WHAT ARE AUTOTRANSFORMERS AND THEIR APPLICATIONS?

Autotransformer is a transformer with a single continuous winding serving both primary and secondary functions, tapped at specific point/s to achieve voltage transformation. Unlike conventional transformers, it retains electrical connection between input and output circuits, enabling more efficient power transfer through direct conduction alongside inductive coupling.

Autotransformers have a single winding serving as both primary and secondary, tapped at intermediate points for different input and output voltages. Power transfers via both electromagnetic induction across the full winding and direct electrical conduction through the shared common section, making them more efficient than two-winding transformers.

Autotransformer diagram features a single continuous winding on a core, with input connected across the full length and output across a tapped common portion. Adjusting the tap position alters the turns ratio, allowing step-up or step-down voltage without full isolation. This design minimizes material use, as only the select voltage portion requires additional taps.

Current in common part of winding carries both primary and secondary currents, while the other part carries only the supply current. The winding wire of the common section is much thicker for this current. In case of multiple taps, different sections carry different current levels, and wires for these sections may be used accordingly. Autotransformers design reduces copper usage by up to 75% for similar ratios compared to regular two-winding transformers. A single copper or aluminum conductor forms the entire winding, while enamel coating on wire provides interlayer insulation. Oil immersion is used in large units for cooling.

Advantages and limitations of autotransformer

Advantages

• Smaller size, lower cost, and higher efficiency due to reduced copper and core losses compared to two-winding transformers.
• Better voltage regulation from lower resistance and leakage reactance.
• Requires less excitation current for operation.

Disadvantages

Fault propagation risk between circuits limits its use in safety-critical applications. Precise tap design to handle load currents without excessive heating.

Three phase autotransformers

A three-phase autotransformer is a transformer with a single winding per phase, enabling efficient voltage adjustment without full electrical isolation. These devices operate on electromagnetic induction principles, commonly configured in star-star connections to minimize phase shifts. They excel in power systems for step-up or step-down applications with small voltage ratios.

Usually, the choice of configuration is start-star type, preferred for balanced systems, both for common 220/400 V applications, as also for high voltage grids (e.g. 66/132 kV or 220/400 kV. 220 / 400 V). Three phase supplies may use delta connection in some cases. For supply voltages of three-phase 440 V system. it is customary to use three single phase variacs in tandem, with wipers of all the three being controlled by one common shaft and knob.

Delta option is not generally preferred due to phase displacement issues. Three-phase autotransformers suit power transmission, distribution voltage compensation, induction motor starting (to limit inrush current), and furnace supplies.

Large autotransformers are in use in industry and grid, right up to MVA range. They can be single phase or three phase, and rated up to several hundred KVA. BHEL has recently made India’s largest autotransformer, rated a staggering 500 MVA.

Applications of autotransformers

• Voltage regulation
• Motor starting
• Lab supplies and testing
• FACTS devices in power systems
• Speed control of heavy-duty fans
• Generate variable voltage in high voltage lab.

Autotransformers suit voltage regulation in power systems, motor starting (reducing inrush current to 50-80% of full load), and variacs for lab testing. They interconnect systems with close voltage levels, such as in FACTS devices for power quality improvement.

Zig-zag transformer

A zig-zag transformer can be considered as autotransformer, since secondary is absent. Each winding is divided in two sections, and interconnections done in a way so as to eliminate triplen harmonics. They eliminate triplen harmonics in nonlinear loads, enhancing power quality. In ungrounded systems, Zig-Zag transformers create a neutral point for steady grounding and safer operation.

Zig-Zag transformers are cheaper than other grounding transformers due to their simpler design & smaller size. They can balance imbalanced loads across phases, decreasing equipment strain and improving system performance. They lower zero-sequence impedance, minimizing voltage drops during unbalanced load conditions.

Their compact and efficient design makes them perfect for space-constrained installations. Zig-Zag transformers are more reliable and require less maintenance due to their simpler design and fewer windings.

Variable autotransformer

The secondary output of autotransformer may be designed to have position its common point variable by means of a sliding contact, which can travel from one end to another. Secondary output voltage can the vary from zero to maximum continuously. Turns ratio can vary with every turn of winding to have fine control on output. Such design can be made by winding the primary around a toroidal core, and a wiper contact can move in arc over the winding by the turn of a knob.