Autotransformer Symbol
Definition: The Autotransformer symbol represents a single-winding transformer in which part of the winding serves as both the primary and secondary, allowing voltage step-up or step-down with a direct electrical connection between input and output, drawn in IEC 60617-06 as a single coil with a tap point and in ANSI/IEEE 315-1975 similarly, with three pins — H1 (high-voltage input), H2 (high-voltage input return), and X1 (tapped output) — used in motor starting, voltage adjustment, and power distribution circuits.
Also known as: auto transformer, autotransformer starter, single-winding transformer, variac (variable type), compensator (motor starting), step-down autotransformer.
What the Autotransformer symbol means
The Autotransformer symbol in a circuit diagram represents a transformer with a single continuous winding where a tap divides the winding into a primary section and a secondary section. Because primary and secondary share turns, the autotransformer is more compact and efficient than a two-winding transformer for modest voltage ratios (less than approximately 2:1 or 1:2). The voltage transformation ratio equals the turns ratio of the tapped section to the total winding.
Autotransformers appear in motor starting circuits (to reduce inrush current by applying a reduced voltage at start-up), in voltage correction and regulation equipment, and in variac-type variable transformers where the tap is moveable. The three-terminal symbol reflects the physical reality: H1 and H2 are the high-voltage (full-winding) terminals, and X1 is the tapped low-voltage terminal. The autotransformer is not isolated — H2 and X1 share a common point, so the secondary circuit is electrically connected to the primary.
How to identify the Autotransformer symbol
The Autotransformer symbol is drawn as a single coil (spiral or semicircular loops representing the winding turns) with a tap arrow or centre connection midway along the coil. H1 and H2 connect to the two ends of the full winding (left side of the symbol), and X1 connects to the tapped point (right side). The symbol resembles a standard transformer but with only one coil drawn (no separate secondary winding), sometimes with a dot or arrow marking the tap position. A variable autotransformer (variac) adds an arrow through the coil to indicate the moveable tap.
Function in a circuit
The autotransformer steps voltage up or down by the ratio of the tapped secondary turns to the total primary turns. For a step-down auto-transformer with turns ratio n = N_secondary / N_primary, the output voltage V_out = n × V_in and the output current I_out = I_in / n (ideal). Because part of the power is transferred by direct conduction through the shared winding (not magnetically coupled), autotransformers are more efficient than two-winding transformers for small voltage ratios. In motor starting, a three-phase autotransformer starter reduces the applied voltage to 50%, 65%, or 80% during start-up, limiting inrush current by the square of the voltage reduction factor.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-06:1996 (Graphical symbols for diagrams — Part 6: Production and conversion of electrical energy) defines the autotransformer symbol as a single coil with a tap, designated by the single-winding convention. IEC 60076-14 covers liquid-immersed autotransformers. The designator is T (transformer). |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 Section 11 defines the autotransformer symbol. It is drawn as two sections of a coil sharing a common connection, or as a single tapped coil. The ANSI representation is functionally identical to IEC; the coil style (open semicircles vs. spiral) may differ slightly. The designator is T (e.g., T1). |
| Key difference | IEC 60617-06 and ANSI/IEEE 315-1975 autotransformer symbols are functionally identical: a single tapped coil with three terminals. IEC style typically uses a rectangle-and-coil or dot-convention to show polarity; ANSI style uses open semicircle coils. Both show the tap connection as the third terminal. The key practical distinction is that IEC standards require polarity dots (•) to be marked on transformer symbols to indicate winding sense. |
Terminals / pins
| Pin | Name |
|---|---|
| h1 | H1 |
| h2 | H2 |
| x1 | X1 |
Typical values
Common voltage ratios: 120/240 V, 208/120 V, 460/400 V, 480/240 V; motor starting taps: 50%, 65%, 80% of full voltage; kVA ratings: 0.5 kVA to 1,000 kVA+; frequency: 50 Hz or 60 Hz; efficiency: 95–99% (higher than two-winding transformers for small voltage ratios); impedance: 2–8% (motor starting types).
Where the Autotransformer symbol is used
- Three-phase autotransformer reduced-voltage starters (compensator starters) for large AC induction motors (above 7.5 kW) to limit starting current
- Variable autotransformers (variacs) in laboratory power supplies and test equipment for continuously adjustable AC output voltage
- Voltage correction in areas where supply voltage is slightly above or below nominal (e.g., 208 V to 120 V conversion in commercial buildings)
- Railway and transit traction power supply systems using 25 kV autotransformer distribution
- Audio and public address amplifier output transformers for impedance matching between stages
- Utility distribution networks where autotransformers provide economical voltage transformation for subtransmission (69 kV to 13.8 kV, for example)
- Power conditioner and voltage stabiliser circuits in markets with unreliable utility voltage
Example
In a star-delta motor starting wiring diagram, an autotransformer starter symbol connects H1 and H2 to the three-phase 460 V supply lines and X1 to the motor terminals. The starter applies 65% of supply voltage (299 V) during acceleration, then disconnects and switches the motor to full voltage once running speed is reached. This reduces starting current from approximately 600% full-load current to 254% (0.65² × 600%), protecting the supply network.
Key facts
- The autotransformer has a single winding with a tap, where the common terminal is shared between the primary (high-voltage) and secondary (low-voltage) circuits — unlike a two-winding transformer which provides galvanic isolation.
- The three terminals on the schematic symbol are H1 (left top, high-voltage input), H2 (left bottom, high-voltage common/return), and X1 (right, tapped output); H2 and X1 are connected to the same physical point on the winding.
- Voltage transformation follows the turns ratio: V_out/V_in = N_secondary/N_primary, where N_secondary is the turns count from tap to the common end (X1 to H2 in a step-down configuration).
- The autotransformer is more compact and efficient than a two-winding transformer for voltage ratios between 0.5:1 and 2:1, because part of the power is conducted (not magnetically coupled), reducing required core and copper material.
- The absence of galvanic isolation is the major safety limitation of autotransformers: a fault in the secondary circuit can result in high primary voltage appearing on the secondary — isolation transformers are mandatory for safety where the secondary circuit is accessible.
- In three-phase motor starting, reduced-voltage autotransformer starters limit starting current by the square of the voltage reduction factor (e.g., 65% voltage tap reduces starting current to 0.65² = 42% of direct-on-line starting current).
- Variable autotransformers (variacs, e.g., Powerstat, Superior Electric) use a toroidal core and a carbon brush moving along a bare winding section, allowing continuous voltage adjustment from 0 V to typically 110–130% of input voltage.
- The designator for an autotransformer in schematics is T (e.g., T1) per ANSI/IEEE 315-1975 and IEC 60617.
Frequently asked questions
What does the autotransformer symbol look like in a circuit diagram?
The autotransformer symbol is a single coil (open semicircles or spiral loops) with a tap connection midway or at a specific point along the winding. H1 and H2 connect to the top and bottom of the full winding on the left side; X1 connects to the tapped point on the right side. Unlike a standard two-winding transformer, only one coil is drawn — there is no separate secondary coil.
What does the autotransformer symbol mean in a schematic?
The autotransformer symbol means the circuit contains a single-winding tapped transformer that steps voltage up or down without galvanic isolation. The voltage ratio equals the fraction of the winding between the tap (X1) and the common terminal (H2) relative to the full winding. The lack of isolation means secondary faults can expose the secondary circuit to primary-side voltage.
What is the difference between an autotransformer and a two-winding transformer?
A two-winding (isolation) transformer has separate primary and secondary windings with no direct electrical connection, providing galvanic isolation. An autotransformer has a single winding with a shared common terminal between primary and secondary — no isolation. Autotransformers are lighter, cheaper, and more efficient for small voltage ratios but are unsafe where isolation from the primary circuit is required.
What are the H1, H2, and X1 pins on the autotransformer symbol?
H1 is the high-voltage input terminal (top of the full winding). H2 is the high-voltage return/common terminal (bottom of the full winding), which is shared with the secondary. X1 is the tapped secondary output terminal. For a step-down autotransformer, supply voltage connects across H1–H2, and the reduced output voltage is taken between X1 and H2 (or X1 and the common point).
What standard defines the autotransformer symbol?
The autotransformer symbol is defined in IEC 60617-06:1996 (Part 6: Production and conversion of electrical energy) as a single tapped coil symbol, and in ANSI/IEEE 315-1975 Section 11 (Transformers). Both standards use the designator T (e.g., T1) for autotransformers. IEC 60076-14 covers performance specifications for autotransformers.
Why is an autotransformer used for motor starting?
An autotransformer reduced-voltage starter applies a fraction of full supply voltage to the motor during start-up, reducing the starting inrush current by the square of the voltage reduction factor. For example, a 65% voltage tap reduces starting current to 42% of direct-on-line (DOL) current. This limits mechanical shock and voltage sag on the supply network when starting large motors.
What is a variac and how does it relate to the autotransformer symbol?
A variac (trade name; also 'powerstat') is a variable autotransformer with a toroidal core and a sliding carbon brush contact on the winding, allowing the tap position (X1) to be moved continuously. The schematic symbol is the same autotransformer symbol but with an arrow through the coil indicating the variable tap. Output voltage is adjustable from 0 V to approximately 130% of input voltage, making variacs essential for laboratory power supplies and equipment testing.
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