With the continuous development of China's power industry, it is becoming increasingly common to use electric motors as power to drive various types of machinery to do work. Especially in rural areas, due to seasonal busy farming and frequent changes in the working location of agricultural machinery, when the motor junction box and terminal post are damaged, spare parts cannot be provided for the time being, so temporary wiring can only be made directly on the six outgoing lines of the motor. This kind of wiring is prone to confusion, and once there is confusion in the head and tail wiring, it will have an impact on the progress of the busy farming season. Moreover, inexperienced people often have a headache when facing the six wires that cannot distinguish the head and tail in this confusion.

Based on years of work experience, I would like to introduce a simple method for using a multimeter to quickly find the connection between the motor head and tail to colleagues in the agricultural power industry, for reference.

1. Self coupling air compressor method

First, use a multimeter in the ohm range (R × 100) to locate the wire ends of the three-phase windings, and then connect any one end of each phase together (i.e. any star connection). Connect the secondary winding of a control transformer with a secondary voltage of 36V or less to the common point of the motor and any of the remaining three wire ends, connect the power supply, and use a multimeter voltage range (depending on the secondary voltage of the control transformer) to measure the remaining two wire ends. If there is no voltage indication, it is the same terminal (i.e. the same phase head or phase tail); If there is a voltage indication, the two wire ends should be swapped. Then, control the secondary of the transformer by connecting one of the two phases that have been measured. Using the same method to measure the head and tail of the secondary winding of the original control transformer, the head and tail of the three-phase motor winding can be quickly distinguished. According to experience, if there is no multimeter available at the moment, the other two phase wire ends can be quickly touched in the above connection sequence. If there is no spark, it proves that they are the same end (i.e. the same winding head or tail). The next inspection method is the same as the multimeter method, except that one can be directly judged from the meter, and the other relies on judging sparks to distinguish between the ends of the motor winding. Of course, if there is no control transformer, it is also possible to use the battery on the tractor or agricultural vehicle as a power source for judgment.

2. Circulation method

The circulation method is based on the principle that the residual magnetism left during the original operation of the motor will generate induced electromotive force to determine the head and tail. Similarly, first use a multimeter to distinguish the windings of each phase, then mark the windings of each phase and connect them in any triangle shape. Place the multimeter range in the DC milliampere range and connect it in series with the triangular connection formed by the three-phase winding. Rotate the motor by hand to dry (the speed should be uniform). If the pointer moves slightly or does not move, it proves that the connection is correct; If the pointer deviates significantly, it indicates that the connection is incorrect. It can be switched phase by phase until the pointer indicates correctly. If the motor is stopped for too long or loses magnetism, this method should not be used.

Of course, there are other methods for determining the head and tail of each phase winding in the electric circuit, such as the lamp cannon method, which will not be introduced one by one here.