The universal base symbol of DC motor in electrical schematics is a circle containing the letter "M". However, to specify the exact internal construction—such as Permanent Magnet (PM), Series, Shunt, or Compound—governing standards (IEC 60617 and IEEE 315) require specific field winding lines and alphanumeric terminal nodes. If you are reading a modern schematic, a circle with an "M" and two terminals (A1, A2) indicates a basic brushed DC motor, while a circle with an "M" and parallel winding lines indicates a shunt-wound motor. This guide provides the exact reference tables, regional variations, and bench-testing procedures you need to correctly identify and wire these machines.
The Complete Symbol of DC Motor Reference Table
The table below maps the physical motor type to its schematic representation under the two dominant global standards. Use this as your primary lookup when tracing control panels or designing motor driver circuits.
| Motor Type | ANSI/IEEE 315 Symbol Description | IEC 60617 Symbol Description | Standard Terminal Labels | Practical Application |
|---|---|---|---|---|
| Permanent Magnet (PM) | Circle with "M", two parallel lines on stator edge indicating magnets | Circle with "M", stator edge marked with PM or magnet symbol | A1 (+), A2 (-) | 12V/24V hobby robotics, automotive wipers, small conveyors |
| Series Wound | Circle with "M", single field winding loop drawn in series with armature | Circle with "M", series field rectangle in line with armature path | A1, A2 (Armature) F1, F2 (Field) |
Traction motors, electric hoists, high starting torque loads |
| Shunt Wound | Circle with "M", field winding loop drawn parallel to the armature | Circle with "M", shunt field rectangle drawn parallel to armature | A1, A2 (Armature) D1, D2 (Shunt Field) |
Industrial machine tools, conveyors requiring constant speed |
| Compound Wound | Circle with "M", showing both series and shunt winding loops | Circle with "M", showing both series and shunt field rectangles | A1, A2 (Armature) D1, D2 (Shunt) F1, F2 (Series) |
Heavy presses, elevators, loads with sudden torque spikes |
| Brushless DC (BLDC) | Circle with "M", often accompanied by a 3-phase inverter block symbol | Circle with "M" and 3-phase winding indicators (U, V, W) | U, V, W (Phases) Hall A, B, C |
Drones, CNC spindles, high-efficiency HVAC fans |
Regional & Standard Variants (IEC vs. ANSI vs. Legacy UK)
While the circle-and-"M" concept is universal, the terminal naming conventions and winding representations shift depending on your region and the age of the schematic.
If you are working in North America or dealing with equipment manufactured by US-based OEMs, you will primarily encounter NEMA and ANSI/IEEE 315 standards. If you are in Europe, Asia, or working with modern globalized PLC panels, IEC 60617 is the governing standard.
- IEC 60617 (Global/European): Uses strict alphanumeric terminal markers. The armature is always A1/A2. The shunt field is D1/D2, and the series field is F1/F2. The symbols themselves tend to use rectangular blocks for windings rather than loops.
- ANSI/IEEE 315 (North American): Often uses sequential numbering (e.g., 1, 2, 3, 4) on older diagrams, though modern NEMA-compliant diagrams have largely adopted the IEC letter codes (A, D, F) to align with global supply chains. Winding symbols are traditionally drawn as curved loops.
- Legacy UK (BS 1643): Pre-harmonization British Standard diagrams used different letter codes entirely. You might see A1/A2 for the armature, but F1/F2 used for the shunt field (instead of D1/D2), and E1/E2 for the series field. If you are retrofitting a machine built in the UK before 1990, verify the standard printed on the title block before wiring.
Rows People Get Wrong (And How to Fix Them)
When reading schematics or wiring physical terminals, these are the most common misinterpretations that lead to blown fuses or runaway motors.
- Confusing Shunt and Series Symbols: In ANSI schematics, a shunt field is drawn parallel to the armature circle, meaning it connects directly across the main DC bus. A series field is drawn in a single line passing through the armature. Wiring a shunt field in series will result in a massive voltage drop and zero starting torque. Wiring a series field in parallel across the bus will cause a dead short and instantly trip the main breaker.
- Assuming "M" Means AC Induction: If the symbol is just a circle with an "M" and lacks the specific DC commutator brush indicators or permanent magnet stator lines, check the power supply rails. If the bus is labeled with a straight line (DC) rather than a sine wave (AC), it is a DC motor. Furthermore, AC motors typically use the letters "M" inside a circle with 3-phase lines (L1, L2, L3) or single-phase designations.
- BLDC vs. Stepper Motor Symbols: Both use electronic commutation, but their symbols differ. A BLDC symbol usually includes a 3-phase AC base (U, V, W) driven by a DC-AC inverter block. A stepper motor symbol typically shows multiple distinct coil rectangles (A, A', B, B') without a continuous rotation indicator.
Decision Path: Selecting the Right Symbol & Terminal Wiring
Use this decision tree to determine the correct schematic symbol to draw, or the correct physical wiring method to execute, based on your application requirements.
| Application Requirement | Motor Type Selection | Schematic Symbol to Draw | Concrete Wiring Pick |
|---|---|---|---|
| Need simple 12V/24V control, low cost, no field power supply available. | Permanent Magnet (PM) | IEC PM Symbol (Circle + M + PM stator lines) | Wire A1 to VCC, A2 to GND via a MOSFET H-Bridge. Use 14 AWG THHN for a 10A load. |
| Need high starting torque for a heavy load (e.g., winch), speed regulation is poor. | Series Wound | IEC Series Symbol (Circle + M + F1/F2 in line) | Wire F1 to DC+, F2 to A1, A2 to DC-. Never run without a mechanical load attached. |
| Need constant speed under varying loads (e.g., lathe, conveyor). | Shunt Wound | IEC Shunt Symbol (Circle + M + D1/D2 parallel) | Wire D1/D2 directly to DC bus. Wire A1/A2 through the speed controller. Always apply field power before armature power. |
| Default / Hobbyist / General Robotics (If unsure) | Permanent Magnet (PM) | IEC PM Symbol | Default Pick: Use IEC PM symbol with A1/A2 terminals, wire using 14 AWG THHN for standard 12V/10A builds. |
Safe Interpretation When Nameplates Are Faded or Missing
On the bench, you will frequently encounter surplus or salvaged DC motors where the physical nameplate is gone, and the terminal block markings (A1, D1, F1) are painted over or faded. You must identify the windings before applying power to avoid destroying the motor or your power supply.
Use a digital multimeter (DMM) in resistance mode to identify the terminals based on these typical thresholds for a standard 24V industrial fractional-horsepower motor:
- Armature (A1 to A2): Expect very low resistance, typically 0.5Ω to 5.0Ω. The armature consists of thick copper wire designed to carry the full load current. If you spin the shaft by hand while measuring continuity, you may see slight fluctuations due to the commutator brushes moving across the segments.
- Shunt Field (D1 to D2): Expect high resistance, typically 50Ω to 300Ω. The shunt field uses thousands of turns of very fine wire to create a magnetic field without drawing excessive current from the main bus.
- Series Field (F1 to F2): Expect near-zero resistance, typically 0.1Ω to 1.0Ω. Like the armature, the series field must carry the full load current, so it is wound with heavy-gauge wire.
- Interpoles / Commutating Poles (B1 to B2): If your motor has four small terminals in addition to the main ones, measure for low resistance (~1Ω). These are interpoles wired in series with the armature to prevent sparking at the brushes.
By cross-referencing your DMM resistance readings with the motor nameplate definitions outlined by EC&M, you can confidently map faded physical terminals back to their correct IEC 60617 schematic symbols. Always verify your findings by applying a low-voltage, current-limited bench supply (e.g., 5V at 2A) to the armature to confirm rotation direction before committing to the final high-voltage wiring.
