Z ZB 120° 240° 36O° A A Z Z B C (a) at=0 (b)ot=120° (c)ot=240° 跳转到第一页跳转到第一页 ωt i i A i B i C O 120° 240° 360° × × × × · × · · · · · (a) ωt = 0° (b) ωt = 120° (c) ωt = 240° A A A X X X B B B Y Y Y C C C × Z Z Z