F3-phase 3-wire Y connection AC voltage controller Resistive load,0°≤a<60° 1 VT VT VT VT 6 VT VT VT L 2 2 兀2兀 5冗 15Power Electron cs i 15 3 -phase 3 phase 3 -wire Y connection wire Y connection AC voltage controller AC voltage controller Resistive load, Resistive load, 0 ° ≤ α < 60 ° α 4 π 3 2 π 3 5 π 3 π 3 0 2 π u ao' u a u ab 2 u ac 2 t 1 t 2 t 3 VT 1 VT 3 VT 6 VT4 VT 6 VT 5 VT 2 VT 5 VT1