直流发电机通过主磁通感应的电动势应存在于（ ）。

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SQL Error: select id,body,options from ***_enewstiku where sourceid='30222' and id < 2444775 and id<>'2444775' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku where sourceid='30222' and id > 2444775 and id<>'2444775' order by id asc limit 5