Constructing and Operating an Autotransformer

The power rating of an autotransformer is higher than that of an equivalent two-winding transformer. Autotransformers are limited by their low impedance and lack of electrical isolation between the primary and secondary circuits.

 

Operation

Transformers typically contain two independent circuits. This includes a primary winding that receives the energy from the source, and a secondary winding that delivers the energy to the load. An autotransformer is a transformer in which the primary and secondary circuits have a portion of their two windings in common. The power rating of an autotransformer is higher than that of an equivalent two-winding transformer.

 

Construction

An autotransformer is usually constructed from a single winding with multiple taps. For a step-down application, the source is applied across the entire coil, which acts as the primary. The load is connected across a portion of the whole winding as shown in the figure. The portion connected to the load acts as the secondary.

 

Figure 1. This autotransformer steps the voltage up with a single copper-saving tapped winding. Image courtesy of All About Circuits

 

The generated electromagnetic field cuts through the entire winding. Therefore, the induced counter voltage is across the entire coil, and the design volts per turn is obtained across each turn of the entire coil. The voltage available to the load is less than the source voltage because there are fewer turns available to the secondary than the primary.

 

Note

According to the NEC®, Article 411, Lighting Systems Operating at 30 Volts or Less, a secondary circuit shall be insulated from the branch circuit by an isolating transformer. Autotransformers cannot be used.

For example, if a step-down autotransformer has 240 turns with a source voltage of 240 V, the source is applied across the entire coil, and the volts per turn is 1 V/turn. If the secondary is tapped to use 120 of the turns, the voltage available to the load is 120 V.

For a step-up application, the source is applied across only part of the coil. The load is connected across the entire coil. The design volts per turn is again obtained across each turn of the entire coil. In this case, the secondary voltage is higher than the source voltage because there are more turns available to the secondary than to the primary.

For example, if a step-up autotransformer has 240 turns and a source voltage of 120 V is applied across only 120 turns of the entire coil, the volts per turn of the coil is 1 V/turn. If the secondary is tapped to use all 240 of the turns, the voltage available to the load is 240 V.

 

Two-Winding Transformer Loading

The current in a standard two-winding transformer is determined by dividing the power rating, in VA, by the voltage, in V. For example, if a transformer primary is rated at 12 kVA (12,000 VA) at 480 V, the maximum current flow is 25 A (12,000 ÷ 480 = 25). See Figure 2. The secondary must be rated at the same 12 kVA. If the secondary is rated to deliver 120 V, the transformer is designed to step down the voltage from 480 V to 120 V and deliver a load current of 100 A (12,000 ÷ 120 = 100).

 

Figure 2. The primary and the secondary have the same power rating.

 

Two-Winding Autotransformers

An autotransformer can also be made from a two-winding transformer by connecting the two windings to form one continuous winding. When tap H2 is connected to tap X2 in the above figure, the windings are in series, and the two-winding transformer operates as an autotransformer. The two windings are on the same core and connected in series. The primary and secondary windings can both be tapped to allow the autotransformer to operate as a step-up transformer or a step-down transformer.

 

Autotransformer Loading

The power rating of a single-coil autotransformer is higher than the rating of the individual windings of an equivalent two-winding transformer. When that same transformer is wired as an autotransformer by connecting H2 to X2, more power can be delivered to the load without exceeding the power rating of the windings.

The individual windings still have the same rating as they did originally, but they are now wired in series. The original primary still carries 25 A, and the original secondary still carries 100 A. The original ratings are still being met.

However, the 25 A current flows through the new connection from the original primary to tap X2 of the original secondary. See Figure 3. The 100 A of induced current in the original secondary combines with the 25 A flowing through the primary, and 125 A flows to the load. As a result of converting the two-winding transformer to an autotransformer, the power available to the load increases from 12 kVA to 15 kVA. The current of the primary flows through the load and increases the kVA available for the load.

 

Figure 3. The current through the primary and the current induced in the secondary combine and flow through the load.

 

This shows that a two-winding transformer wired as an autotransformer can deliver more power to a load than when wired as a two-winding transformer. Alternatively, a single-coil autotransformer can be designed specifically for the application. This single-coil transformer can be smaller and less expensive than the two-winding transformer.

 

Limitations

Autotransformers have several limitations that prevent their widespread use in power circuits. A significant limitation of an autotransformer compared to a conventional transformer is its low impedance. Another limitation is a lack of isolation between the primary and the secondary.

 

Low Impedance

The impedance of an autotransformer is less than the impedance of a two-winding transformer. For example, a two-winding transformer may have an impedance of about 3% of the rated primary voltage. When wired as an autotransformer, the same windings have an impedance that is equal to 3% of the ratio of the difference between the primary and secondary voltages to the primary voltage. The reduced impedance is calculated as follows:

$${{Z}_{A}}=Z\times \frac{{{V}_{P}}-{{V}_{S}}}{{{V}_{P}}}$$

where

Z_A = autotransformer impedance (in %)

Z = two-winding transformer impedance (in %)

V_P = primary voltage (in V)

V_S = secondary voltage (in V)

For example, if the primary voltage is 240 V, the secondary voltage is 120 V, and the two-winding transformer impedance is 3%, the autotransformer impedance is calculated as follows:

$${{Z}_{A}}=Z\times \frac{{{V}_{P}}-{{V}_{S}}}{{{V}_{P}}}=3\%\times \frac{240-120}{240}=1.5\% $$

Short circuits are a significant concern. Because the impedance of an autotransformer is lower than that of a two-winding transformer, higher current flows when a short circuit fault occurs. See Figure 4. Hence, an autotransformer should be provided with some external series impedance such as the impedance from other transformers or reactors or the impedance of the power lines.

 

Figure 4. The low impedance of an auto-transformer allows higher short circuit current flow than a two-winding transformer.

 

Lack of Electrical Isolation

Autotransformers share part of the windings between the primary and secondary. The delta-wye or zigzag winding of standard transformers is no longer present. Therefore, autotransformers are not able to block the flow of transients and harmonics throughout a 3-phase power system.

During a fault condition, there can be an open circuit in the common part of the winding. In such a case, anyone coming in contact with the secondary is subject to the primary voltage. In addition, the primary may become grounded and establish a high voltage between one of the low-voltage conductors and ground. If the winding shorts to the ground, it affects both windings, and the load could be subjected to high voltage. See Figure 5. The entire winding of an autotransformer should be designed with the insulation level required for the highest voltage expected.

 

Figure 5. A short circuit from the primary winding to the ground can subject the load to the full voltage.

 

For very large autotransformers, the cost of extra insulation increases the cost of the autotransformer faster than the reduction in copper reduces the cost. At a certain point, typically about 100 kVA or at a turns ratio of about 4:1, it becomes less expensive to use a two-winding transformer than an autotransformer.