外文翻译--基本的AC驱动

The Basic AC Drives

AC drives, inverters, and adjustable frequency drives are all terms that are used to refer to equipment designed to control the speed of an AC motor. The term SIMOVERT is used by Siemens to identify a Siemens Motor inverter (AC drive).AC drives receive AC power and convert it to an adjustable frequency, adjustable voltage output for controlling motor operation. A typical inverter receives 380 VAC, three-phase,50 Hz input power and in turn provides the proper voltage and frequency for a given speed to the motor. The three common inverter types are the variable voltage inverter, current source inverter (CSI), and pulse width modulation (PWM).Another type of AC drive is a cycle converter. These are commonly used for very large motors and will not be described in this course. All AC drives convert AC to DC, and then through various switching techniques invert the DC into a variable voltage, variable frequency output. Variable Voltage Inverter

The variable voltage inverter uses an SCR converter bridge to convert the incoming AC voltage into DC. The SCRs provide a means of controlling the value of the rectified DC voltage from 0 to approximately 600 VDC. The L1 choke and C1

capacitor(s) make up the DC link section and smooth the converted DC voltage. The inverter section consists of six switching devices. Various devices can be used such as thyristors, bipolar transistors, MOSFETS, and IGBTs. The following schematic shows an inverter that utilizes bipolar transistors. Control logic (not shown) uses a microprocessor to switch the transistors on and off providing a variable voltage and frequency to the motor.

This type of switching is often referred to as six-step because it takes six 60°steps to complete one 360°cycle. Although the motor prefers a smooth sine wave, a six-step output can be satisfactorily used. The main disadvantage is torque pulsation which occurs each time a switching device, such as a bipolar transistor, is switched. The Pulsations can be noticeable at low speeds as speed variations are sometimes referred to as cogging. The non-sinusoidal current waveform causes extra heating in the motor requiring a motor derating. Current Source Inverter

The current source inverter (CSI) uses an SCR input to produce a variable voltage DC link. The inverter section also uses SCRs for switching the output to the motor. The current source inverter controls the current in the motor. The motor must be care fully matched to the drive. Current spikes, caused by switching, can be seen in the output. At low speeds current pulses can causes the motor to cog. Pulse Width Modulation

Pulse width modulation (PWM) drives, like the Siemens MICROMASTER and MASTERDRIVE VC, provide a more sinusoidal current output to control frequency and voltage supplied to an AC motor. PWM drives are more efficient and typically provide higher levels of performance. A basic PWM drive consists of a converter, DC link, control logic, and an inverter. Converter and DC Link

The converter section consists of a fixed diode bridge rectifier which converts the three-phase power supply to a DC voltage. The L1 choke and C1 capacitor(s) smooth the converted DC voltage. The rectified DC value is approximately 1.35 times the line-to-line value of the supply voltage. The rectified DC value is approximately 650 VDC for a 480 VAC supply. Control Logic and Inverter

Output voltage and frequency to the motor are controlled by the control logic and

inverter section. The inverter section consists of six switching devices. Various devices can be used such as thyristors, bipolar transistors, MOSFETS and IGBTs. following schematic shows an inverter that utilizes IGBTs. The control logic uses a microprocessor to switch the IGBTs on and off providing a variable voltage and frequency to the motor. IGBTs

IGBTs (insulated gate bipolar transistor) provide a high switching speed necessary for PWM inverter operation. IGBTs are capable of switching on and off several thousand times a second. An IGBT can turn on in less than 400 nanoseconds and off in approximately 500 nanoseconds. An IGBT consists of a gate, collector and an emitter. When a positive voltage (typically +15 VDC) is applied to the gate the IGBT will turn on.This is similar to closing a switch. Current will flow between the collector and emitter. An IGBT is turned off by removing the positive voltage from the gate. During the off state the IGBT gate voltage is normally held at a small negative voltage (-15 VDC) to prevent the device from turning on. PWM Output

There are several PWM modulation techniques. It is beyond the scope of this

book to describe them all in detail. The following text and illustrations describe a typical pulse width modulation method. An IGBT (or other type switching device) can be switched on connecting the motor to the positive value of DC voltage (650 VDC from the converter). Current flows in the motor. The IGBT is switched on for a short period of time, allowing only a small amount of current to build up in the motor and then switched off. The IGBT is switched on and left on for progressively longer periods of time, allowing current to build up to higher levels until current in the motorreaches a peak. The IGBT is then switched on for progressively shorter periods of time, decreasing current build up in the motor. The negative half of the sine wave is generated by switching an IGBT connected to the negative value of the converted DC voltage.

PWM Voltage and Current

The more sinusoidal current output produced by the PWM reduces the torque pulsations, low speed motor cogging, and motor losses noticeable when using a six-step output.

The voltage and frequency is controlled electronically by circuitry within the AC drive. The fixed DC voltage (650 VDC) is modulated or clipped with this method to provide a variable voltage and frequency. At low output frequencies a low output voltage is required. The switching devices are turned on for shorter periods of time. Voltage and current build up in the motor is low. At high output frequencies a high voltage is required. The switching devices are turned on for longer periods of time, allowing voltage and current to build up to higher levels in the motor.

中文译文

基本的AC驱动

AC驱动是一种逆变器，是可调节的频率驱动，可以被用于提供按设备的期望来控制交流电机的速度。西门子(AC驱动)是使用用语SIMOVERT来辨认的西门子电机逆变器。AC驱动接受交流电源并且把它转换成一可调整的频率，可调整的电压来控制电机的输出。一台典型的逆变器接受380 V三相交流电压，对电机来说分别是50Hz输入电源和反过来提供适当的电压和频率来达到特定的速度。三共同性反转类型是可控的电压型逆变器，当前有源逆变器(CSI)和脉冲宽度调制(PWM)。另外一种AC驱动是循环逆变器。这些通常被用在大型电机中，在这个过程中是不可控的。所有AC驱动都是把交流转换成直流，然后通过各种各样开关技术把直流逆变为一种可调的电压、频率的电源。 可控电压型逆变器

可控电压型变换器是使用SCR管的整流桥把交流电压转换成成直流。 SCR管提供控制被矫正的直流电压的值大约从0V到600 V。L1开关和C1电容器组成直流环节部分使其转换成平滑的直流电。逆变器部分包括六个开关设备。例如可控硅整流器、双极性晶体管、MOSFETS和IGBT管等各种各样的设备都可以使用。根据图所显示运用双极晶体管的一台逆变器。控制逻辑(没显示)使用一个微处理器控制开关开断来提供可变的电压和频率给电机。

此种开关通常有六个步骤，因为它采取六个60°步骤来完成一个360°周期。虽然电机应该使用光滑的正弦波，但是六步的输出可以令人更满意地使用了。 这种开关的缺点主要是每一个开关装置容易发生转矩脉动，例如一双极性晶体管开关装置。在的低速电机中脉动是明显的。非正弦信号的电流波形导致额外发热。 电流源型逆变器

电流源型逆变器(CSI)使用SCR输出产生一个易变的直流电压环节。对电机来说逆变器部分也使用SCR作为开关装置。电流源型逆变器控制电机的电流。马达必须配以合适的驱动。从输出中可以看出是由变换造成。在低速电流脉冲由电机中的齿轮引起。 脉宽调制

脉宽调制(PWM)驱动，如西门子MICROMASTER和MASTERDRIVE VC，提供更多正弦电流输出控制频率和电压的供应给交流电机。PWM驱动是高效率和典型的提供高水平的控制。一个基本的PWM驱动包括整流器、直流环节、逻辑控制和逆变器. 整流器和直流环节

整流器部分含有一个固定的二极管桥式整流器把三相电源转换成直流电压。L1电阻和C1电容器使被转换的直流电压比较光滑。被矫正的直流电压值大约是1.35倍电源电压的线间短路混线两线间电压值。被矫正的直流电压值大约是514 V由380 V供应。 逻辑控制和逆变器

电机所需的输出电压和频率是由逻辑控制和逆变器部分控制的。逆变器部分包括六个开关装置。如可控硅整流器、双极晶体管、MOSFETS和IGBTs这样的各种各样的设备都可以使用。根据概要显示要运用IGBTs的一台变换器。控制逻辑使用一个微处理器接通IGBTs并且提供一个易变的电压和频率给电机。

IGBTs

IGBTs (绝缘栅双极型功率管)提供一个高速度开关必须为PWM变换器操作。IGBTs 能够每秒能关断数千次。其次，IGBT可能是在不少于大约400到500纳秒打开。IGBT包括一个栅极、一个源极和一个发射极。当一个正电压(典型的+15 V)被用于将起动的IGBT的栅极，这是一个类似闭合的开关。电流将流动在这个源极和发射极之间。IGBT是通过取消通过这个栅极的正电压来关闭的。在状态期间IGBT 栅极电压通常设在一个负电压(-15V)来防止其打开。 PWM输出

这有几个PWM模块化技术。它是在这本书的全部范围之外详细描述的。是根据文本和例证描述一典型脉冲宽度模块化的方法。IGBT (或其他类型开关设备) 能在连接被交换直流电压的正值到马达 (从逆变器的514 V)。电流流动到马达中。在一个短的时期内IGBT被接通，仅准许少量的加强的电流在马达形成然后关闭。IGBT被接通和断开之间有各很长且缓慢的时期，允许电流建立由更高的水平决定的，直到当前在马达到达峰顶。然后IGBT被接通的短周期内，在马达中减少电流的建立。负正弦波的一半是通过交换IGBT的连接引起的直流电压的负值被转换引起的。 PWM电压和电流

由PWM生产的正弦的电流减少扭矩脉动、低速电机的镶齿效应，当用到六步输出时，电机的损失明显极少。

电压和频率通过在AC驱动内的电路来控制。固定的直流电压(514 V)是调制或截去以这种方法来提供可变的电压和频率。需要在低频率输出一个低电压时，开关装置的单位时间内开关时间更短，电压和电流在电机中是很低的。需要在高频率输出一个高电压时，开关装置的单位时间内开关时间较长。允许较高的电压和电流在电机中。