单项选择题一台直流电动机的电枢回路额定电压为UN,额定转速为nN，额定理想空载转速为<br/> <img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20251103/8789772_0.jpg"><br/> <img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20251103/8789772_0_1.jpg">