In an analysis and discussion about the winding connection methods of three-phase motors, an engineer raised a question regarding the number of winding terminal leads.
For most three-phase motors windings, different connection methods—delta (Δ) and star (Y)—can be adopted depending on the line voltage. When a star connection is used, only three terminal leads are required. However, for a delta connection, is it possible to internally connect the windings and still bring out only three terminal leads? Theoretically, this is feasible, yet most electric motor manufacturers, as well as basic design standards, typically design and produce motors with six terminal leads.
The reasons can be analyzed from the following perspectives:
1. Dual Voltage Compatibility: With six terminal leads, the motor can accommodate two different line voltage requirements. This is particularly useful for line-frequency motors, allowing them to meet both starting and operational demands.
2. Avoiding Circulating Currents in Special Applications: In certain operating conditions, to prevent potential circulating currents in a delta connection, users may opt for a star connection by adjusting the voltage.
3. Manufacturing and Testing Convenience: During motor production, having six terminal leads facilitates easy inspection of turn-to-turn, phase-to-phase, and ground insulation in the three-phase windings. If an internal delta connection is used with only three leads, phase-to-phase and ground insulation checks become less straightforward—especially for detecting phase-to-phase insulation issues.
Based on the above analysis, whether for star- or delta-connected motors, using six terminal leads allows the final connection to be implemented on the terminal board. However, this approach requires slightly more components for wiring.
If only three terminal leads are needed for practical applications, a comprehensive inspection of turn-to-turn, phase-to-phase, and ground insulation must be conducted before making the connections. Only after ensuring the motor’s electrical performance meets requirements should further processing proceed. Additionally, once an internal connection is finalized, the motor can only operate at a single voltage.
