... 3150DZ, ZJ70/4500DZ, ZJ90/5850DZ AC variable-frequency motor drive drilling rig model: ZJ40/2250DB, ZJ50/3150DB, ZJ70/4500DB, ZJ90/ ... stepless speed regulation, so as to gain excellent drilling performance. The drilling rig driven by AC variable frequency motor employs VFD and ... be equipped with a height indicator for the travelling block, so as to improve the drilling safety. The drawworks is equipped ...