The benefits of keeping 6 terminals for three-phase motors

During an analysis and discussion on the connection method of three-phase motor windings, a netizen raised a question about the number of winding output terminals.

For most three-phase motor windings, we can use different connection methods according to the line voltage, namely delta connection and star connection. When the winding adopts star connection, there are only 3 output terminals. When the winding adopts delta connection, can we use inner connection to lead out 3 output terminals? Theoretically, there is no problem, but why do most motor manufacturers, including some basic design specifications, design and manufacture according to 6 output terminals?

We can analyze it from the following aspects:

First, when six output terminals are used, the corresponding motor can meet the power supply requirements of two line voltages, especially for industrial frequency motors, which can achieve the dual requirements of starting and running.

Second, for some special operating scenarios, in order to avoid the circulating current problem that may be caused by the triangle connection, the user may adopt the star connection method by adjusting the voltage.

Third, we look ahead to the motor manufacturing process. When six terminals are used, the three-phase winding can be inspected at any time for inter-turn, inter-phase and ground insulation. Once the internal connection method is used with three terminals, the motor’s inter-phase insulation and ground insulation inspection will not be very direct. In particular, inter-phase insulation problems cannot be discovered.

In view of the above analysis, whether it is a star-connected motor or a delta-connected motor, if it is processed according to 6 output terminals, the final application connection can be achieved on the terminal board, but the number of parts required for wiring will be relatively more.

According to the actual application requirements of the motor, when three output terminals are required, the insulation between turns, between phases and to the ground of the three-phase winding must be fully checked before connection, and then subsequent processing is performed to ensure that the electrical performance of the motor meets the requirements. Moreover, once the connection is completed using the internal connection method, the motor can only operate at one voltage.