变压器带载运行时，励磁磁动势是（   ）产生的。

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69508' and id < 4502872 and id<>'4502872' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69508' and id > 4502872 and id<>'4502872' order by id asc limit 5