WAZIPOINT Engineering Science & Technology: Conversion of 2-Winding Transformer into Auto-transformer

## Conversion of 2-Winding Transformer into Auto-Transformer

Any two-winding transformer can be converted into an auto-transformer either step-down or step-up. The figure below shows such a transformer with its polarity markings. Suppose it is a 20-kVA,

You can convert a two-winding transformer into an Autotransformer by connecting its winding in series. But while touching the windings keep the polarity into account because it will change its capacity. suppose you have a 240/120V two-winding transformer by connecting these two windings with additive polarity the voltage at the output terminal will be 240+120=360V, then the transformer will be a step-up transformer. Similarly connecting these two windings in subtractive polarity output voltage will be 240-120.

### How does the autotransformer convert into two winding transformers?

An autotransformer is essentially a 2-winding transformer with both primary and secondary windings. As the terminal markings show, the primary winding is X connected to terminals 1 and 2. The secondary winding is Y connected to terminals 3 and 4. Both windings share a common core leg in the magnetic circuit so that the sum of the amp-turns on that leg equals zero as in a standard 2-winding. When connected as an auto, terminals 1 and 4 are connected together. The low-voltage side is across terminals 1 and 2 while the high-voltage side is across terminals 2 and 3.

The primary and secondary windings are sometimes referred to as the common (excitation) winding and the series winding respectively. For large power applications, a third winding called the tertiary winding is added for the purposes of harmonic suppression and voltage stabilization. Only by physically adding another winding. If you do this make sure the winding is sufficiently isolated. Do not just wind enameled wire on top of the existing winding. It needs more insulation than that to be safe.

### Factors to Consider When Converting a General Transformer to an Auton Transformer.

Converting a 2-winding transformer into an auto-transformer involves rewiring the windings to create a single winding with multiple tapping points, instead of having two separate windings. This can be done by connecting a portion of the winding as the common winding for both the input and output sides. Auto-transformers are often used in applications where voltage transformation is required but without complete electrical isolation between the input and output sides. Here's how the conversion process generally works:

• Identify the Windings: In a 2-winding transformer, there are typically two windings: the primary winding and the secondary winding. The primary winding receives the input voltage, and the secondary winding provides the output voltage.
• Determine the Auto-Transformer Configuration: Decide on the desired auto-transformer configuration. You need to determine the tapping points on the winding to achieve the required voltage transformation. The auto-transformer will have a single winding, and the tapping points will determine the voltage levels.
• Disconnect Secondary Winding: If you're converting the transformer completely into an auto-transformer, you will need to disconnect the secondary winding completely. This means removing the secondary winding's connections from the circuit.
• Connect Windings: Connect the primary winding and the remaining portion of the secondary winding together. The point of connection depends on the desired auto-transformer configuration. The voltage levels at the different tapping points will be determined by the turn ratio of the windings at those points.
• Add Tapping Points: If the auto-transformer is designed to have multiple voltage levels, add tapping points to the winding at appropriate locations. These tapping points can be used to achieve different voltage transformations.
• Insulation and Safety: Ensure that all connections are properly insulated and secured to prevent electrical hazards. Safety measures are crucial since the primary and secondary sides are no longer electrically isolated.
• Testing: After the rewiring is complete, the auto-transformer should be thoroughly tested to verify its performance and safety. This may include testing voltage levels, current carrying capacity, and insulation resistance.

It's important to note that the primary advantage of an auto-transformer is its potential for cost savings due to the reduced amount of winding material needed compared to a traditional isolation transformer. However, the drawback is that the primary and secondary sides are not electrically isolated, which could be a concern in certain applications where isolation is critical.

Before attempting to convert a transformer into an auto-transformer, make sure you have a good understanding of electrical circuits, safety protocols, and transformer operation. If you're not confident in performing this conversion, it's recommended to seek assistance from a qualified electrical engineer or professional.