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Single phase autotransformer with ring-lug barrier strip.


Quality Transformer and Electronics custom-designs and manufactures Autotransformers. These transformers, are used in power applications to interconnect systems operating at different voltage levels where isolation is not required. An autotransformer, also called an autoformer, is used to step voltages up or down. The primary and secondary windings share common turns and thus provide no isolation. Autoformers are typically smaller for an equivalent power level when compared with a transformer with isolated primary and secondary windings and are therefore often less expensive.

Our Capabilities


The Autotransformer Connection

Also called an Autoformer

Autotransformers are used in applications where electrical isolation is not a critical requirement. In Autotransformers, the primary and secondary circuits have part of a winding in common, resulting in a reduction in size and cost. When the ratio V2:V1 is 0.3 or more, Autotransformers are used with advantage. It is important to note that as input and output voltages are closer to one other, savings in material are higher. Common applications are motor starters, boosters, and/or static balancers.

There are several advantages in using an autotransformer. The voltage/current transformation and impedance conversion aspects of a two winding transformer are retained but with lesser material and weight, translating into lower costs. Common electrical differences between the isolated transformer and the autotransformer include:

  • the losses are reduced which will increase the efficiency,
  • reactance is reduced resulting in better regulation characteristics, and
  • lower exciting current will occur.

All these benefits are enhanced as the voltage ratio approaches unity.

However, there are also disadvantages to be considered with this type of arrangement, such as:

  • loss of electrical isolation and a larger short circuit current,
  • larger short circuit forces on the winding as well, and
  • common neutral/ground for primary and secondary.


  • Single phase autotransformer with mounting dimensions designed to meet customer specifications.
    3.5 kVA Autotransformers
    Power Rating 3.5 kVA
    Frequency 50/60 Hz
    No. of Phases 1
    Primary 208 Vrms
    Secondary 300 Vrms
    • CE Mark
    • NRTL to UL506
    • EN 60742
    This autotransformer is used in the semiconductor capital equipment industry to help step-up voltage for a piece of equipment.
  • Side angle of Autotransformer.
  • View of 3.5 kVA single-phase autotransformer.
  • Electrical label on autotransformer. Included is certification marks.
  • Autotransformer drawing designed in Autocad.

Example 2


Power Rating 160 kVA
Frequency 50/60 Hz
Phases Three
Primary 380 Vrms +/- 10% taps
Secondary 480 Vrms @ 1,000 kVA
Notes For use in thin film coating equipment. Designed for high leakage inductance to control harmonic currents associated with a six pulse rectifier.
  • CE Mark
  • NRTL to UL506
  • EN 60742


Width 85.00"
Length 55.00"
Height 85.00"
Weight 6,900 lbs
Three phase autotransformer with touch-safe lexan cover.

How to Order

What You'll Need

Our engineers are well-versed and knowledgeable and will be able to help guide you through the design and specification process. However, they will need to know a few parameters before they can design your transformer. Below is a list of the things they will need to know in order to design your Autotransformer:

  • Power Rating of the transformer (required)
  • Operating frequency (required)
  • Input and output voltages (required)
  • Output current (required)
  • Termination requirements (i.e., ring-lug compatibility, etc.)

If you do not know how to spec out a transformer, or need more information, we have many ways of helping you figure it out. Please feel free to contact us via the form below, by phone number, or by email address below and one of our technical assistants or engineers will be more than happy to assist you. You can also visit our Resources Center to learn more about your product options.

How to Contact Us

Quality Transformer and Electronics
963 Ames Avenue
Milpitas, CA 95035
United States

Phone: +1 (408) 263-8444
Fax: +1 (408) 263-8448


Feel free to contact us via the form below. One of our representatives will contact you right away ready to assist you.