule All control components like contactors, relays, push buttons etc. are designated by alphabets. For example In a forward and reverse operation of motor there are two contactors. Forward contactor is designated as F and reverse contactor is designated as R. All contacts of various contactors, relays shown in the diagram are in de energized state. The various contacts of contactor are designated by numerals. For example various contacts of a contactor F will be represented as F 1, F 2, F 3 and so on. The control wires are also designated with numbers in the diagram. These numbers are marked on wires by inserting ferrules on the wires. Dr.M.S.Narkhede, LEE, GP Mumbai 15

16 The thumb rule is to mark two supply leads of control transformer as L 1 and L 2. Here L 1 acts as phase supply and L 2 is generally grounded. The control diagrams are classified in to following 3 types. Wiring Diagram Elementary line diagram or schematic diagram Wireless connection diagram In wiring diagram the components are shown exactly according to their physical relationship and location in the panel. In schematic diagram representation of circuit is done in proper electrical sequence. In wireless connection diagram only location of components is shown. It is not used frequently. Dr.M.S.Narkhede, LEE, GP Mumbai 16

17 Dr.M.S.Narkhede, LEE, GP Mumbai 17

18 Control Circuit for Skip Hoist The figure shows the working of skip hoist. Dr.M.S.Narkhede, LEE, GP Mumbai 18

19 Overhead Crane Dr.M.S.Narkhede, LEE, GP Mumbai 19

20 Power circuit of Crane Dr.M.S.Narkhede, LEE, GP Mumbai 20

21 Conveyer Dr.M.S.Narkhede, LEE, GP Mumbai 21

22 Control circuit for conveyers Dr.M.S.Narkhede, LEE, GP Mumbai 22

23 A cam for operating conveyer circuit automatically Control Transformer Control Transformers are to supply power to control and / or auxiliary equipment not intended for direct connection to the main source. Control Transformers are specifically used to provide control supply voltage for the control circuits of AC motor starters, i.e. Starter coils, timers, indicating lamps electronics protection relays etc. Control Transformers for contractor applications have to Energize the coil and Maintain a contact for definite period of time. The initial energizing of the coil takes 5 to 40 mil. seconds and requires 3 to 10 times the normal current. While calculating the VA rating of the Control transformer, this factor should be taken in to account. They are available for single phase and three phase inputs and are in the range from 15 VA to 30 KVA. Dr.M.S.Narkhede, LEE, GP Mumbai 23

24 Control Transformer Control Transformers are to supply power to control and / or auxiliary equipment not intended for direct connection to the main source. Control Transformers are specifically used to provide control supply voltage for the control circuits of AC motor starters, i.e. Starter coils, timers, indicating lamps electronics protection relays etc. Control Transformers for contractor applications have to Energize the coil and Maintain a contact for definite period of time. The initial energizing of the coil takes 5 to 40 mil. seconds and requires 3 to 10 times the normal current. While calculating the VA rating of the Control transformer, this factor should be taken in to account. They are available for single phase and three phase inputs and are in the range from 15 VA to 30 KVA. Control Transformer Construction Control Transformers are two winding transformers with both windings made of copper. Power House transformers are made of using high quality materials. Electrolytic grade super enameled wire with best insulations are used. Core material are CRGO EI or Strip cores. Appropriate class of insulation materials are used. Transformers are impregnated in insulating varnish for long lasting performance. Input and out are terminated on Bakelite board with proper size of terminals. Dr.M.S.Narkhede, LEE, GP Mumbai 24

25 Control Transformer Information required while ordering: A) Input supply- Voltage, Frequency and Number of Phases i.e. Single or Three Phases. In case of Three Phase system Type of connection- Star / Delta / Vector group. B) Out put supply- voltage and current in of each secondary winding( if multiple out put windings are required). C) Class of insulations: D) Service conditions if other than standard service conditions. Dr.M.S.Narkhede, LEE, GP Mumbai 25