调速系统中调频器的作用为(    )。

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69507' and id < 4502805 and id<>'4502805' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69507' and id > 4502805 and id<>'4502805' order by id asc limit 5