[第十章频率响应多频正弦稳态电路09.ppt

当频率较低时,ωL1/(ωC),电抗X为负值,电路呈容性。因而电流超前于电压ùS, 如图(a)所示。随着频率的逐渐升高,|X|减小, 从而阻抗的模值也减小,电流的模值增大。 当电源角频率改变到某一值ω0时, 使ω0L = 1/(ω0C), 这时电抗X等于零, 阻抗的模|Z|达最小值。这时电流达最大值, 且与电源电压ùS同相。其相量关系如图(b)所示。 1. X、|Z|随角频率变化的情况 Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. 如电源频率继续升高, 则ωL1/(ωC), 电抗为正值, 电路呈感性。因而电流落后于电压 , 其相量关系如图(c)所示。 X、|Z|随角频率变化的情况 Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. 2. 电路的串联谐振频率 当回路电抗等于零, 电流与电源电压同相时, 称电路发生了串联谐振。 这时的频率称为串联谐振频率, 用f0表示, 相应的角频率用ω0表示。 电路发生串联谐振时, 有 X = ?0L - 1/(?0C) = 0 故得 由上式可知, 电路的谐振频率仅由回路元件参数L和C决定, 而与激励无关, 但仅当激励源的频率等于电路的谐振频率时, 电路才发生谐振现象。谐振反映了电路的固有性质。 Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. 除改变激励频率使电路发生谐振外, 实际中，经常通过改变电容或电感参数使电路对某个所需频率发生谐振, 这种操作称为调谐。譬如,收音机选择电台就是一种常见的调谐操作。 谐振时的感抗与容抗数值相等, 其值称为谐振电路的特性阻抗, 用ρ表示, 即 3. 谐振电路的调谐和特性阻抗 Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. 在工程中, 通常用电路的特性阻抗ρ与回路的电阻r的比值来表征谐振电路的性质, 此比值称为串联谐振电路的品质因数用Q表示（品质因数和无功功率符号相同， 注意不要混淆）。即： 它是一个无量纲的量。 4. 串联谐振电路的品质因数 Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. *