Good Example Of Research Paper On Principles Of Construction

Variable Frequency Drives

Introduction
VFD is a system of control rotor speed asynchronous (or synchronous) motor. It consists of the motor and the inverter. Frequency converter (inverter) is a device consisting of a rectifier (DC bridge), which converts AC power to DC power, and an inverter (converter, sometimes with PWM), which converts direct current into alternating required frequency and amplitude. Output thyristors (GTO) or IGBT transistors provide the necessary current to power the motor. To avoid the inverter overloading with great length between the inverter and the feeder chokes are put. To reduce to reduce EMI it is being used EMC-filter.
With scalar control harmonic currents motor phases are generated. Vector control is a control method of synchronous and asynchronous motors, not only form the harmonic currents (voltage) phases, but also provide control of the rotor flux (torque on the motor shaft).

In converters with a direct connection inverter is controlled rectifier. The control system in turn unlocks the group of thyristors, and connects the stator windings of the motor to the mains. Thus, the output voltage is formed from the "cut" of the input sinusoid voltage portions. Output voltage frequency can be equal to or higher than the mains frequency. It is in the range from 0 to 30 Hz. As a consequence - a small range of engine speed control (no more than 1: 10). This limitation does not allow applying such converters in modern variable frequency drives with a wide range of adjustment of parameters. Using unclosable thyristors requires relatively complex control systems that add value to the transmitter. "Cutting" sine wave inverter output with direct connection is a source of higher harmonics, which cause additional losses in electric motors, electric machine to overheat, reducing the time, very strong interference in the mains. The use of compensating devices increases the cost, weight, dimensions, reduction in the efficiency of the overall system.
The most widely used in modern variable frequency drive is converters with a pronounced DC link. In this class of converters is used a double conversion of electrical energy: the input sinusoidal voltage is rectified by a rectifier filters filters, and then converted back to AC inverter voltage variable frequency and amplitude. Double conversion of energy reduces the efficiency and to some deterioration in weight and size parameters with respect to the direct-coupled inverters.

The Use of VFDs

The classical method of controlling the supply pump units involves throttling the flow lines and the regulation of the number of working units on any technical parameters (e.g. pressure in the pipeline). Pumping units in this case are selected on the basis of certain characteristics of the calculated (usually with a margin for performance) and continuously operate at a constant speed, without changing costs caused by the varying consumption. At a minimum flow pumps continue to operate at a constant speed, creating excessive pressure on the network (the cause of accidents), the useless spent a considerable amount of electricity. Thus, for example, occurs during the night when the water consumption decreases sharply. The main effect is not due to energy savings, but due to a significant reduction in the cost of repair of water supply networks.
The appearance of the variable frequency drive maintains a constant pressure directly at the consumer. A wide used in the world practice has received a frequency regulated electric drive with induction motor for general industrial use. Frequency regulation of shaft speed induction motor by means of an electronic device is called a frequency converter. The above effect is achieved by changing the frequency and amplitude of the three-phase voltage supplied to the motor. Thus, by changing the parameters of the supply voltage (frequency control), one can make the engine speed both below and above the nominal value. In the second zone (frequency above nominal) of the maximum torque is inversely proportional to the shaft rotational speed.
The method of frequency conversion is based on the following principle. Typically, the frequency of commercial power is 50 Hz. For example, take the pump with a two-pole motor. In view of the sliding speed of the motor is about 2800 (depending on capacity) revolutions per minute and gives the output of the pump unit rated head and capacity (as is his nominal parameters, according to the passport). When using the inverter frequency and lower amplitude supplied to a variable voltage, respectively, the motor speed will drop, and therefore change the performance of the pump unit. Information about the pressure in the network enters the inverter unit from a special pressure sensor mounted at the consumer on the basis of these data converter appropriately changes the frequency supplied to the motor.

Conclusion

Modern frequency converter has a compact design, dust- and waterproof housing, user-friendly interface. It can be used in the most demanding and challenging environments. The power range is very wide, ranging from 0.18 to 630 kW or more at a standard power 220/380 V and 50-60 Hz. Practice shows that the use of frequency converters in pumping stations allows you to:
save energy (with significant changes flow) by adjusting the electric power depending on the actual water consumption (saving effect of 20-50%);
reduce the consumption of water by reducing leaks in excess of line pressure when the flow rate of water consumption is actually small (an average of 5%);
reduce costs (the main economic effect) on emergency repairs of the equipment (the entire infrastructure of water supply due to a sharp decrease in the number of accidents caused by water hammer in particular, which often happens in the case of an uncontrollable drive (it is proved that the resource of the equipment increases of at least 1, 5 times);
increase a pressure above normal in case of need;
automate the systems of water supply, thereby reducing the payroll service and staff on duty, and eliminate the influence of the "human factor" on the system, which is also important.
According to the various reports, the payback period for the implementation of VFDs is from 3 months to 2 years.

Works Cited

Bose, Bimal K. Power Electronics and Motor Drives Advances and Trends. Amsterdam: Elsevier/Academic, 2006. Print.
Jaeschke, Ralph L. Controlling Power Transmission Systems. Cleveland: Penton/IPC, 1978. Print.
Siskind, Charles S. Electrical Control Systems in Industry. New York: McGraw-Hill, 1963. Print.