28.空芯线圈的作用是什么？
["(1)\u5728\u7535\u6c14\u5316\u533a\u6bb5\u9010\u6bb5\u5e73\u8861\u4e24\u94a2\u8f68\u7684\u7275\u5f15\u7535\u6d41\u56de\u6d41\uff1b(2)\u5b9e\u73b0\u4e0a\u4e0b\u884c\u7ebf\u8def\u95f4\u7684\u7b49\u7535\u4f4d\u8fde\u63a5\uff1b(3)\u6539\u5584 \u7535\u6c14\u7edd\u7f18\u8282\u7684Q\u503c\uff0c\u4fdd\u8bc1\u5de5\u4f5c\u7a33\u5b9a\u6027\u3002"]