若异步电动机在频率为50Hz时额定转速为960r/min，则电机极数为（   ）。

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69445' and id < 4497232 and id<>'4497232' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69445' and id > 4497232 and id<>'4497232' order by id asc limit 5