Asynchronous motor contactor interlocking positive &negative circuit control lines

What is interlock ( Interlock) 吗? Draw the asynchronous motor contactor interlocking positive &negative control circuit to explain, and indicate the interlocking contactor in the picture. When a contactor to action, through its auxiliary normally closed contacts make another contactor cannot function called interlocking (electric action Or interlock) 。 Three-phase asynchronous motor contactor interlocking positive &negative control circuit in order to guarantee a contactor to action, the other a contactor not to action, in order to avoid the power of the interphase short circuit, the control circuit of the forward the concatenated reversing contactor KM2 auxiliary normally closed contact, while in the control circuit of the inversion of the concatenated forward contactor KM1 auxiliary normally closed contact.