The standard schematic symbol for a generator is a circle containing the uppercase letter "G" under US standards (IEEE 315), or a circle with internal waveform and field annotations under international standards (IEC 60617). On single-line power diagrams, it is typically represented by a simple circle with a "G" or a simplified alternator icon, often mechanically linked to a prime mover symbol like a diesel engine or turbine.
Complete Generator Symbol & Designation Reference Table
The table below maps the visual geometry and standard alphanumeric tags for rotating electrical machines. Use this as your primary bench and jobsite reference when reading schematics or drafting single-line diagrams.
| Generator Type | IEEE 315 (US) Visual | IEC 60617 (Intl) Visual | Standard Letter Tag | Typical Application |
|---|---|---|---|---|
| Generic DC Generator | Circle with "G" | Circle with "G" and straight line (-) | G, GS | Older traction systems, exciter supplies |
| AC Alternator (Synchronous) | Circle with "G" and "~" | Circle with sine wave inside | G, GA | Standby backup, grid-tie, micro-hydro |
| Permanent Magnet Gen (PMG) | Circle with "G" and magnet icon | Circle with "G" and permanent magnet symbol | GPM, PM | Wind turbines, portable inverter gensets |
| Motor-Generator (M-G) Set | Two overlapping circles ("M" and "G") | Two adjacent circles linked by dashed line | MG | Frequency conversion, isolated UPS flywheels |
| Tachogenerator | Circle with "TG" | Circle with "TG" and tachometer symbol | TG, BR | Speed feedback loops on DC motor drives |
Regional Variants and Rows People Get Wrong
While the basic circle remains universal, the internal annotations shift depending on whether your region defaults to IEEE 315 (North America) or IEC 60617 (Europe, UK, Australia, and most global industrial projects). The old UK BS 3939 standard is largely obsolete but still appears on legacy plant drawings; it used distinct semi-circle arcs for AC machines that modern IEC standards have replaced with the internal sine wave.
The "Rows People Get Wrong" Notes
- Current Transformers (CTs) vs. Generators: On crowded single-line diagrams, a CT is often drawn as two overlapping circles or a circle with a single straight line passing completely through it. Technicians frequently mistake a CT array for a small multi-winding generator. If the line passes through the circle rather than terminating at it, it is a transformer/CT, not a generator.
- Inverters vs. Generators: A "G" strictly denotes a rotating electromechanical machine. Solid-state solar or battery inverters are drawn as rectangles with a DC input line and an AC output line (often containing a sine wave or a chopper symbol). Never label an inverter with a "G" tag on a formal schematic.
- Synchronous Condensers: This machine is physically an AC synchronous generator running without a prime mover, used purely for power factor correction (VAR support). On schematics, it is often drawn with a motor symbol ("M") or a capacitor symbol, despite being constructed identically to an AC alternator.
- Exciter Windings: Large AC alternators require a DC field excitation. Schematics will show a secondary, smaller circle attached to the main "G" circle. This is the exciter (often a PMG or a static excitation transformer). Do not confuse the exciter symbol for a separate, independent generator.
Identifying Generators When Markings Are Faded or Missing
On older machinery or salvaged equipment, physical nameplates corrode and schematic ink fades. If you are staring at an unmarked rotating machine and need to confirm if it is a generator or a motor before applying power or connecting a load, use these bench diagnostics:
- Count and Label the Terminals: A standard 3-phase AC generator stator will have leads labeled U, V, W (and sometimes U1/U2, V1/V2 for dual-voltage zig-zag or star/delta configurations). A DC generator will have heavy armature terminals (A1, A2) and lighter shunt field terminals (F1, F2). Induction motors typically just have T1 through T9.
- The Residual Magnetism Spin Test: Disconnect all leads. Set your multimeter to the DC millivolt (mV) range. Spin the machine shaft by hand or with a low-speed drill. A DC generator or a synchronous AC generator with a permanent magnet exciter will generate a small residual voltage (typically 2mV to 50mV) due to remnant magnetism in the iron core. A standard squirrel-cage induction motor will read exactly 0.00 mV.
- Check for a Prime Mover Coupling: Generators are driven. Look at the drive-end (DE) bearing housing. If it is flanged directly to a diesel block, a water turbine draft tube, or a belt-drive pulley system with a high-ratio step-up, it is a generator. Motors are typically coupled to loads (pumps, compressors) via the drive end, but the presence of a governor linkage or fuel rack on the coupled machine confirms a generator set.
Generator Symbol FAQ
What is the symbol for a standby backup generator on a single-line diagram?
On a single-line diagram, a standby backup generator is drawn as a circle with the letter "G" inside, mechanically linked (via a dashed or solid line) to a prime mover symbol—usually a rectangle labeled "Diesel" or a stylized turbine icon. Crucially, the generator symbol will be connected to the main bus through an Automatic Transfer Switch (ATS) symbol, which looks like a switch with an interlocking mechanical linkage pointing to both the utility and generator feed lines.
How do I distinguish an AC alternator symbol from a DC generator symbol?
Look inside the circle. Under IEEE 315, an AC alternator will have a tilde (~) next to the "G", while a DC generator will have a straight line (-) or no internal modifier. Under IEC 60617, the AC alternator contains a full sine wave drawing, whereas the DC generator contains a solid straight horizontal line. If the internal markings are entirely missing, check the alphanumeric tag: "GA" usually denotes an AC alternator, while "GS" or a plain "G" defaults to DC in older American schematics.
What does a circle with an M and G stacked inside mean?
A single circle with "M" stacked over "G" (or two overlapping circles containing M and G) represents a Motor-Generator (M-G) set. This is an electromechanical device where an electric motor drives a generator on the same shaft. Historically, these were used to convert 60Hz utility power to 50Hz for specialized equipment, or to provide massive rotational inertia (flywheel effect) for early rotary UPS systems to bridge the gap between a utility failure and a diesel engine starting.
Is the generator symbol the same for solar inverter schematics?
No. The "G" symbol is strictly reserved for rotating electromechanical machines (alternators, dynamos, PMGs). A solar inverter or battery inverter is a solid-state power electronics device. On a schematic, an inverter is drawn as a rectangle. The left side of the rectangle will show a DC input (often with a battery or solar array symbol), and the right side will show an AC output (often with a sine wave inside the rectangle or on the output line). Labeling an inverter as a "G" will cause confusion during arc flash studies and relay coordination.
