Frequency inverter drives

Figure 3-14. Typical invester AC motor drive consisting of rectifier-DC link, adjustable-frequency inverter, and induction of synchronous motor [10].

The paper will then focus on the application of higher frequency modulated line inverters drives to actively reduce the harmonic content seen at the Point of Common Coupling whilst providing the ability to support VAR compensation and greater tolerance to supply variations. 

The use of an adjustable-frequency power supply (ac inverters) in conjunction with the induction motor results in an adjustable-speed drive system.The speed of the motor is adjusted by controlling the output frequency of the ac inverter.Thus the induction motor can be used on many adjustable-speed apphcations. The speed-torque motor performance at different input motor frequencies is Ulustrated by Fig. 5.127a. This permits the apphcation of induction motor adjustable-speed systems to loads such as fans and pumps for flow control with considerable power savings over fixed-speed systems with dampers or valves for flow control. Figure 5.127Z) shows the performance of a 10-hp motor adjustable-frequency system driving a fan load. 

In addition to the above inverter systems there is one more system, called a cyclo converter system. These drives tire employed for very large motors, when IGBTs in such ratings are a limitation. It converts the fixed a.c. supply frequency to a variable frequency, generally lower than rated, directly and without rectifying it to a d.c, source. They are basically frequency converters. This system is more complex and expensive and has only... 

A relatively new innovation for use in electric motor compressor drives is the variable frequency power source. Fundamentally, the power source converts an existing three-phase source into DC then uses an inverter to convert back to a variable frequency supply. Thyristors or transistors are used to switch the output at the required frequency. 

Inverter-AC Motor Drives. An adjustable-frequency control of AC motors provide efficient operation with the use of brushless, high-performance induction, and synchronous motors. A typical system is shown in Figure. 3-14. Such a system consists of a rectifier (which provides DC power from the AC line) and an inverter (which converts the DC power to acljustable-frequency AC power for the motor). Inverter cost per kilowatt is about twice that of controller rectifiers thus the power convertor for an AC drive can approach three times the cost of a DC drive. 

Harmonic number (h) refers to the individual frequency elements that comprise a composite waveform. For example, h = 5 refers to the fifth harmonic component with a frequency equal to five times the fundamental frequency. If the fundamental frequency is 60 Hz, then the fifth harmonic frequency is 5 x 60, or 300 Hz. The harmonic number 6 is a component with a frequency of 360 Hz. Dealing with harmonic numbers and not with harmonic frequencies is done for two reasons. The fundamental frequency varies among individual countries and applications. The fundamental frequency in the U.S. is 60 Hz, whereas in Europe and many Asian countries it is 50 Hz. Also, some applications use frequencies other than 50 or 60 Hz for example, 400 Hz is a common frequency in the aerospace industry, while some AC systems for electric traction use 25 Hz as the frequency. The inverter part of an AC adjustable speed drive can operate at any frequency between zero and its full rated maximum frequency, and the fundamental frequency then becomes the frequency at which the motor is operating. The use of harmonic numbers allows us to simplify how we express harmonics. The second reason for using harmonic numbers is the simplification realized in performing mathematical operations involving harmonics. 

Compare the plots of quantum and semiclassical theory that are shown in Fig. 2. The relatively faster rates and weaker driving force dependence in the inverted region for the correlation calculated by using the quantum expression is caused by nuclear tunneling in high-frequency vibrational modes that are coupled to ET.)... 

The transistors of the output-side inverter are pulse-width modulated in such a way that in every phase, a near-sinusoidal current is created after smoothing by motor inductivity. The frequency of this current can be varied to adjust the speed of the motor. The switching frequency of small to medium frequency converters is on the order of 2 to 8 kHz and adjustable. In frequency converter operation, this frequency is quite audible. The larger the motor, the smaller the switching frequency selected. In large drives, switching frequency drops to 400 Hz. 

Voltage peaks also arise from the semiconductors switching in the inverter stressing the insulation of connected motors. For this reason, motors intended for inverter operation are better insulated. Sometimes - depending on the length of the motor cable - output filters are provided between the frequency converter and the motor to protect the latter. Such measures should be undertaken when standard motors used initially at constant speed on the mains are retrofitted with an inverter for variable drive. 

Modern frequency converters now make it possible to handle smaller automation tasks via the motor drive alone. Here, the controller intended for controlling the inverter assumes monitoring and control tasks that it can communicate externally via the available (or expandable) I/O. Moreover, it is also possible to connect various bus systems. Sometimes the functioning of small programmable logic controls can be integrated into the inverter. Parameters and control function programming can be entered via a small user interface. It may be even more convenient to connect to a laptop with an RS 232 interface. 

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