... ZJ70/4500DZ, ZJ90/5850DZ AC variable-frequency motor drive drilling rig model: ZJ40/2250DB, ZJ50/3150DB, ZJ70/4500DB, ZJ90/5850DB ... electromechanical drilling rig model: ZJ40/2250LDB, ZJ50/3150LDB, ZJ70/4500LDB mechanical drive ... so as to gain excellent drilling performance. The drilling rig driven by AC variable frequency motor employs VFD and digital ...