Autotransformer, primary and secondary windings are connected both electrically and magnetically, reducing the cost compared to conventional transformers.

Unlike the previous voltage transformer with two electrically insulated windings, called primary and secondary, an Autotransformator has only one voltage winding that is common to both sides.This single winding is "poured" at various points along its length to provide a percentage of the primary voltage source along its secondary load.Then the autotransformer has the usual magnetic nucleus, but only one winding, which is common in both primary and secondary circuits.

Therefore, in an autotransformer, the primary and secondary windings are connected both electrically and magnetically.The main advantage of this type of transformer design is that it can be made much cheaper for the same VA value, but the biggest disadvantage of an autotransformer is that a traditional double-winded transformer does not have primary/secondary winding insulation.

The winding section, which is specified as the primary part of the winding, is connected to the AC power supply so that the secondary is part of this primary winding.An autotransformer can also be used to increase or decrease the feed voltage by reversing the connections.If the primary total is winding and is connected to a source and is connected to only part of the secondary circuit winding, the secondary voltage is "lowered", as shown.

Autotransformer Design


The primary current I P flows in the direction of the arrow from a single winding, while the secondary current I S flows in the opposite direction.Therefore, in the part of the winding that produces secondary voltage, the current flowing from the V S winding is the difference between I P and I S.

Autotransformercan be made with more than one single tow point.Self-transformers can be used to provide different voltage points along the winding or to increase the supply voltage according to the supply voltage V P as shown.

Multi Touch Point Autotransformer


The standard method for marking the windings of an automatic transformer is to label it in uppercase letters (uppercase).For example, to define the supply end, click A , B , Z, and so on.Typically, the public neutral link is marked N or n.For secondary grid pulls, the final additional numbers are used for all guide tow points along the primary winding of automatic transformers.These numbers usually start with " 1 " and continue in ascending order for all guide points, as shown.

Autotransformer Terminal Signs


An autotransformer is mainly used for adjusting line voltages to change its value or keep it constant.If the voltage setting is up or down, a small amount, then the transformer ratio is almost equal to the small V P and V S.I P and I S currents are almost equal.

Therefore, the part of the winding that carries the difference between the two currents can be made from a much smaller conductor size, since the currents are much smaller, saving on the cost of an equivalent double-winded transformer.

However, the regulation of an autotransformer for a certain VA or KVA degree, its leakage inductee and physical size (since there is no second winding), is less than a double-winded transformer.

Autotransformers are clearly much cheaper than traditional double-winded transformers with the same VA rating.When deciding to use an autotransformator, it is usual to compare its cost with that of an equivalent pair of wound types.

This is done by comparing the amount of copper saved in the winding.If the "n" ratio is defined as the high voltage ratio of low voltage, it can be shown that the savings in copper are: n*100%.For example, copper savings for two autotransformators will be as follows:


Autotransformer Question Example 1

To increase a voltage from 220 volts to 250 volts, an autotransformer is required.The total number of coil turns in the transformer main winding is 2000. When the output is rated 10KVA and determine the amount of copper saved, the position of the primary stage point, primary and secondary currents.


Thus, the primary current flows 45.4 amps, the secondary current attracted by the load flows 40 amps and 5.4 amps from the common winding.The saved copper is 88%.

Disadvantages of an Autotransformer

  • The main disadvantage of an autotransformer is that a traditional double-winded transformer does not have primary to secondary winding insulation.Then an autotransformer can not be safely used to reduce higher voltages to much lower voltages, which are suitable for smaller loads.
  • If the secondary side winding is open circuit, the load current stops flowing through the primary winding and stops the transformer movement, causing full primary voltage to be applied to the secondary terminals.
  • If the secondary circuit is exposed to a short circuit state, the primary current resulting from the increased flux connection damaging the autotransformer will be much larger than an equivalent double-winded transformer.
  • Since the neutral connection is common in both primary and secondary windings, the grounding of the secondary winding is automatically primary, since there is no insulation between the two windings.Double-winded transformers are sometimes used to isolate equipment from the soil.

Autotransformer transmission lines have many uses and applications, including the start of induction motors used to regulate voltage, and can be used to transform voltages when the second ratio is in the first close union.

From conventional two winding transformers, by serially connecting the primary and secondary windings and depending on how the connection is made, the secondary voltage can be added to or removed from the primary voltage.

Variable Autotransformer

In addition to being a fixed or gradual secondary that produces a voltage output at a certain level, there is another useful application of automatic transformer type regulation, which can be used to produce a variable AC voltage from a fixed voltage AC source.This type of Variable Autotransformator is usually used in schools and laboratories and is more commonly known as Variac.

The structure of the variable autotransformer or variance is the same as the fixed type.As with the automatic transformer, a single primary winding is used wrapped around a laminated magnetic core, but instead of being fixed to a predetermined stage point, the secondary voltage is drawn with a carbon brush.

This carbon brush is rotated or allowed to slide along an exposed part of its primary winding, contacting it while moving, providing the required voltage level.

Then a variable autotransformer contains a variable tap in the form of a carbon brush that shifts the primary winding up and down, which controls the length of the secondary winding, and therefore the secondary output voltage is completely variable from the primary feed voltage value to zero volts.

The variable autotransformer is usually designed with a significant number of primary windings to produce a secondary voltage that can be adjusted from a few volts per turn to the fractions of a volt.This is achieved because the carbon brush or slider is always in contact with one or more rotations of the primary winding.Because the primary coil turns are placed evenly along its length.Then the output voltage becomes proportional to the angular rotation.

Variable Autotransformer

We can see that the variable is able to adjust the voltage to the load from zero to the nominal feed voltage without problems.If the supply voltage is interrupted at some point along the primary winding, potentially the output secondary voltage may be higher than the actual feed voltage.Variable autotransformators can also be used to dim lights, and when used in such applications they are sometimes referred to as "dimmerstat".

Variants are also very useful in electrical and electronics workshops and laboratories, as they can be used to provide a variable AC source.However, care should be taken with proper fuse protection to ensure that there is no higher supply voltage in secondary terminals under fault conditions.

Autotransformer has many advantages over traditional double wound transformers.They are generally more efficient for the same VA value, are smaller in size and require less copper in their structure, so their costs are lower compared to double-winded transformers with the same VA value.In addition, core and copper losses are lower due to less resistance and leakage reassurance, which provides superior voltage regulation than the equivalent two-winded transformer I 2 R.

In the next lesson about transformers, we will look at another transformer design around its core that does not have a traditional primary winding winding.This type of transformer is often called The Current Transformer and is used to feed the amperemeter and other such electrical power indicators.