A 3 phase 6 lead motor wiring diagram exposes the start and finish of all three internal stator windings, allowing you to wire the motor externally in either a Star (Wye) or Delta configuration. While 9-lead motors are common for dual-voltage applications in North America, the 6-lead motor is the global standard for single-voltage Star-Delta reduced-voltage starting, or for 230V Delta / 400V Star dual-voltage setups common in IEC regions. Understanding how to trace this diagram from the main disconnect down to the terminal block is critical to preventing catastrophic winding failures during the Star-to-Delta transition.
Decoding the Diagram: Symbols and Physical Terminals
Before tracing the wires, you must translate the schematic symbols to the physical device. On a wiring diagram, the motor windings are typically represented by three parallel circles or zig-zag lines, each with two nodes. Contactors are labeled with KM (e.g., KM1 for Main, KM2 for Delta, KM3 for Star), and overloads are marked as OL or F.
Physically, the motor terminal block will feature six brass or copper studs. Depending on the manufacturing standard, these are labeled differently:
- IEC Standard (Global/Modern): U1, V1, W1 (Winding Starts) and U2, V2, W2 (Winding Finishes).
- NEMA Standard (North America Legacy): T1, T2, T3 and T4, T5, T6.
Node-by-Node Trace: Source to Motor Terminals
To wire a 6-lead motor for Star-Delta starting, the power path must be traced sequentially from the source to the load, ensuring the ground path remains entirely independent of the switching logic.
- Main Disconnect / Breaker: Three-phase power (e.g., 480V or 400V L1, L2, L3) enters the main disconnect. The Protective Earth (PE) or ground wire is bonded directly to the panel's ground bar.
- Main Contactor (KM1): L1, L2, and L3 feed into the line side of KM1. When energized, KM1 passes power to the load side.
- Overload Relay (OL): The load side of KM1 feeds directly into the line side of the thermal or electronic overload relay. This protects the motor from sustained overcurrent.
- Motor Terminals (Starts): The load side of the OL relay connects directly to the motor's winding starts: U1, V1, and W1. These connections remain solid throughout both the Star and Delta running phases.
- Star Contactor (KM3): During the starting phase, KM3 closes, creating a dead short across U2, V2, and W2. This forms the neutral point of the Star configuration, applying reduced voltage (Line Voltage / √3) to each winding.
- Delta Contactor (KM2): After a timed transition (typically 3 to 10 seconds), KM3 opens and KM2 closes. KM2 bridges the windings in a triangle: U1 to W2, V1 to U2, and W1 to V2. Full line voltage is now applied across each winding.
- Ground Path (PE): The equipment grounding conductor runs directly from the main panel ground bar to the motor frame's external ground lug. It does not pass through the breaker, contactors, or overloads. This ensures equipotential bonding regardless of the contactor state.
Terminal Mapping and Coil Verification
Incorrectly mapping the winding starts and finishes will result in a dead short when the Delta contactor (KM2) engages. Use the table below to map your connections, and verify them with a multimeter before applying power.
| Lead Function | IEC Terminal | NEMA Terminal | Internal Winding Pair | Delta Bridging Connection |
|---|---|---|---|---|
| Phase A Start | U1 | T1 | U1 — U2 | U1 bridges to W2 |
| Phase B Start | V1 | T2 | V1 — V2 | V1 bridges to U2 |
| Phase C Start | W1 | T3 | W1 — W2 | W1 bridges to V2 |
| Phase A Finish | U2 | T4 | U1 — U2 | U2 bridges to V1 |
| Phase B Finish | V2 | T5 | V1 — V2 | V2 bridges to W1 |
| Phase C Finish | W2 | T6 | W1 — W2 | W2 bridges to U1 |
How to Verify Connections with a Meter
Before terminating wires at the contactors, isolate the motor and perform a continuity and insulation test. According to Fluke's guidelines on motor testing, verifying coil integrity prevents immediate failure upon energization.
- Coil Continuity (Ohms): Set your multimeter to the lowest Ohms range (e.g., Fluke 87V). Measure across U1 to U2, V1 to V2, and W1 to W2. You should read a low, balanced resistance (typically 0.1Ω to 5.0Ω depending on motor horsepower). A reading of OL (Open Loop) indicates a blown internal winding.
- Phase Isolation: Measure cross-phase (e.g., U1 to V1, V1 to W1). The meter must read OL. Any low resistance reading here means the internal insulation between windings has failed.
- Insulation Resistance (Megger): Using a 500V DC insulation tester, place the negative lead on the motor frame (bare metal) and the positive lead on U1. Apply voltage for 60 seconds. Per the NEMA MG-1 standard, the resistance should be well over 1 Megohm. Repeat for V1 and W1.
Frequently Asked Questions
Can I use a 6 lead motor for dual voltage (230V/460V) wiring?
Generally, no. A common misconception is that any motor with multiple leads can be rewired for dual voltage. In North America, dual-voltage 230V/460V operation typically requires a 9-lead motor (T1-T9) because the internal windings must be split into parallel sets for low voltage and series sets for high voltage. A true 6-lead motor is usually designed for a single voltage and is brought out to six terminals specifically to allow external Star-Delta starting. The exception is IEC motors wound for 230V Delta / 400V Star, which use 6 leads to switch between those two specific European grid voltages.
What happens if I swap the start and finish leads (e.g., U1 and U2) on the Delta contactor?
If you reverse the polarity of even one winding pair (connecting U1 to U2 instead of the correct bridging sequence), the magnetic vectors of the stator will oppose each other rather than rotating smoothly. When the timer transitions from Star to Delta, the motor will experience a massive current spike, likely stalling instantly, tripping the main breaker, or violently jerking the rotor. Always verify the start/finish mapping with an ohmmeter and label the wires before the final Delta termination.
How do I identify the leads if the physical U1-W2 tags are missing?
If the terminal block is unmarked, you must identify the three coil pairs and their polarities. First, use an ohmmeter to find the three pairs that show continuity (Pair A, Pair B, Pair C). Next, use a 12V DC battery and a center-zero analog voltmeter (or a digital meter with a fast min/max capture). Connect the battery momentarily across Pair A. If you connect the positive battery terminal to a lead and the meter kicks positive on a secondary pair, the lead connected to the positive meter probe is the "Start" (U1/V1/W1) and the other is the "Finish" (U2/V2/W2). This inductive kick test establishes the relative polarity required for a safe Delta closure.
Can I run a 6 lead motor directly on a Variable Frequency Drive (VFD)?
Yes, but you must wire the motor in the Delta configuration for the VFD's output voltage. For example, if you have a 230V Delta / 400V Star motor and you are running it on a standard 230V 3-phase VFD output, you must link the terminals in Delta (U1-W2, V1-U2, W1-V2) at the motor peckerhead. Do not use the Star-Delta contactor logic with a VFD; the VFD handles the soft-start and reduced voltage ramping internally via PWM. Bypass the KM2/KM3 contactors entirely and feed the VFD output directly to the Delta-linked terminals.
