本文主要內容是變頻調速，變頻調速(VFD)是一種電機控制驅動裝置，它可以改變提供給電機的頻率。它也被稱為變速驅動器。變頻調速通常用來驅動感應電動機。隨著變頻技術的發展，調速性能可以與直流電機媲美。與工頻供水相比，變頻供水系統具有明顯的優點。本篇論文代寫行情文章由澳洲論文人EducationRen教育網整理，供大家參考閱讀。
Variable frequency drive (VFD) is a motor controlling drive device that can vary the frequency supplied to an electric motor. It is also known as variable speed drive. VFD is usually used to drive an induction motor. With the development of frequency conversion technology, the speed control performance can be as good as a direct current motor. As compared to power frequency water supply, a variable frequency water supply system has obvious advantages as follows.
Saves electricity
Prolongs the life of the pump and the motor
Keeps pumps working efficiently
Reduces the impact to grid by soft-starting the motor
Controls pump to deliver constant pressures
Constant pressure water supply is based on the technology of frequency conversion. In this constant, pressure water supply application. To meet the variable demand of water supply and keep pipe network pressure constant, a pressure controller and pressure sensors are needed. To reduce the cost, in most cases a VFD is used to drive a motor instead of several motors.
VFD is a frequency conversion device which is used to drive an induction motor. It is comprised of integrated circuit and power electronics. Its heart is a central processing unit (CPU). The frequency of grid is fixed. In Canada, the power frequency is 60Hz. It is known that the synchronous speed of an induction motor is:
ns = 120f/P
where ns = synchronous speed (rpm)
f = motor stator frequency (Hz)
P = number of magnetic poles
The rotating speed of an induction motor nr is a function of its synchronous speed ns. Their relationship is:
nr = ns(1-s)
= 120f/P (1-s)
Where s = slip (usually 0.02 to 0.06)
From the above equation, a result can be found out that the speed is proportional to the frequency supplied to the motor, inversely proportional to the number of its magnetic poles. Therefore, we can vary the speed by changing either the frequency or the number of poles. Once an induction motor has been built, the number of its magnetic poles is fixed. The easy way to vary the speed of an induction motor is to change the frequency supplied to the motor.
Figure 4 shows the basic theory of a VFD. A VFD includes a rectifier, a DC link circuit, an inverter and control circuit at least.
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