The standard three phase motor symbol is a circle containing an 'M' (NEMA/IEEE) or '3~' (IEC), intersected by three incoming power lines. However, the exact terminal designations, internal connection symbols, and grounding representations change drastically depending on whether you are reading a European IEC 60617 print or a North American IEEE 315 / NEMA schematic. Below is the direct reference data you need to read, interpret, and wire these motors safely.
Master Reference Table: Three Phase Motor Symbols
Use this table to decode the motor symbols on your schematic. The table covers the primary induction and synchronous variants you will encounter in industrial and commercial panels.
| Motor Type | IEC 60617 Symbol / Text | NEMA / IEEE 315 Symbol / Text | Terminal Designations | Practical Application & Meaning |
|---|---|---|---|---|
| 3-Phase Squirrel Cage Induction | Circle with '3~' or 'M 3~' | Circle with 'M' or '3Ø' | IEC: U, V, W NEMA: T1, T2, T3 |
The workhorse of industry. Symbol implies a standard direct-on-line (DOL) or VFD-driven rotor with no external electrical connections to the rotating part. |
| 3-Phase Wound Rotor (Slip Ring) | Circle with '3~' + 3 rotor lines exiting to resistor bank | Circle with 'M' + 3 rotor lines (often labeled R1, R2, R3) | Stator: U, V, W Rotor: K, L, M (IEC) or R1, R2, R3 (NEMA) |
Used for high starting torque/low inrush current applications (cranes, hoists). The extra lines represent the slip rings connecting to an external resistance bank. |
| 3-Phase Synchronous Motor | Circle with '3~' and a DC excitation symbol on the rotor | Circle with 'M' and a DC field coil symbol | Stator: U, V, W Field: F1, F2 |
Runs at exact synchronous speed. The DC excitation symbol indicates the requirement for a separate DC supply to the rotor field windings. |
| Star/Delta (Wye/Delta) Starter Representation | Motor symbol with a bridged 'Y' and 'Δ' switching contactor diagram | Motor symbol with 'Y' and 'Δ' or 6-lead/9-lead terminal map | 6-lead: U1/V1/W1 & U2/V2/W2 9-lead: T1 through T9 |
Indicates a reduced-voltage starting method. The symbol explicitly shows the transition from a Star (Wye) configuration for starting to Delta for running. |
Regional Standards: IEC vs. NEMA vs. Legacy UK
Knowing which standard applies to your region prevents catastrophic miswiring. A 400V IEC motor wired using NEMA assumptions can result in a dead short or a burned-out winding.
- IEC 60617 (Europe, Global, Modern AU/NZ): Uses alphanumeric designations (U, V, W for phases; 1 for start, 2 for finish). A 6-lead motor will have U1, V1, W1 and U2, V2, W2. The symbols are minimalist, relying on the '3~' text inside the circle to denote three-phase AC.
- NEMA / IEEE 315 (US, Canada): Uses 'T' designations. A standard single-voltage motor uses T1, T2, T3. A dual-voltage 9-lead motor uses T1 through T9. The symbols often include the letter 'M' and may explicitly draw the internal coil connections for 9-lead motors.
- Legacy UK (BS 3939): Phased out in favor of IEC, but still found in older British plants. Used A, B, C for phases with red/yellow/blue color coding. If you encounter A1/A2, B1/B2, C1/C2, treat them exactly as U1/U2, V1/V2, W1/W2 under modern IEC standards.
Rows People Get Wrong (And How to Fix Them)
When reading schematics or looking at terminal blocks, these specific table rows and symbol variations cause the most field errors.
Row 1 Confusion: 6-Lead vs. 9-Lead Internal Connections
People often see a standard squirrel cage symbol and assume a simple 3-terminal connection. If you open the peckerhead (terminal box) and find 6 or 9 wires, the basic symbol on the schematic was an oversimplification. The Fix: If you have 6 leads (IEC), you must manually jumper them for Star (U2-V2-W2 tied together) or Delta (U1-W2, V1-U2, W1-V2). If you have 9 leads (NEMA), refer to the internal Wye or Delta map printed on the inside of the terminal box cover.
Row 4 Confusion: The Grounding vs. Bonding Terminal
Schematics frequently omit the Protective Earth (PE) or Equipment Grounding Conductor (EGC) terminal from the primary motor symbol to reduce visual clutter. The Fix: Always look for a separate symbol—a circle with three descending lines of decreasing width (IEC PE symbol) or a simple ground symbol—pointing to the motor casing. Never use the motor's internal winding neutral point as a ground path.
Faded Markings: Safe Interpretation & Multimeter Testing
If you are retrofitting an old motor and the nameplate is missing or the terminal block markings are faded, you must identify the windings before energizing. Guessing the phasing will cause the motor to run backward or trip the breaker instantly.
- De-energize and Lockout: Shut off the disconnect, apply a LOTO lock, and verify zero voltage with a non-contact voltage tester and a multimeter.
- Find the Pairs (Continuity): Set your multimeter to continuity or low ohms. Test all combinations of the 6 leads. You will find exactly three pairs that show continuity (e.g., Lead 1 & 4, Lead 2 & 5, Lead 3 & 6). These are your individual coil windings.
- Identify Start vs. Finish (Polarity): If the pairs are unmarked, you can determine polarity using a 9V battery and an analog meter (or digital meter with a min/max capture). Connect the meter to one pair. Briefly tap the 9V battery to a second pair. If the meter deflects positive when the battery positive is applied, the battery-positive lead and the meter-positive lead are both 'Start' (1) ends.
- Label Immediately: Use heat-shrink tubing or numbered wire markers to label U1/U2, V1/V2, W1/W2 (or T1-T6) before reassembling the terminal box.
Decision Path: Selecting Your Wiring Configuration
Use this decision tree to terminate your schematic reading into a concrete wiring action. Follow the path based on your region, voltage, and lead count.
| Condition / Input | Path | Concrete Action & Jumper Configuration |
|---|---|---|
| Region: US/Canada (NEMA) Voltage: 480V Leads: 9-Lead Dual Voltage |
High-Voltage Wye | Action: Wire for High Voltage Wye. Jumpers: Tie T4-T7, T5-T8, T6-T9 together and insulate. Power: Apply L1 to T1, L2 to T2, L3 to T3. |
| Region: US/Canada (NEMA) Voltage: 240V Leads: 9-Lead Dual Voltage |
Low-Voltage Delta | Action: Wire for Low Voltage Delta. Jumpers: Tie T1-T6-T7, T2-T4-T8, T3-T5-T9 together. Power: Apply L1 to T1/6/7 node, L2 to T2/4/8 node, L3 to T3/5/9 node. |
| Region: EU/Global (IEC) Voltage: 400V Leads: 6-Lead |
Delta Run (DOL) | Action: Wire for standard Delta operation. Jumpers: Install 3 horizontal jumpers: U1-W2, V1-U2, W1-V2. Power: Apply L1 to U1, L2 to V1, L3 to W1. |
| Region: EU/Global (IEC) Voltage: 400V (VFD Driven) Leads: 6-Lead |
Star (Wye) for VFD | Action: Wire for Star to reduce voltage stress on older insulation when using modern VFDs. Jumpers: Tie U2-V2-W2 together. Power: Apply VFD outputs to U1, V1, W1. |
For authoritative reference on schematic standards, consult the IEEE 315 Graphic Symbols for Electrical and Electronics Diagrams for North American prints, and refer to NFPA 79 (Electrical Standard for Industrial Machinery) for panel integration and safety grounding requirements. Always verify local AHJ requirements before finalizing industrial motor installations.
