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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.
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.