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class="site-content clr"> <article id="post-5324"> <header class="entry-header clr"> <h1 class="single-post-title entry-title" itemprop="headline">What is an autotransformer and how does it work?</h1> </header> <div class="entry-content clr" itemprop="text"> <div class="code-block code-block-2" style="margin: 8px 0; clear: both;">   <ins class="adsbygoogle" style="display:block" data-ad-client="ca-pub-4567036280313464" data-ad-slot="7226276104" data-ad-format="auto" data-full-width-responsive="true"></ins> <script> (adsbygoogle = window.adsbygoogle || []).push({}); </script></div> <p>An autotransformer, also known as<strong> variac or dimmerstat</strong>, is a single winding transformer which has a common primary and secondary winding. The primary and secondary winding in them is not electrically isolated. They are cheaper, efficient and have a better voltage regulation than the normal transformers. They are much useful in cases in which the voltage ratio is less than 2 and the electrical isolation of winding is not essential.</p> <blockquote class="wp-block-quote"><p>Note: The term ‘Auto’ in ‘autotransformer’ does not refer to automatic output voltage variation. </p></blockquote> <div class="ub_table-of-contents" data-showtext="show" data-hidetext="hide" data-scrolltype="auto" id="ub_table-of-contents-08213244-2dd2-40ef-8823-e9a3ec455fbe" data-initiallyhideonmobile="false" data-initiallyshow="true"><div class="ub_table-of-contents-header"> <div class="ub_table-of-contents-title">Table of contents</div></div><div class="ub_table-of-contents-container ub_table-of-contents-1-column "><ul><li><a href="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/autotransformers-variac-dimmerstat/#0-construction">Construction</a><ul><li><a href="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/autotransformers-variac-dimmerstat/#1-core">Core</a></li><li><a href="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/autotransformers-variac-dimmerstat/#2-winding">Winding</a></li><li><a href="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/autotransformers-variac-dimmerstat/#3-moving-contact-knob-arrangement">Moving-contact-knob arrangement</a></li></ul></li><li><a href="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/autotransformers-variac-dimmerstat/#4-working-of-autotransformers">Working of autotransformers</a></li><li><a href="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/autotransformers-variac-dimmerstat/#5-copper-savings-in-autotransformer">Copper savings in autotransformer</a></li><li><a href="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/autotransformers-variac-dimmerstat/#6-applications">Applications</a></li><li><a href="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/autotransformers-variac-dimmerstat/#7-advantages-and-disadvantages-of-autotransformers-over-two-winding-transformer">Advantages and disadvantages of autotransformers over two winding transformer</a><ul><li><a href="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/autotransformers-variac-dimmerstat/#8-advantages">Advantages</a></li><li><a href="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/autotransformers-variac-dimmerstat/#9-disadvantages">Disadvantages</a></li></ul></li></ul></div></div> <h2 id="0-construction">Construction</h2> <p>An <strong>autotransformer</strong> consists of three parts: core, winding and moving contact – knob arrangement. </p> <div class="wp-block-image"><figure class="aligncenter size-large is-resized"><picture class="wp-image-5327"> <source type="image/webp" data-lazy-srcset="https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction-1024x477.png.webp 1024w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction-300x140.png.webp 300w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction-768x358.png.webp 768w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction.png.webp 1350w" srcset="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20512%20239'%3E%3C/svg%3E" data-lazy-sizes="(max-width: 512px) 100vw, 512px"/> <img src="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20512%20239'%3E%3C/svg%3E" alt="Parts of autotransformer" width="512" height="239" data-lazy-srcset="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction-1024x477.png 1024w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction-300x140.png 300w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction-768x358.png 768w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction.png 1350w" data-lazy-sizes="(max-width: 512px) 100vw, 512px" data-lazy-src="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction-1024x477.png"/> </picture> <noscript><picture class="wp-image-5327"> <source type="image/webp" srcset="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction-1024x477.png.webp 1024w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction-300x140.png.webp 300w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction-768x358.png.webp 768w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction.png.webp 1350w" sizes="(max-width: 512px) 100vw, 512px"/> <img src="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction-1024x477.png" alt="Parts of autotransformer" width="512" height="239" srcset="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction-1024x477.png 1024w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction-300x140.png 300w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction-768x358.png 768w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Autotransformer-construction.png 1350w" sizes="(max-width: 512px) 100vw, 512px"/> </picture> </noscript></figure></div> <h3 id="1-core">Core</h3> <p>The core is made of high-quality Cold Rolled Grain Oriented (CRGO) silicon steel in order to minimize core losses. The core could be laminated, split, or toroidal type</p> <h3 id="2-winding">Winding</h3> <p>As said earlier, autotransformers have a single copper conductor wound over the core which acts as the primary and secondary.</p> <h3 id="3-moving-contact-knob-arrangement">Moving-contact-knob arrangement</h3> <p>A rotary arm like arrangement with carbon brush at its tip and a knob at the top is present in autotransformers. The carbon brush slides over the windings when the knob is rotated. Smooth control of voltage can be attained with the help of the sliding brush.</p> <div class="wp-block-image"><figure class="aligncenter size-large is-resized"><picture class="wp-image-5330"> <source type="image/webp" data-lazy-srcset="https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1.jpg.webp 500w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1-300x300.jpg.webp 300w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1-150x150.jpg.webp 150w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1-50x50.jpg.webp 50w" srcset="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20250%20250'%3E%3C/svg%3E" data-lazy-sizes="(max-width: 250px) 100vw, 250px"/> <img src="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20250%20250'%3E%3C/svg%3E" alt="Three phase autotransformer" width="250" height="250" data-lazy-srcset="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1.jpg 500w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1-300x300.jpg 300w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1-150x150.jpg 150w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1-50x50.jpg 50w" data-lazy-sizes="(max-width: 250px) 100vw, 250px" data-lazy-src="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1.jpg"/> </picture> <noscript><picture class="wp-image-5330"> <source type="image/webp" srcset="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1.jpg.webp 500w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1-300x300.jpg.webp 300w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1-150x150.jpg.webp 150w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1-50x50.jpg.webp 50w" sizes="(max-width: 250px) 100vw, 250px"/> <img src="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1.jpg" alt="Three phase autotransformer" width="250" height="250" srcset="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1.jpg 500w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1-300x300.jpg 300w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1-150x150.jpg 150w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/three-phase-variable-auto-transformer-variac-500x500-1-50x50.jpg 50w" sizes="(max-width: 250px) 100vw, 250px"/> </picture> </noscript></figure></div> <p>In the case of three-phase autotransformers, three single-phase autotransformers are arranged one above the other, as in the picture above, and controlled by a single knob.</p> <h2 id="4-working-of-autotransformers">Working of autotransformers</h2> <p>As mentioned earlier, in an autotransformer single winding serves as both primary and secondary. The terminals A and C acts as the input (primary) and the terminals E and C acts as the secondary. E is connected to the sliding carbon brush.</p> <figure class="wp-block-image size-large"><picture class="wp-image-5333"> <source type="image/webp" data-lazy-srcset="https://www.electricalclassroom.com/wp-content/uploads/2020/11/image-1.png.webp 885w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/image-1-300x125.png.webp 300w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/image-1-768x321.png.webp 768w" srcset="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20885%20370'%3E%3C/svg%3E" data-lazy-sizes="(max-width: 885px) 100vw, 885px"/> <img width="885" height="370" src="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20885%20370'%3E%3C/svg%3E" alt="Working of autotransformer" data-lazy-srcset="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/wp-content/uploads/2020/11/image-1.png 885w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/image-1-300x125.png 300w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/image-1-768x321.png 768w" data-lazy-sizes="(max-width: 885px) 100vw, 885px" data-lazy-src="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/wp-content/uploads/2020/11/image-1.png"/> </picture> <noscript><picture class="wp-image-5333"> <source type="image/webp" srcset="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/image-1.png.webp 885w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/image-1-300x125.png.webp 300w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/image-1-768x321.png.webp 768w" sizes="(max-width: 885px) 100vw, 885px"/> <img width="885" height="370" src="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/image-1.png" alt="Working of autotransformer" srcset="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/image-1.png 885w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/image-1-300x125.png 300w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/image-1-768x321.png 768w" sizes="(max-width: 885px) 100vw, 885px"/> </picture> </noscript></figure> <p>A part of energy transfer between the primary and the secondary is through conduction and the rest is through induction. In the schematic shown below, Let N<sub>p</sub> be the total number of turns wound over the transformer core, the brush is moved to point C to obtain a secondary turns N<sub>s</sub>. V<sub>p</sub> and V<sub>s</sub> are the voltage measured across the primary and secondary currents respectively. I<sub>p </sub>and I<sub>s</sub> are the primary and secondary currents.</p> <figure class="wp-block-image size-large is-resized"><picture class="wp-image-5336"> <source type="image/webp" data-lazy-srcset="https://www.electricalclassroom.com/wp-content/uploads/2020/11/Schematic-Diagram-of-autotransformer.png.webp 602w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/Schematic-Diagram-of-autotransformer-300x208.png.webp 300w" srcset="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20301%20209'%3E%3C/svg%3E" data-lazy-sizes="(max-width: 301px) 100vw, 301px"/> <img src="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20301%20209'%3E%3C/svg%3E" alt="Schematic Diagram of autotransformer" width="301" height="209" data-lazy-srcset="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Schematic-Diagram-of-autotransformer.png 602w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/Schematic-Diagram-of-autotransformer-300x208.png 300w" data-lazy-sizes="(max-width: 301px) 100vw, 301px" data-lazy-src="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Schematic-Diagram-of-autotransformer.png"/> </picture> <noscript><picture class="wp-image-5336"> <source type="image/webp" srcset="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Schematic-Diagram-of-autotransformer.png.webp 602w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Schematic-Diagram-of-autotransformer-300x208.png.webp 300w" sizes="(max-width: 301px) 100vw, 301px"/> <img src="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Schematic-Diagram-of-autotransformer.png" alt="Schematic Diagram of autotransformer" width="301" height="209" srcset="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Schematic-Diagram-of-autotransformer.png 602w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Schematic-Diagram-of-autotransformer-300x208.png 300w" sizes="(max-width: 301px) 100vw, 301px"/> </picture> </noscript></figure> <p>The voltage and current transformation ratio (turns ratio) is same as that of the two winding transformer. </p> <p>The number of turns between the points A and B is N<sub>p</sub>, the number of turns between A and C is (N<sub>p</sub>-N<sub>s</sub>). Since all the turns link the same flux in the transformer core,</p> <p>The turns ratio ‘a’ can be varied by moving the carbon brush through the winding and changing the position of the point C.</p> <p>Turns ratio, a <img src="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20128%2030'%3E%3C/svg%3E" class="ql-img-inline-formula " alt="= \frac{V_p}{V_s} = \frac{N_p}{N_s} = \frac{I_s}{I_p}" title="Rendered by QuickLaTeX.com" height="30" width="128" style="vertical-align: -10px;" data-lazy-src="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/wp-content/ql-cache/quicklatex.com-eb624cc2b6422c723e116a0f2397ce04_l3.svg"/><noscript><img src="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/ql-cache/quicklatex.com-eb624cc2b6422c723e116a0f2397ce04_l3.svg" class="ql-img-inline-formula " alt="= \frac{V_p}{V_s} = \frac{N_p}{N_s} = \frac{I_s}{I_p}" title="Rendered by QuickLaTeX.com" height="30" width="128" style="vertical-align: -10px;"/></noscript></p> <p>The turns ratio of the upper part of the transformer is given by</p> <p class="ql-center-displayed-equation" style="line-height: 44px;"><span class="ql-right-eqno"> </span><span class="ql-left-eqno"> </span><img src="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20246%2044'%3E%3C/svg%3E" height="44" width="246" class="ql-img-displayed-equation " alt="\[ \frac{V_p - V_s }{V_p} = \frac{N_p-N_s}{N_p} = (1-\frac{1}{a}) \]" title="Rendered by QuickLaTeX.com" data-lazy-src="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/wp-content/ql-cache/quicklatex.com-86015f920f26de101d3b4fe1cb0c6743_l3.svg"/><noscript><img src="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/ql-cache/quicklatex.com-86015f920f26de101d3b4fe1cb0c6743_l3.svg" height="44" width="246" class="ql-img-displayed-equation " alt="\[ \frac{V_p - V_s }{V_p} = \frac{N_p-N_s}{N_p} = (1-\frac{1}{a}) \]" title="Rendered by QuickLaTeX.com"/></noscript></p> <h2 id="5-copper-savings-in-autotransformer">Copper savings in autotransformer</h2> <p>Copper conductor contributes to the larger part of manufacturing cost of a transformer. As the primary and secondary of the autotransformers share the same winding, a huge cost savings can be achieved.</p> <p>The weight of the copper winding depends on the length and thickness of the conductor used. Therefore it is proportional to the number of turns of copper used in the transformer. </p> <p class="has-text-align-center"><strong>The weight of copper ∝ (length of cu. wire × cross-sec. area of cu. wire</strong>)</p> <p>The area of cross- section of copper wire is directly proportional to the amount of current it can conduct.</p> <p>Therefore, the weight of copper, N <strong>∝</strong> N <strong>×</strong> I</p> <p>The weight of copper (W<sub>AT</sub>) needed for an autotransformer is proportional to:</p> <p class="has-text-align-center">W<sub>AT</sub> <strong>∝</strong> (N<sub>p</sub> – N<sub>s</sub>).I<sub>p</sub> + Ns.(I<sub>s</sub> – I<sub>p</sub>) </p> <p class="has-text-align-center">(i.e.) W<sub>AT</sub> <strong>∝</strong> N<sub>p</sub>.I<sub>p</sub> + Ns.I<sub>s</sub> – 2N<sub>s</sub>.I<sub>p</sub></p> <p class="has-text-align-left">Since N<sub>p</sub>.I<sub>p</sub> = Ns.I<sub>s</sub>, </p> <p class="has-text-align-center">W<sub>AT</sub> <strong>∝</strong> 2(N<sub>p</sub> – N<sub>s</sub>).I<sub>p</sub> ——— Equation (1)</p> <p>Now, let us calculate the weight of copper (W<sub>TW</sub>) required for a two winding transformer.</p> <p class="has-text-align-center">W<sub>TW</sub> <strong>∝</strong> N<sub>p</sub>.I<sub>p</sub> + Ns.I<sub>s</sub></p> <p>Since N<sub>p</sub>.I<sub>p</sub> = Ns.I<sub>s</sub>,</p> <p class="has-text-align-center">W<sub>TW</sub> <strong>∝</strong> 2N<sub>p</sub>.I<sub>p</sub> ——— Equation (2)</p> <p>Dividing equation (1) by equation (2),</p> <p class="ql-center-displayed-equation" style="line-height: 44px;"><span class="ql-right-eqno"> </span><span class="ql-left-eqno"> </span><img src="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20215%2044'%3E%3C/svg%3E" height="44" width="215" class="ql-img-displayed-equation " alt="\[ \frac{W_A_T}{W_T_W} = \frac{N_p-N_s}{N_p} = 1 - \frac{1}{a} \]" title="Rendered by QuickLaTeX.com" data-lazy-src="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/wp-content/ql-cache/quicklatex.com-7605617d9e872aafff19e5850707617b_l3.svg"/><noscript><img src="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/ql-cache/quicklatex.com-7605617d9e872aafff19e5850707617b_l3.svg" height="44" width="215" class="ql-img-displayed-equation " alt="\[ \frac{W_A_T}{W_T_W} = \frac{N_p-N_s}{N_p} = 1 - \frac{1}{a} \]" title="Rendered by QuickLaTeX.com"/></noscript></p> <p>This means if the turns ratio is 2, the weight of copper required for an autotransformer is half of that required for two winding transformers.</p> <p><strong>Read More</strong>: <a href="/web/20210602232644/https://www.electricalclassroom.com/transformer-ratio-calculator-online/" class="rank-math-link">Transformer turns ratio</a></p> <h2 id="6-applications">Applications</h2> <ul><li>Autotransformers are widely used in laboratories to obtain variable AC voltage for lab apparatus. </li><li>They are used to start induction and synchronous motors.</li></ul> <h2 id="7-advantages-and-disadvantages-of-autotransformers-over-two-winding-transformer">Advantages and disadvantages of autotransformers over two winding transformer</h2> <h3 id="8-advantages">Advantages</h3> <ul><li>For the same VA rating, an autotransformer requires less copper and less core material.</li><li>Also, it requires less exciting current, has an ohmic loss and is lighter as compared to a two winding transformer. </li><li>Autotransformers have higher efficiency and lower leakage impedance than two winding transformers. </li><li>They have a better voltage regulation than two winding transformers.</li></ul> <h3 id="9-disadvantages">Disadvantages</h3> <ul><li>They are preferred for a voltage ratio of less than 2. </li><li>Break in the part of the winding that is used as both primary and secondary will result in the transformer acts as an inductor in series with the load. </li><li>Autotransformers do not isolate primary and secondary circuits. Hence any failures in the transformer can result in the application of full primary voltage to the load.</li><li>Due to lesser impedance, the short circuit current in autotransformer is higher than that in two winding transformer.</li></ul> <figure class="wp-block-image size-large"><a href="https://web.archive.org/web/20210602232644/https://www.globalspec.com/learnmore/electrical_electronic_components/transformers/autotransformers" target="_blank" rel="noopener"><img width="1024" height="1024" src="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%201024%201024'%3E%3C/svg%3E" alt="Variable autotransformer" class="wp-image-5363" data-lazy-srcset="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Variable_autotransformer_0-220_V_4_A_880_VA-1096db1caf15f41346309ffc2b03b9dd64ab-1024x1024.png 1024w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/Variable_autotransformer_0-220_V_4_A_880_VA-1096db1caf15f41346309ffc2b03b9dd64ab-300x300.png 300w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/Variable_autotransformer_0-220_V_4_A_880_VA-1096db1caf15f41346309ffc2b03b9dd64ab-150x150.png 150w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/Variable_autotransformer_0-220_V_4_A_880_VA-1096db1caf15f41346309ffc2b03b9dd64ab-768x768.png 768w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/Variable_autotransformer_0-220_V_4_A_880_VA-1096db1caf15f41346309ffc2b03b9dd64ab-1536x1536.png 1536w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/Variable_autotransformer_0-220_V_4_A_880_VA-1096db1caf15f41346309ffc2b03b9dd64ab-2048x2048.png 2048w, https://www.electricalclassroom.com/wp-content/uploads/2020/11/Variable_autotransformer_0-220_V_4_A_880_VA-1096db1caf15f41346309ffc2b03b9dd64ab-50x50.png 50w" data-lazy-sizes="(max-width: 1024px) 100vw, 1024px" data-lazy-src="https://web.archive.org/web/20210602232644/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Variable_autotransformer_0-220_V_4_A_880_VA-1096db1caf15f41346309ffc2b03b9dd64ab-1024x1024.png"/><noscript><img width="1024" height="1024" src="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Variable_autotransformer_0-220_V_4_A_880_VA-1096db1caf15f41346309ffc2b03b9dd64ab-1024x1024.png" alt="Variable autotransformer" class="wp-image-5363" srcset="https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Variable_autotransformer_0-220_V_4_A_880_VA-1096db1caf15f41346309ffc2b03b9dd64ab-1024x1024.png 1024w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Variable_autotransformer_0-220_V_4_A_880_VA-1096db1caf15f41346309ffc2b03b9dd64ab-300x300.png 300w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Variable_autotransformer_0-220_V_4_A_880_VA-1096db1caf15f41346309ffc2b03b9dd64ab-150x150.png 150w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Variable_autotransformer_0-220_V_4_A_880_VA-1096db1caf15f41346309ffc2b03b9dd64ab-768x768.png 768w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Variable_autotransformer_0-220_V_4_A_880_VA-1096db1caf15f41346309ffc2b03b9dd64ab-1536x1536.png 1536w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Variable_autotransformer_0-220_V_4_A_880_VA-1096db1caf15f41346309ffc2b03b9dd64ab-2048x2048.png 2048w, https://web.archive.org/web/20210602232644im_/https://www.electricalclassroom.com/wp-content/uploads/2020/11/Variable_autotransformer_0-220_V_4_A_880_VA-1096db1caf15f41346309ffc2b03b9dd64ab-50x50.png 50w" sizes="(max-width: 1024px) 100vw, 1024px"/></noscript></a></figure> <div class="code-block code-block-7" style="margin: 8px auto; 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