不论是负载作星形边接还是三角型连接，每相的有功功率、无功功率和视在功率都可以分别按下式决定：

                                               P_相=U_相I_相cosφ_相

                                               Q_相=U_相I_相sinφ_相

                                               S_相=U_相I_相

        而三相负载总功率则等于各相功率之和。即：

                                               P=P_A＋P_B＋P_C

                                               Q=Q_A＋Q_B＋Q_C

                                               S=S_A＋S_B＋S_C

        在负载对称和电压对称的情况下，三相制的三种功率各等于单相功率的三倍，即：

                                               P=3P_相=U_相I_相cosφ_相

                                               Q=3Q_相=U_相I_相sinφ_相

                                                S=3S_相=U_相I_相

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