三相异步电动机变极调速中的功率与转矩问题讨论

Power and Torque Problems in Pole-changing Adjustable-speed Motor with Three-phase

很多教科书在介绍三相异步电动机变极调速(比如Y/YY调速)时,既设定了变极前后线电压不变,在特性参数保持不变的两个半相绕组由串联变为并联情况下,又假定变极后的线电流变为原来的2倍,若仅用简单的阻抗串并联的观点分析很容易引起误解与矛盾.基于感应电动机的T型等效电路,本文计算分析了三相异步电动机变极前后的线电流、输出电磁功率、电磁转矩及功率因数随两个参数变化关系.结果不仅澄清了很多教科书上关于Y/YY变极调速前后线电压不变而线电流加倍这一问题的本质,而且表明转差率是影响三相异步电动机线电流、电磁功率与转矩及功率因数的重要因素,单星形(Y)到双星形(YY)的变极调速是一种对转差率有要求(SYY>>SY/2)的恒转矩调速.计算结果还表明,相对转差率而言,T型等效电路中励磁回路的电抗大小变化对电动机运性特性影响较小.

Based on the T-shape equivalent circuit of induction motor, the linear current, output electromagnetic power, electromagnetic torque and power factor before and after pole-changing were calculated and analyzed in detail. The results not only clarified the essence of linear current double in the Y/YY pole-changing adjutable-speed procedure when kept the linear voltage unchanged, but also revealed that slip rate was the important factor influencing the linear current, electromagnetic power and torque, and power factor of three-phase motor. It was shown that Y/YY pole-changing adjustable-speed was one type of torque-unchanged adjustable-speed under a certain situation（i.e slip rate sYYsY/2）. The calculated result also revealed that, in contrast to the slip rate, the influence of excitation reactance of T-shape equivalent circuit on the running properties of motor was weak.