{"id":43,"date":"2019-07-04T10:59:50","date_gmt":"2019-07-04T14:59:50","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/?post_type=chapter&#038;p=43"},"modified":"2019-10-11T14:37:10","modified_gmt":"2019-10-11T18:37:10","slug":"instrument-transformers","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/chapter\/instrument-transformers\/","title":{"raw":"Instrument Transformers","rendered":"Instrument Transformers"},"content":{"raw":"<span>Measuring and controlling circuits where very high currents or voltages are present cannot be done with standard multi-meters. The pressure on the meters would be too great and cause damage to both the meter and the person operating it.\u00a0<\/span><span>We use instrument transformers to step the values down to values that are safer to measure.\u00a0Instrument transformers are designed to step down large voltages and currents at a fixed, known proportion.<\/span>\r\n\r\n<span>The two most common types of instrument transformers are:<\/span>\r\n<ul>\r\n \t<li>\r\n<div><span>Current transformers (CTs)<\/span><\/div><\/li>\r\n \t<li>\r\n<div><span>Potential transformers (PTs)<\/span><\/div><\/li>\r\n<\/ul>\r\n<span>Polarity of instrument transformers is commonly denoted with dots showing like polarity.<\/span>\r\n<h2 align=\"left\"><strong><span>Current Transformers\u00a0<\/span><\/strong><\/h2>\r\n<span>A current transformer (CT) usually has its ratio fixed. For example, a CT has a ratio of 50:5. This is still a 10:1 transformer, but the ratio is given at something to 5.<\/span>\r\n\r\n<span>It is dangerous to open the secondary of a CT. Since there is no load on the secondary of a CT, the current in the secondary is determined by the current on the primary.\u00a0With the secondary open, there is no CEMF to hold back the primary core flux, and therefore the secondary must be short-circuited to prevent very large voltages from being a danger.<\/span>\r\n\r\n<span>The core is driven to saturation very quickly.<\/span>\r\n\r\n<span>As the max number of flux lines collapses 120 times per second, a very high voltage is induced on the secondary terminals.\u00a0<\/span><span>This can be VERY dangerous. Never work on a CT that has its secondary open as there can be a very high voltage present.<\/span>\r\n<h2 align=\"left\"><strong><span>Potential Transformers<\/span><\/strong><\/h2>\r\n<span>A potential transformer (PT) is usually constructed to have a secondary voltage of 120 V for standardization purposes.\u00a0<\/span>Much like a CT, a PT will have its ratio stated as something to 120 volts. For example, it may have a ratio of 12KV:120. This would mean the mathematical ratio is 100:1.\r\n\r\nA PT acts in the same fashion as a step-down transformer.\r\n<h2><span>Multiplier Ratio<\/span><\/h2>\r\n<span>The reading of a wattmeter connected to the secondary circuits of a PT and a CT would need to be multiplied by the ratio of both transformers.<\/span>\r\n\r\n<span>For example: 50:1 x 20:1 = 1000<\/span>\r\n\r\nThis is known as its multiplier.\r\n<div class=\"textbox textbox--exercises\"><header class=\"textbox__header\">\r\n<h2 class=\"textbox__title\" style=\"text-align: center\"><span style=\"color: #ffffff\">Example<\/span><\/h2>\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\n[caption id=\"attachment_149\" align=\"aligncenter\" width=\"870\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-content\/uploads\/sites\/777\/2019\/07\/Instrument-Transformers-e1566424411566-870x1024.png\" alt=\"\" width=\"870\" height=\"1024\" class=\"wp-image-149 size-large\" \/> Figure 23. Instrument transformer circuit[\/caption]\r\n<ul>\r\n \t<li><span>If the voltmeter reads 117 volts, what is the primary voltage? (6000:120 = 50:1. \u00a0115 x 50 = 5750 V)<\/span><\/li>\r\n \t<li><span>If the ammeter reads 4.6 A, what is the primary current? (50:5 = 10:1 \u00a04.6 A x 10 = 46 A)<\/span><\/li>\r\n<\/ul>\r\n<ul>\r\n \t<li><span>What is the multiplier for the wattmeter? (50:1 x 10:1 = 500:1)<\/span><\/li>\r\n<\/ul>\r\n<ul>\r\n \t<li><span>What is the true power delivered to the load? (46 A x 5750 V = 264,500 watts)<\/span><\/li>\r\n \t<li><span>What is the reading of the wattmeter? (264,500\/500 = 529 W)<\/span><\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<h2 style=\"text-align: center\"><strong>Video Alert!<\/strong><\/h2>\r\nThis video will walk through the concepts of instrument transformers and how to perform the calculations necessary.\r\n\r\n[embed]https:\/\/youtu.be\/fXQa0aEhfmU[\/embed]\r\n<h1>Attributions<\/h1>\r\n<a href=\"https:\/\/youtu.be\/fXQa0aEhfmU\">Instrument transformers<\/a>\u00a0video by <a href=\"https:\/\/www.youtube.com\/channel\/UCRm79BcxKhOWRj6CQAB_Cqg\">The Electric Academy<\/a> is under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">Creative Commons Attribution License<\/a>.","rendered":"<p><span>Measuring and controlling circuits where very high currents or voltages are present cannot be done with standard multi-meters. The pressure on the meters would be too great and cause damage to both the meter and the person operating it.\u00a0<\/span><span>We use instrument transformers to step the values down to values that are safer to measure.\u00a0Instrument transformers are designed to step down large voltages and currents at a fixed, known proportion.<\/span><\/p>\n<p><span>The two most common types of instrument transformers are:<\/span><\/p>\n<ul>\n<li>\n<div><span>Current transformers (CTs)<\/span><\/div>\n<\/li>\n<li>\n<div><span>Potential transformers (PTs)<\/span><\/div>\n<\/li>\n<\/ul>\n<p><span>Polarity of instrument transformers is commonly denoted with dots showing like polarity.<\/span><\/p>\n<h2 style=\"text-align: left;\"><strong><span>Current Transformers\u00a0<\/span><\/strong><\/h2>\n<p><span>A current transformer (CT) usually has its ratio fixed. For example, a CT has a ratio of 50:5. This is still a 10:1 transformer, but the ratio is given at something to 5.<\/span><\/p>\n<p><span>It is dangerous to open the secondary of a CT. Since there is no load on the secondary of a CT, the current in the secondary is determined by the current on the primary.\u00a0With the secondary open, there is no CEMF to hold back the primary core flux, and therefore the secondary must be short-circuited to prevent very large voltages from being a danger.<\/span><\/p>\n<p><span>The core is driven to saturation very quickly.<\/span><\/p>\n<p><span>As the max number of flux lines collapses 120 times per second, a very high voltage is induced on the secondary terminals.\u00a0<\/span><span>This can be VERY dangerous. Never work on a CT that has its secondary open as there can be a very high voltage present.<\/span><\/p>\n<h2 style=\"text-align: left;\"><strong><span>Potential Transformers<\/span><\/strong><\/h2>\n<p><span>A potential transformer (PT) is usually constructed to have a secondary voltage of 120 V for standardization purposes.\u00a0<\/span>Much like a CT, a PT will have its ratio stated as something to 120 volts. For example, it may have a ratio of 12KV:120. This would mean the mathematical ratio is 100:1.<\/p>\n<p>A PT acts in the same fashion as a step-down transformer.<\/p>\n<h2><span>Multiplier Ratio<\/span><\/h2>\n<p><span>The reading of a wattmeter connected to the secondary circuits of a PT and a CT would need to be multiplied by the ratio of both transformers.<\/span><\/p>\n<p><span>For example: 50:1 x 20:1 = 1000<\/span><\/p>\n<p>This is known as its multiplier.<\/p>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h2 class=\"textbox__title\" style=\"text-align: center\"><span style=\"color: #ffffff\">Example<\/span><\/h2>\n<\/header>\n<div class=\"textbox__content\">\n<figure id=\"attachment_149\" aria-describedby=\"caption-attachment-149\" style=\"width: 870px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-content\/uploads\/sites\/777\/2019\/07\/Instrument-Transformers-e1566424411566-870x1024.png\" alt=\"\" width=\"870\" height=\"1024\" class=\"wp-image-149 size-large\" srcset=\"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-content\/uploads\/sites\/777\/2019\/07\/Instrument-Transformers-e1566424411566-870x1024.png 870w, https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-content\/uploads\/sites\/777\/2019\/07\/Instrument-Transformers-e1566424411566-255x300.png 255w, https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-content\/uploads\/sites\/777\/2019\/07\/Instrument-Transformers-e1566424411566-768x904.png 768w, https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-content\/uploads\/sites\/777\/2019\/07\/Instrument-Transformers-e1566424411566-65x76.png 65w, https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-content\/uploads\/sites\/777\/2019\/07\/Instrument-Transformers-e1566424411566-225x265.png 225w, https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-content\/uploads\/sites\/777\/2019\/07\/Instrument-Transformers-e1566424411566-350x412.png 350w\" sizes=\"auto, (max-width: 870px) 100vw, 870px\" \/><figcaption id=\"caption-attachment-149\" class=\"wp-caption-text\">Figure 23. Instrument transformer circuit<\/figcaption><\/figure>\n<ul>\n<li><span>If the voltmeter reads 117 volts, what is the primary voltage? (6000:120 = 50:1. \u00a0115 x 50 = 5750 V)<\/span><\/li>\n<li><span>If the ammeter reads 4.6 A, what is the primary current? (50:5 = 10:1 \u00a04.6 A x 10 = 46 A)<\/span><\/li>\n<\/ul>\n<ul>\n<li><span>What is the multiplier for the wattmeter? (50:1 x 10:1 = 500:1)<\/span><\/li>\n<\/ul>\n<ul>\n<li><span>What is the true power delivered to the load? (46 A x 5750 V = 264,500 watts)<\/span><\/li>\n<li><span>What is the reading of the wattmeter? (264,500\/500 = 529 W)<\/span><\/li>\n<\/ul>\n<\/div>\n<\/div>\n<h2 style=\"text-align: center\"><strong>Video Alert!<\/strong><\/h2>\n<p>This video will walk through the concepts of instrument transformers and how to perform the calculations necessary.<\/p>\n<p><iframe loading=\"lazy\" id=\"oembed-1\" title=\"Instrument transformers\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/fXQa0aEhfmU?feature=oembed&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<h1>Attributions<\/h1>\n<p><a href=\"https:\/\/youtu.be\/fXQa0aEhfmU\">Instrument transformers<\/a>\u00a0video by <a href=\"https:\/\/www.youtube.com\/channel\/UCRm79BcxKhOWRj6CQAB_Cqg\">The Electric Academy<\/a> is under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">Creative Commons Attribution License<\/a>.<\/p>\n","protected":false},"author":422,"menu_order":11,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-43","chapter","type-chapter","status-publish","hentry"],"part":3,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-json\/pressbooks\/v2\/chapters\/43","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-json\/wp\/v2\/users\/422"}],"version-history":[{"count":12,"href":"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-json\/pressbooks\/v2\/chapters\/43\/revisions"}],"predecessor-version":[{"id":238,"href":"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-json\/pressbooks\/v2\/chapters\/43\/revisions\/238"}],"part":[{"href":"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-json\/pressbooks\/v2\/parts\/3"}],"metadata":[{"href":"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-json\/pressbooks\/v2\/chapters\/43\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-json\/wp\/v2\/media?parent=43"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-json\/pressbooks\/v2\/chapter-type?post=43"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-json\/wp\/v2\/contributor?post=43"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/singlephasetransformers\/wp-json\/wp\/v2\/license?post=43"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}