3. The working principle of butt-joint welding is to fuse two wire harness using immediate high current produced by positive and negative short circuit . Its cost is low and easy to operate. 

But when welding enamelled wires and lead wires, empty soldering will occur , which is not easy to find for welder.  Empty soldering makes the motor power can't reach its rating and some inferior-quality motors will be produced.  At the same time,  the electric spark is harmful to the operators. Long time operations even cause some operators transient blindness.