The standard schematic dc motor symbol is a circle containing the letter "M" (for a generic motor) or specific internal winding indicators like "A" (armature) and "F" (field). However, on the physical terminal box, DC motors do not use the L1/L2/L3 designations common to AC motors. Instead, they rely on specific alphanumeric terminal markings (A1/A2 for armature, F1/F2 for shunt field, S1/S2 for series field) governed by strict regional standards. Misinterpreting these symbols or swapping regional standards is a primary cause of commutator flashovers and burnt field windings in legacy and imported machinery.
Complete DC Motor Symbol & Terminal Reference Table
Before wiring any DC motor, you must identify the governing standard stamped on the nameplate. The table below maps the schematic symbols to their physical terminal designations under the two dominant global standards: NEMA MG-1 (North America) and IEC 60034-8 (International/Europe).
| Winding / Component | Schematic Symbol | NEMA MG-1 Terminals | IEC 60034-8 Terminals | Function & Wiring Practice |
|---|---|---|---|---|
| Armature | Circle with 'A' or 'M' | A1, A2 | A1, A2 | The main rotating winding connected to the commutator. Carries full load current. Requires heavy-gauge wire and low-resistance connections. |
| Shunt Field | Coil with 'F' | F1, F2 | F1, F2 | High-resistance, many-turns winding wired in parallel with the armature. Controls base speed and flux. Never open-circuit while running. |
| Series Field | Coil with 'S' | S1, S2 | D1, D2 | Low-resistance, heavy-wire winding in series with the armature. Provides high starting torque. Noticeable divergence between NEMA and IEC here. |
| Interpole (Commutating) | Small coil with 'B' | B1, B2 | B1, B2 | Narrow poles between main poles to prevent arcing. Must be wired in series with the armature, never the shunt field. |
| Compensating Winding | Zig-zag line 'C' | C1, C2 | C1, C2 | Embedded in the main pole faces to counteract armature reaction under heavy loads. Wired in series with the armature and interpoles. |
Regional Standard Variants: NEMA vs. IEC vs. Legacy UK
When repairing imported machinery or integrating global supply chains, assuming a single universal standard will lead to catastrophic wiring errors. The most critical divergence occurs in the series field and legacy British color codes.
| Standard / Region | Series Field Designation | Wire Color Codes (Legacy/Control) | When It Applies to You |
|---|---|---|---|
| NEMA MG-1 (US/Canada) | S1, S2 | Black (Line), Red (Armature), White (Field) | Motors manufactured in North America, or imported motors specifically rewound for the US market. |
| IEC 60034-8 (EU/Global) | D1, D2 | Brown (Line), Black (Armature), Grey (Field) | Modern European imports, Asian-manufactured OEM components, and global standardized industrial drives. |
| Legacy UK (Pre-IEC) | Z1, Z2 (Series) Y1, Y2 (Shunt) |
Red/Yellow/Blue (Phases), Black (Neutral) | Vintage British machinery (pre-1970s) still in service in older UK manufacturing plants or former Commonwealth nations. |
According to the NEMA MG-1 standard documentation, North American motors strictly enforce the S1/S2 nomenclature for series fields. Conversely, the IEC 60034-8 specification assigns D1/D2 to the series field to avoid confusion with the 'S' often used for stator or switchgear in other IEC diagrams. If you are wiring an IEC motor using a NEMA schematic, you must mentally map S1/S2 to D1/D2.
Rows People Get Wrong & Faded Marking Protocols
Even with the reference table in hand, bench technicians frequently make two specific errors when terminating DC motors. Furthermore, on older motors, the stamped terminal markings are often obscured by decades of grease, carbon dust, or paint.
The Two Most Common Wiring Mistakes
- Wiring Interpoles in Parallel: The interpole winding (B1/B2) exists solely to neutralize the magnetic distortion caused by armature current. It must be wired in series with the armature. If you accidentally wire it in parallel with the shunt field, it will draw massive current, overheat rapidly, and fail to prevent commutator arcing, leading to a flashover.
- Reversing Shunt Field Polarity on Compound Motors: In a cumulative compound motor, the magnetic flux of the series field must aid the shunt field. If you swap S1/S2 (or D1/D2) without adjusting the armature leads, the motor becomes a differential compound motor. It will exhibit severe speed instability, potentially overspeeding and destroying itself mechanically when the load is suddenly removed.
The shunt field winding (F1/F2) is a massive inductor. If you break the F1/F2 circuit while the motor is energized or spinning, the collapsing magnetic field will induce a voltage spike that can exceed 1000V, instantly destroying solid-state motor controllers and posing a lethal shock hazard. Always use a field discharge resistor (freewheeling diode in DC drives) across F1/F2, and never open the field contactor before the armature contactor during shutdown sequences.
Safe Interpretation Protocol for Faded or Missing Markings
When you inherit a surplus DC motor with unreadable terminal blocks, do not guess. Use a digital multimeter (DMM) to identify the windings based on their inherent resistance characteristics. Ensure the motor is completely disconnected from any power source before testing.
- Identify the Shunt Field (F1/F2): Set your DMM to the highest resistance range (e.g., 2kΩ). Probe the terminals in pairs. The shunt field consists of thousands of turns of fine wire. You will read a high resistance, typically between 50Ω and 800Ω depending on the motor voltage rating. Mark these terminals as your field.
- Identify the Armature (A1/A2): Set your DMM to the lowest resistance range (e.g., 200Ω or use the continuity beeper). The armature uses thick copper bars and heavy brushes. You will read a very low resistance, usually under 1.0Ω. Furthermore, if you manually rotate the motor shaft while probing, you may see the resistance fluctuate slightly as the brushes cross the commutator segments. Mark these as your armature.
- Identify the Series/Interpole Fields (S1/S2 / B1/B2): These windings are also low resistance (typically 1Ω to 15Ω), but they are completely isolated from the commutator. If you read a low resistance but rotating the shaft causes zero fluctuation in the reading, you have found a series or interpole winding. To distinguish between the two, consult the manufacturer's datasheet for the exact ohmic value, or trace the physical wires: interpole wires usually route to the smaller, narrower poles located between the main stator poles.
For comprehensive schematic drafting rules, refer to the IEEE 315 standard for graphic symbols, which dictates exactly how these internal windings must be represented on electrical prints to ensure cross-disciplinary clarity between mechanical and electrical engineering teams.
