Abstract: The efficiencies of a pump and an electric motor cannot be calculated separately by traditional method if the pump is powered by the electric motor.In the variable frequency speedcontrolling system of an asynchronous electric motor,the speed of the electric motor cannot be determined directly by the changed frequency,and the output power of the electric motor cannot be determined.If a pump is powered by an electric motor,the axle power cannot be determined because of having a rotary joint between pump and electric motor.Based on the theory of electric motor and the characteristics of pump,a new method for calculating output power and speed of an electric motor with changed frequency is presented.The efficiencies of a pump and an electric motor can be calculated separately by using the new method.The formulas for calculating the ratio of rotation difference and speed of electric motor after changing the frequency were established.The idea that the ratio of rotation difference of a variable frequency conversion speed controlling electric motor is constant can be negated.

Key words: variable frequency speed-controlling system, asynchronous electric motor, pump, efficiency, axle power, calculation method

摘要： 在异步电机变频调速系统中,频率改变后,电机的转速不能由频率直接确定,因而不能确定电机输出的机械功率。电机拖动泵类负载时,由于泵与电机对轴联接,因此泵的轴功率也不能确定,电机和泵的效率也不能单独计算。依据电机理论和泵的特性,分析并得出了在频率改变后,由频率计算电机转速及输出机械功率的方法。利用该方法可以单独计算电机和泵的效率。另外,还分析得出了频率改变后电机转差率和转速的计算公式,并同时否定了变频调速电机转差率不变的论点。

关键词: 变频调速系统, 异步电动机, 泵, 效率, 轴功率, 计算方法