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Auto Transformer vs Isolation Transformer


What is auto type transformer? Auto transformers have one voltage winding, shared by both sides.  The term "auto" refers to a single coil acting alone, not to any kind of automatic mechanism.  You tap the winding along its length to provide some of the primary voltage across its second load.  You should use auto transformers in applications where you need to step up or step down on three-phase line voltages.


Advantages of an auto transformer:

  • Cheap and more efficient
  • Less leakage inductance between primary and secondary, thus less losses
  • Simple construction to the OEM
  • Smaller footprint for same VA rating


The primary purpose of an auto transformer is to regulate the voltage of transmission lines and can be used to transform voltages.  With only one winding, an auto transformer adjusts the voltage automatically according to load.  These transformers call for AC currents to operate correctly and will not function on direct current.


Application for auto transformers can include:

  • Used as a starter to give up to 50 to 60% of full voltage to the starter of a squirrel cage induction motor during starting up
  • Used to give a small boost to a distribution cable, to correct the voltage drop 
  • Used as a voltage regulator
  • Used in power transmission and distribution systems, audio system and railways simply to act as a voltage changer


Now what is an isolation transformer?  This is a true transformer in a sense it does transformer the voltage between windings, it is an electrical transformer with a primary and secondary coil winding.  These windings are separated by insulation which limits the risk of electrocution when the active parts and earth are touched simultaneously.  They deliver a better power quality, safer, fewer surges pass through them and are typically not as noisy as other transformer types.


Advantages of an isolation Transformer:

  • Better power quality
  • Noise reduction
  • Reduces surges
  • Safety


The primary purpose of an isolation transformer is to isolate circuits.  These transformers are designed and manufactured with attention to capacitive coupling between the two windings.  Capacitance between primary and secondary windings would also couple alternating current (AC) current from the primary to the secondary.


Applications for isolation transformers can include:

  • For preventing data corruption and failure in computers using hard disc
  • The protection of telecommunication and data communication equipment
  • For the patient's safety while using machines like ECG, EEG or EMG pacemakers or any electrosurgical aid 
  • Clear recording in audio and video recording equipment
  • For error free data in process control instruments 
  • The elimination of earthing problem in electrical installations
  • The protection of CNC/PLC equipment from surges/spikes
  • Reduction of electrical noise or harmonics in the system for a specific design or level


Contact 3L2R Inc. today for an evaluation of your specific transformer needs, whether it is an isolation transformer vs and auto transformer


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Mike Raposo
November 17, 2021
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