{"id":125,"date":"2020-01-13T13:51:54","date_gmt":"2020-01-13T18:51:54","guid":{"rendered":"https:\/\/opentextbc.ca\/basicmotorcontrol\/chapter\/power-circuit\/"},"modified":"2020-08-20T12:23:18","modified_gmt":"2020-08-20T16:23:18","slug":"power-circuit","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/basicmotorcontrol\/chapter\/power-circuit\/","title":{"raw":"Power Circuit","rendered":"Power Circuit"},"content":{"raw":"The [pb_glossary id=\"243\"]power circuit[\/pb_glossary] of a three-phase motor consists of the following items:\n<ul>\n \t<li>3PST three-phase disconnect<\/li>\n \t<li>[pb_glossary id=\"209\"]Overcurrent[\/pb_glossary] protection<\/li>\n \t<li>[pb_glossary id=\"233\"]Horsepower-rated motor starter contacts[\/pb_glossary]<\/li>\n \t<li>Overload relay heaters<\/li>\n<\/ul>\n<img class=\"aligncenter wp-image-124 size-large\" src=\"https:\/\/opentextbc.ca\/accessibilitytoolkit\/wp-content\/uploads\/sites\/314\/2020\/01\/Power-Circuit-3-1-e1581616309290-1024x444.png\" alt=\"\" width=\"1024\" height=\"444\">\n\nIn practical applications, these four components would often be mounted in a combination starter, which might house a [pb_glossary id=\"250\"]circuit breaker[\/pb_glossary] and [pb_glossary id=\"227\"]motor starter[\/pb_glossary] in the same cabinet.\n\nIn order for the motor to operate, the 3-phase disconnect must be closed, the [pb_glossary id=\"211\"]fuses[\/pb_glossary] must be in good working order and the power contacts must be closed. The start and stop action of the motor is determined by the [pb_glossary id=\"242\"]control circuit[\/pb_glossary], which energizes the magnetic motor starter, pulling in or dropping out the [pb_glossary id=\"263\"]armature[\/pb_glossary] that then opens or closes the power contacts.\n\nThe current that goes to the motor then runs through the heater elements of the [pb_glossary id=\"241\"]overload relay[\/pb_glossary]. If the motor draws too high a current for too long, the heater element causes a [pb_glossary id=\"230\"]normally closed contact[\/pb_glossary] to open, which then interrupts the current to the motor starters coil.\u00a0 This causes the power contacts to automatically open and turn off the motor.\n\nThis relationship also provides [pb_glossary id=\"215\"]low-voltage protection (LVP)[\/pb_glossary] in the case of a power outage. Motors controlled from magnetic starters need to be manually reset following a period of low [pb_glossary id=\"219\"]voltage[\/pb_glossary].\n\nThe motor starters that we will be focusing on are \u201cacross-the-line\u201d magnetic motor starters. This refers to them using full-line voltage to start the motor.\n\nAcross-the-line motor starters draw large [pb_glossary id=\"207\"]inrush currents[\/pb_glossary] when energized. To minimize inrush currents and severe starting torques, some circuits use a variety of \u201csoft-start\u201d devices, but these devices are beyond the scope of this text.\n\nPower circuits are identical for most 3-phase motor starters, so it is common not to refer to them in [pb_glossary id=\"245\"]schematic drawings[\/pb_glossary].","rendered":"<p>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_125_243\">power circuit<\/a> of a three-phase motor consists of the following items:<\/p>\n<ul>\n<li>3PST three-phase disconnect<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_125_209\">Overcurrent<\/a> protection<\/li>\n<li><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_125_233\">Horsepower-rated motor starter contacts<\/a><\/li>\n<li>Overload relay heaters<\/li>\n<\/ul>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-124 size-large\" src=\"https:\/\/opentextbc.ca\/accessibilitytoolkit\/wp-content\/uploads\/sites\/314\/2020\/01\/Power-Circuit-3-1-e1581616309290-1024x444.png\" alt=\"\" width=\"1024\" height=\"444\" srcset=\"https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-content\/uploads\/sites\/314\/2020\/01\/Power-Circuit-3-1-e1581616309290-1024x444.png 1024w, https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-content\/uploads\/sites\/314\/2020\/01\/Power-Circuit-3-1-e1581616309290-300x130.png 300w, https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-content\/uploads\/sites\/314\/2020\/01\/Power-Circuit-3-1-e1581616309290-768x333.png 768w, https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-content\/uploads\/sites\/314\/2020\/01\/Power-Circuit-3-1-e1581616309290-1536x666.png 1536w, https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-content\/uploads\/sites\/314\/2020\/01\/Power-Circuit-3-1-e1581616309290-2048x887.png 2048w, https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-content\/uploads\/sites\/314\/2020\/01\/Power-Circuit-3-1-e1581616309290-65x28.png 65w, https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-content\/uploads\/sites\/314\/2020\/01\/Power-Circuit-3-1-e1581616309290-225x97.png 225w, https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-content\/uploads\/sites\/314\/2020\/01\/Power-Circuit-3-1-e1581616309290-350x152.png 350w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><\/p>\n<p>In practical applications, these four components would often be mounted in a combination starter, which might house a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_125_250\">circuit breaker<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_125_227\">motor starter<\/a> in the same cabinet.<\/p>\n<p>In order for the motor to operate, the 3-phase disconnect must be closed, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_125_211\">fuses<\/a> must be in good working order and the power contacts must be closed. The start and stop action of the motor is determined by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_125_242\">control circuit<\/a>, which energizes the magnetic motor starter, pulling in or dropping out the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_125_263\">armature<\/a> that then opens or closes the power contacts.<\/p>\n<p>The current that goes to the motor then runs through the heater elements of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_125_241\">overload relay<\/a>. If the motor draws too high a current for too long, the heater element causes a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_125_230\">normally closed contact<\/a> to open, which then interrupts the current to the motor starters coil.\u00a0 This causes the power contacts to automatically open and turn off the motor.<\/p>\n<p>This relationship also provides <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_125_215\">low-voltage protection (LVP)<\/a> in the case of a power outage. Motors controlled from magnetic starters need to be manually reset following a period of low <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_125_219\">voltage<\/a>.<\/p>\n<p>The motor starters that we will be focusing on are \u201cacross-the-line\u201d magnetic motor starters. This refers to them using full-line voltage to start the motor.<\/p>\n<p>Across-the-line motor starters draw large <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_125_207\">inrush currents<\/a> when energized. To minimize inrush currents and severe starting torques, some circuits use a variety of \u201csoft-start\u201d devices, but these devices are beyond the scope of this text.<\/p>\n<p>Power circuits are identical for most 3-phase motor starters, so it is common not to refer to them in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_125_245\">schematic drawings<\/a>.<\/p>\n<div class=\"glossary\"><span class=\"screen-reader-text\" id=\"definition\">definition<\/span><template id=\"term_125_243\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_125_243\"><div tabindex=\"-1\"><p>In contrast to the control circuit, the power circuit provides the large values of voltage and current used by the motor itself. Must be equipped with overcurrent and overload protection, and horsepower-rated contacts in the control gear equal to the voltage and current ratings of the motor.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_125_209\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_125_209\"><div tabindex=\"-1\"><p>A sharp and fast rise in current over a short period of time (fractions of a second) where the value of current is far greater than the nominal line current.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_125_233\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_125_233\"><div tabindex=\"-1\"><p>A set of contacts that has been designed to make or break the flow of current to a motor and can handle loads up to a specified amount of horsepower at a specific voltage. The size of contacts are determined by the size of the motor they are controlling.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_125_250\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_125_250\"><div tabindex=\"-1\"><p>An automatic device that is designed to safely disconnect circuits under fault conditions. Most circuit breakers provide Overload and Overcurrent protection, and are rated in Volts, Amps and Horsepower.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_125_227\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_125_227\"><div tabindex=\"-1\"><p>A device that controls the flow of electrical power to a motor. It is designed to safely start and stop a motor, and provide <strong>overload protection<\/strong>.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_125_211\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_125_211\"><div tabindex=\"-1\"><p>An insulated tube containing a strip of conductive metal that has a lower melting point than either copper or aluminum. It protects a circuit from damage because it will melt in overload or overcurrent situations and break the connection with the rest of the circuit.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_125_242\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_125_242\"><div tabindex=\"-1\"><p>In contrast to the Power Circuit, the Control Circuit consists of inputs, in the form of switches, pushbuttons or pilot devices, which when activated, can either directly, or through a magnetic motor starter, energize a load. The Control Circuit often operates at a lower voltage than the Power Circuit for safety and ease of installation.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_125_263\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_125_263\"><div tabindex=\"-1\"><p>With respect to magnetic contactors, the armature or plunger is the movable part of the magnetic circuit. When a coil is energized the armature is pulled in, opening and\/or closing a set or sets of contacts.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_125_241\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_125_241\"><div tabindex=\"-1\"><p>A heater element paired with normally-closed contacts that open once the heater gets too hot. Two types of relays are the bimetallic strip and the melting solder pot.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_125_230\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_125_230\"><div tabindex=\"-1\"><p>A contact that under normal conditions has continuity through it. When the contact changes its state it interrupts the flow of current by opening its contacts. Can be associated with pushbuttons, pilot devices or magnetic contactors.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_125_215\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_125_215\"><div tabindex=\"-1\"><p>Circuits with low-voltage protection will not automatically turn back on when voltage is restored following a power outage. Examples include the microwave or power tools.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_125_219\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_125_219\"><div tabindex=\"-1\"><p>The difference in electric potential between two points, which is defined as the work needed per unit of charge to move a test charge between the two points. It is measured in volts (V).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_125_207\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_125_207\"><div tabindex=\"-1\"><p>The initial high value of current produced when an inductive load is first energized.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_125_245\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_125_245\"><div tabindex=\"-1\"><p>A diagram that shows how a circuit works logically and electrically. It uses symbols to identify components and interconnecting lines to display the electrical continuity of a circuit. It is often used for troubleshooting purposes. Also known as a ladder diagram.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><\/div>","protected":false},"author":90,"menu_order":2,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-125","chapter","type-chapter","status-publish","hentry"],"part":121,"_links":{"self":[{"href":"https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-json\/pressbooks\/v2\/chapters\/125","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-json\/wp\/v2\/users\/90"}],"version-history":[{"count":2,"href":"https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-json\/pressbooks\/v2\/chapters\/125\/revisions"}],"predecessor-version":[{"id":294,"href":"https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-json\/pressbooks\/v2\/chapters\/125\/revisions\/294"}],"part":[{"href":"https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-json\/pressbooks\/v2\/parts\/121"}],"metadata":[{"href":"https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-json\/pressbooks\/v2\/chapters\/125\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-json\/wp\/v2\/media?parent=125"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-json\/pressbooks\/v2\/chapter-type?post=125"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-json\/wp\/v2\/contributor?post=125"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/opentextbc.ca\/basicmotorcontrol\/wp-json\/wp\/v2\/license?post=125"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}