AutotransformerEdit
An autotransformer is a type of transformer that uses a single continuous winding with a tapping to provide both the input and the output voltages. The winding is shared between the primary and secondary circuits, unlike an isolating transformer where the windings are separate. This arrangement can make autotransformers more compact and economical for certain voltage changes, but it also means there is no galvanic isolation between the input and output. In practice, autotransformers are common in power distribution, commercial equipment, and specialized motor-starting circuits where a modest voltage adjustment is needed without significant added weight or cost. transformer alternating current
Autotransformers achieve voltage transformation by applying the high-side voltage across a portion of the winding and taking the desired low- or high-side voltage from a tap along that same winding. The basic electrical relationship can be described by the voltages V_H (high side), V_L (low side), and V_S (the series portion between the tap and the end of the winding). The common portion of the winding carries the load current, while the series portion provides the voltage difference that establishes the transformation. This single-winding approach reduces copper and core material for a given power level, leading to a smaller and cheaper device in many cases. However, the loss structure and safety implications differ from those of an isolating transformer, and the lack of isolation is a major consideration in system design. See also voltage and galvanic isolation.
Construction and principle
Autotransformers consist of a single winding that is tapped at one or more points to create the input and output connections. The portion of the winding connected to both the input and output is the common section, while the portion beyond the tap acts as the series winding. The voltage ratio is determined by the tap position, with the nominal transformation described by a_high / a_low ≈ V_H / V_L. In practical terms, a given autotransformer will be designed for a specific high-voltage rating and a tap that provides the desired output voltage. For a discussion of the topology and its magnetics, see magnetic coupling and electromagnetism as they pertain to power transformers. See also the idea of a tapped winding in tap changer technology.
Autotransformers can be classified by their intended voltage relationship, such as step-up autotransformers (raising voltage) and step-down autotransformers (reducing voltage). They may also incorporate on-load tap changers (OLTC) to adjust the output voltage under load, which is common in power distribution substations. See on-load tap changer for more details. In many equipment applications, the term “autotransformer” is used interchangeably with specific form factors, including high-voltage and low-voltage configurations.
In terms of construction materials, copper windings and a magnetic core are typical, with insulation rated for the operating voltages. Insulation class, creepage, and clearance standards must account for the absence of galvanic isolation, as well as the proximity of high and low conductors within the same winding. See also insulation (electrical) and electrical clearance.
Ratings and efficiency
A key characteristic of autotransformers is that they can deliver higher apparent power (kVA) for a given physical size than a purely isolating transformer of the same core, because part of the power is transferred conductively through the common winding rather than entirely by magnetic coupling. In practice, this means autotransformers are often favored when the voltage change is modest relative to the nominal voltage and when space and weight savings are important. The magnetically transferred portion of the power—the part that goes through the transformer's magnetic circuit—still accounts for copper and core losses, while the conductive portion reduces the copper loss that would occur if a separate winding were used for primary and secondary.
A critical design consideration is the absence of isolation. While this can be beneficial from a system integration standpoint, it means that any fault on the high-voltage side can propagate to the low-voltage side and vice versa. Protective measures, proper grounding, and clear safety clearances become essential. For a contrast with fully isolated devices, see isolation transformer.
The efficiency of an autotransformer is typically high, particularly for small voltage differences and moderate load. The efficiency gains arise from reduced winding material and a lower hot-spot risk in some designs. Designers also assess losses associated with the connection points at the taps, the performance of switchgear in protecting the device, and the impact of any leakage inductance on dynamic responses.
Applications
Autotransformers find use in a variety of settings where a modest voltage adjustment is desirable without the need for full galvanic isolation. Common applications include:
- Distribution voltage regulation in substations, where the voltage must be kept within tight bounds across fluctuating load conditions. On-load tap changers enable automatic adjustments as system voltage varies. See on-load tap changer and distribution transformer for related topics.
- Motor starting, particularly for large induction motors, where an autotransformer starter reduces inrush current by briefly applying a reduced voltage during the start-up phase with a fast transition to full line voltage when the motor nears operating speed. See motor starting and induction motor for context.
- Voltage boosting or stepping down in equipment that requires a specific nominal voltage different from the available supply, such as some industrial controls or power supplies in legacy systems.
- Aircraft and shipboard power systems in which compact, rugged voltage adjustment is needed without full isolation, under carefully specified safety regimes.
In contrast to isolated transformers, autotransformers are not typically used where complete electrical isolation is a priority, such as in instrumentation circuits that require galvanic isolation between measurement and power sources. For related technologies used when isolation is essential, see isolation transformer.
Protection, safety, and standards
Because the windings in an autotransformer are physically continuous, accidental contact with exposed parts or inadvertent cross-connections can create shock hazards if safety clearances are not maintained. Protective devices, proper enclosure, and robust insulation are necessary to prevent arcing and insulation breakdown. In industrial environments, coordination with upstream protection, fusing, and overcurrent devices is essential to avoid damage to the equipment or personnel.
Standards and best practices for autotransformers align with broader electrical engineering guidelines, including those governing transformers, insulation, and high-voltage equipment. See references to IEC and ANSI standards for transformers and electrical equipment, as well as general electrical safety guidelines.