单项选择题 The initial voltage of self-excitation generator set up by（）

单项选择题 “380V/220V Y/△” noted on the nameplate of a non-synchronous motor means that（）

单项选择题 The nameplate of an AC non-synchronous motor shows that running speed is 1460r/minfrequency 50HzYou can infer that the number of magnetic poles is（）