semiconductor materials and process of manufacturing. 在电力电子系统，中半导体开关是非常重要和关键部件。半导体开关将要替换机械开关,但半导体材料的性质和生产过程严重限制了他们。
Switching losses开关损耗
Power losses in the power eletronic converters are comprised of the Switching losses and parasitic losses. 电力电子转换器的功率损耗分为开关损耗和寄生损耗
the parasitic losses account for the losses due to the winding resistances of the inductors and transformers,the dielectric losses of capacitors,the eddy and the hysteresis losses. 寄生损失的绕组电感器、变压器的阻力、介电损耗的电容器,涡流和磁滞损耗
the switching losses are significant and can be managed. 这个开关损耗是非常重要的,可以被处理。they can be further divided into three components:(a)the on-state losses,(b)the off-state losses and the losses in the transition states. 他们可以分为三个部分: 通态损耗，断态损耗和转换过程中产生的损耗。
On-State Losses通态损耗：
The electrical switches conduct heavy current and have nonzero voltage across the switch in the on-state.The on-state power losses are given by Pon=Uson if.这个电子开关能导通大电流，并且在通态时有非零的压降。这个通态功率损耗的公式为Pon=Uson if.
The Uson and If are respectively the switch voltage in the on-state and the forward current through the switch.For example,the typical power diodes and the power transistors have nearly 0.5 to l volt across them in the on-state.The forward currents can be hundreds to thousands of amperes.The on-state power losses are very significant. 其中Uson是通态时开关上的压降，if是流过开关的电流。例如，典型的功率二极管和功率晶体管有近似0.5~1伏的通态压降。而电流会有数百到数千安培。这个通态损耗非常重要。
Off-State Losses断态损耗
The electrical switches withstand high voltages and have nonzero leakage current through the switch in the off-state.The off-state power lesses are given by Poh=Uoff ir在关断状态时，电子开关到经受得起高电压，并会有非零的漏电流。断态损耗的公式
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为Poh=Uoff ir.
The Usoff and Ir are respectively the reverse bias voltage in the off-state and the reverse current through the switch.For example,the typical power diodes and the power transistors have high reverse voltages in hundreds to thousands of volts and microamps to milliamps through them in the off state. 其中Uoff在断态时的反向偏置电压，ir是流过开关的反向漏电流。例如，典型的功率二极管和功率晶体管有很高的反向压降几百到几千伏和几微安到几毫安的漏电流。
Transition-State Losses转换损耗：
The practical switching devices have limited capabilities of rate of voltage transition and the rate of current steering.These nonabrupt transition rates give rise to power losses in the switching devices.We will examine these switching losses in two cases separately:the inductive and capacitive loads. 在实际的开关装置限制了电压变换率和电流变化率。非突变引起了开关装置的功率损耗。我们测试开关损耗时分两种情况：感性负载和容性负载。
Switching with Inductive Load接感性负载的开关：
The indutor is assumed to be large so that the current through it in steady state is nearly constant Io.Assume that initially the switch is off.The inductor current is +Io and freewheels through diode V1.When the switch is turned on,the current through the switch begins to build up linearly(an assumption)to+Io while the diode V1 is still on.The on diode has zero voltage across it(an ideal diode),hence,the voltage on the switch is held constant at+Us.When the current buildup is over,the diode V1 ceases to conduct and the voltage on the switch ramps linearly(again an assumption)down to zero. 假设电感无穷大，即在稳定时流经电感的电流是恒定的Io，假定开始时开关处于关断状态。电感电流为+Io惯性流过二极管V1。当开关闭合后，电流流经开关开始建立线性上升+Io此时二极管扔导通。二极管压降为0，此时开关两端电压维持在+Us当电流建立完成后，二极管V1截止，开关两端电压线性下降为0。
When the switch is turned off ,the voltage begins to build up linearly to +us while the diode V1 is off. while the diode is off the current through the switch equals the inductor current,which is constant I0 After the switch voltage reaches
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aero, the current through the switch begins to decrease below I0,as the remaining current is now steered through the diode V1 which has now turned on The current through the swithch ramps down to zero ultimately. Switching waveforms with inductive load are shown in Fig.3-1开关打开后，开关两端电压线性上升至Us。此时二极管仍截止，二极管截止，流过开关的电流相当于流过电感的电流，维持在恒定的Io。开关电压到0时，通过开关的电流开始上升到Io以下。此时余留的电流正转向二极管V1，V1导通。最终通过开关的电流下降到0，开关过程的波形的电感负载波形见图3-1 The switching losses are given by : Psw=1/2UsIo[???.]fs 开关损耗的公式是Psw=1/2UsIo[???.]fs
The switching power losses increase linearly with the switching requency like in the resistive case but about six times more. The upper bound on the switching frequency is also about half. 开关功率损耗线性增加随着开关频率 此时的损耗要比阻性负载损耗的6倍还多。当f取最大时 Psw=1/2UsIo。 Switching with capacitive Load开关接有容性负载
The capacitor is assumed to be large so that the voltage through is in stedy state is nearly constant U0.Assume that initially the switch is on,hence,the cuttent through the switch is IS.The capacitor voltage is U the voltage across the switch is zero and the diode V1is reverse biased.When the switch is turned off,the switch voltage begins to ramp up to+U0 while the diode V1 is still off.During this buildup,the current through the switch is held constant at Is.Wheng the voltage buildup is over,the diode V1begins to conduct and the voltage on the switch is clamped at U0,and the current through the switch ramps linearly(again an assumption)down to zero. 假设电容器很大，致使在稳定状态下其两端电压接近为常数Uo。假使开关初始状态为闭合，则通过开关的电流为Is。电容器的电压为Uo开关两端的电压为零，二极管V1反向偏置。当开关打开后，开关两端电压开始上升到Uo，此时二极管一直关断。当电压等于Uo时，二极管V1开始导通并且开关两端电压被钳位在Uo。流过开关的电流线性下降到零。
When the switch is closed,the current begins to build up linearly to Is while the diode V1 is still on .The voltage on the switch remains clamped at U0.After the switchcurrent reaches Is ,the diode turns off and the voltage on the switch begins to ramp down to zero. 当开关闭合，电流开始上升至Is，
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此时V1仍然导通，开关两端电位被钳位在Uo。当开关电流等于Is时，二极管关断并且开关两端电压线性下降至0。
The switching power losses in the case of capacitive load also have similar dependence as in the case of inductive loads. 当开关闭合，电流开始上升至Is，此时V1仍然导通，开关两端电位被钳位在Uo。当开关电流等于Is时，二极管关断并且开关两端电压线性下降至0。
The switching power losses in the case of capacitive load also have similar dependence as in the case of inductive loads. 接容性负载情况下的开关功率损耗的决定因素与感性负载情况下有相似之处。
The switching losses can be usually minimized in two ways; 1divert the energy from the switch to a loss or non-loss circuit or 2switch at either zero current or at zero voltage.The first is called snubbering and the later is known as zero-voltage and zero-current swithching. 开关损耗一般被归为两点：1。开关的能量转化成了损耗或无损耗电流或开关的过0电流或过0电压，第一个被称为缓冲，最后一个被成为过0电流或过0电压的开关。
The basic principle of operation of an induction machine is illustrated by the revolving horseshoe magnet and copper-disk experiment pictured in Fig.4-1. When the horseshoe magnet is rotated, the moving magnetic field passing across the copper disk induces eddy currents in the disk. These eddy currents are in such a direction as to cause the disk to follow the rotation of the horseshoe magnet. With the direction of rotation shown in the figure, the eddy currents will be as displayed in Fig.4-1 according to Fleming's right-hand rule
一台感应电机的基本工作原理如图4－1中的旋转的U形磁铁和铜圈盘的实验图所示。当U形铁旋转时，运动的磁场穿过铜圆盘从而在铜圆盘中产生涡流。这些涡流的方向是这样的以致于使圆盘顺着U
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形磁铁的转向旋转。在图示的旋转方向下，根据弗莱明右手定则涡流的方向将如图4－1所示。
Fleming's right-hand rule: Place the thumb and the first and second fingers of the right hand so that all three are mutually perpendicular. With the hand in this position, the first finger is pointed in the direction of the field, the thumb is in the direction of motion of the relative motion of the conductor, and the second finger is the direction of the induced voltage. Note that the relative motion of the conductor is opposite to the rotation of the direction of rotation of the magnetic field.
弗莱明右手定则：伸出右手大拇指、食指以及中指并使它们相互垂直。保持右手处于这种状态，使食指指向磁场的方向，拇指指向导体相对运动的运动方向，那么中指所指的方向就是感应电压的方向，要注意的是导体的相对运动与磁场的旋转方向的运动相反。
By applying Fleming's right-hand rule, the force on the copper disk is determined to be in the direction of rotation of the magnet.
通过应用弗莱明右手定则，我们确定作用在铜圆盘上的力与磁铁的旋转方向一致
Fleming's left-hand rule: Place the thumb and the first and second finger of the left hand so that all three are mutually perpendicular to each other. With the first finger in the direction of the field and the second finger in the direction of the current, the thumb indicates the direction of the force. 弗莱明左手定则：伸出左手大拇指、食指以及中指并使它们相互垂直。
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