Practical Work Week 10 4.2 Contactor
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Week 10 Practical Work 4.2 Contactor A contactor is activated by a control input which is a lower voltage / current than that which the contactor is switching. 4.2.1 Disassembling and re-assembling the contactor Introduction Contactors are magnetic switches. They are used for switching a power circuit remotely. Contactors come in many forms with varying capacities and features. Unlike a circuit breaker a contactor is not intended to interrupt a short circuit current. Contactors are used to control electric motors, lighting, heating, capacitor banks, and other electrical loads. Even though the contactors are rugged and reliable because they do not contain delicate mechanisms, they are in fact subjected to many dangers which should be avoided in order to ensure a proper functioning. In this section, we will try to investigate the various troubles which may be encountered when dealing with contactors. These troubles occur in the various parts constituting the contactor as shown in figure (4-3). Line L1 Line Load T1 L3 Stationary contact L3 T1 Movable contact T2 L2 L1 Line L2 T2 T3 T3 Core Coil Pushbutton open Pushbutton open Armature Armature ON POSITION OFF POSITION Figure (4-3): The various parts constituting the contactor. 56 Week 10 Practical Work The main parts of a contactor are the electromagnet used to obtain the electromagnetic forces, the poles used to supply the power circuits with the required energy and the auxiliary contacts used in the control circuits. An exploded view of a contactor is shown in figure (4-4). Contactor Overview 2 3 1 4 9 8 5 7 6 1. Contactors up to 75 KW 5. Overload relays 2. Suppressors 6,7,9. Auxiliary contact 3. Amplifier modules 8. Pneumatic timer 4. 4th pole Figure (4-4): An exploded view of a contactor 57 Week 10 Practical Work Apparatus 1. One contactor 2. Tools box Procedure : 1. Disassembling the contactor 2. Identify the all parts of the contactor as shown in figure (4-3) and (4-4). 3. Re-assembling the contactor 4.2.2 Troubleshooting and maintenance a. Electromagnet If the magnetic circuit vibrates, check: The voltage of the main supply. An electromagnet vibrates when it is operated at a voltage lower than that for which it is designed. That no foreign matter is lodged between the moving and the fixed parts of the armature. The condition of the armature surfaces. These should never be painted, scraped or filed. If they are particularly dirty, clean them with trichloroethylene. b. Coil If it becomes necessary to replace a coil, in case the control circuit voltage is changed for example, the new coil must be chosen in relation to the actual supply voltage. c. Contacts 58 Week 10 Practical Work The knowledge of the controlled power circuit and its application is used to determine the service life of the contacts or to select a contactor as a function of the required number of switching operations The number of switching operations is determined by the type of application according to the manufacturer's specifications. A periodic check of the compression travel is necessary to reveal the degree of wear. During the period of use, never try to adjust the compression travel. On the other hand, when the compression travel becomes below 50 % of the initial value when the contactor is new, replace the contacts. After every change of contacts: - The contacts must be aligned, adjusting the initial compression value (according to the specification data sheet). - It is preferable to check the pressure of each contact. - It is essential to check the tightness of the setting screws and nuts. d. Auxiliary contacts The auxiliary contacts generally require no maintenance. Do not modify the travel of auxiliary contacts. This has been determined by the manufacturer according to the contact function (hold in, sequence control, economy resistor switching, signalling, etc ...). e. Enclosures Periodically, lubricate hinges and locking devices. On airtight enclosures, check the condition of the sealing devices (compression glands, cable box ...). Check also the condition of the gaskets. 59 Week 10 Practical Work 4.2.3 Use the contactor in the control circuit Introduction Contactors are extensively used in applications which require switching of high power circuits from a low power circuit. There is a number of ways to accomplish this activity; in this practical work, we will investigate some of the many techniques used to connect contactors to enable the switching activity to be carried out properly and safely. The most used techniques are: 1- Remote control by two spring push buttons. 2- Remote control by several spring push buttons. Apparatus 1. Thermal overload relay. 2. Spring return push buttons. 3. Triple pole contactor. 4.2.3.1 Remote control by two spring push buttons : Procedure 1. Connect the circuit as shown in fig. (4-5). When push button S2 is pressed, the coil of C1 is energized and the normally open auxiliary contact C11 (13-14) closes ensuring self-holding of the coil of contactor C1, The poles of the contactor close. If push button S2 is pressed, the contactor is de-energized and its poles and contacts return to their initial states. 2. Connect the control circuit to the power supply. 3. Press on S2 and release. 4. Make sure that the poles of the contactor "have closed. 60 Week 10 Practical Work Q1 95 96 11 S1 12 13 13 S2 14 C11 14 A1 C1 A2 Q2 (2) (1) (1) - Control circuit (2) - Poles of the contactor with the thermal overload relay Figure (4-5) : Circuit of two spring return push buttons 5. Press on S1 and release. Make sure that the poles of the contactor have opened If the circuit does not operate, several preliminary checks should be carried out: Check for the presence of voltage at the incoming terminals using a test lamp or a voltmeter. Check to see that the switch Q1 used to connect the circuit to the power supply closes correctly. Make sure that the control circuit fuses have not blown. 61 Week 10 Practical Work Check that the overload relay is set (the relay is set by pressing the red push button located on the relay). When all preliminary checks have been carried out and the contactor still does not operate, then the following troubleshooting procedure should be carried out: Switch the power supply off. Check the tightness of all terminals and the continuity of all devices (the start push button should be pressed when investigating its continuity). If the contactor does not close while all the other parts function properly, connect a voltmeter across the coil terminals then connect the control circuit to the power supply. Two cases are possible : There is no voltage: Check again the continuity of the control circuit. The voltage is normal : - Check that the voltage indicated on the coil is the same as that measured by the voltmeter. - Make sure that the coil connections are tight. - Change the coil if the above two checks pass, then re-operate the circuit. 4.2.3.2 Remote control by several spring push buttons Circuit operation, fig (4-6): In the circuit diagram, two push buttons S3 and S4 are connected in parallel for the energization, and two push buttons S1 and S2 are connected in series for the contactor de-energization. Usually, one of the energization push buttons and one of the de-energization push buttons are Used for the remote control operation. Repeat the previous procedure for the two spring return push buttons circuit, except that in this case, four push buttons should be checked. 62 Week 10 Practical Work Q1 95 96 11 S1 12 11 S2 12 13 S4 14 13 S3 14 13 C11 14 (2) A1 C1 A2 Q2 (1) (1) - Control circuit (2) - Poles of the contactor with the thermal overload relay Figure (5-6): Circuit of several spring return push buttons Worksheet 10 63 Week 10 Practical Work Solve the following questions: 1. Connect the circuit as shown in fig. (4-5) with supply, and answer the following. a) What happened if you press on S1. ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... b) If the circuit does not operate, write down the preliminary checks should be carried out: ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... 64 Week 10 Practical Work ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... c) What happened if you press button S2 ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... 65 Week 10 Practical Work 2. Connect the circuit as shown in fig. (4-6) with supply, and answer the following. a) What happened if you press on S1. ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... b) When all preliminary checks have been carried out and the contactor still does not operate, then state the troubleshooting procedure should be carried out: 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................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... c) What happened if you press button S2 ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... d) What happened if you press button S3 ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... e) What happened if you press button S4 ................................................................................................................................................... ......................................... ................................................................................................................................................... 67 Week 10 Practical Work ......................................... ................................................................................................................................................... ......................................... ................................................................................................................................................... ......................................... 68
