09-08-2012, 09:52 AM
VARIABLE FREQUENCY DRIVE
variable frequency drive.docx (Size: 277.61 KB / Downloads: 105)
INTRODUCTION
In today’s industrial and commercial world, variable frequency drives (also known as VFD’s, Frequency Drives, AC motor controllers, and Inverters) are becoming a more and more important method of speed control. They are used for all but the most demanding speed control applications. Large rolling mills are among the few places where DC motors and controls are still used. One of the reasons for this is that AC motors and their associated controls are much more reliable than the DC motors and the associated controls that to a large extent they have replaced. In current market VFD’s ranging from ¼ hp to 30,000 hp are available for use.
In 1960 the first inverter were made and had limited applications due to small size and reliability in 1980’s when high power transistor were available, larger inverters were made and many more applications opened up. Reliability and Mean Time between Failures (MTBF) was still a problem. All of these earlier devices used linear amplifiers and controls for their basic operation. Small potentiometers and dip switches were used to set their operating characteristics. In the 1990’s digital controls began to be used more and more in Inverters. Solid state devices were developed that allowed higher voltage and current ratings. Inverters are used in larger motors. Micro processors made the Inverter a versatile device. For many applications, the Inverter can be removed from a packing box, wired to a motor, and turned on and operated without additional set up.
: VARIABLE FREQUENCY DRIVE
Motors are designed to run at a constant speed. However, motor drive systems are often operated at part or variable load. In particular, fans and pumps can have highly irregular load profiles. This means, the motors on these systems either run at constant speed by passing the excess capacity, or use some form of capacity regulation such as dampers, valves, or inlet guide vanes, all of which are very inefficient.
System output can be controlled by adjusting the speed of the motor using one of different types of Adjustable Speed
• Variable Frequency Drives (VFDs)
Motion control is required in large number of electrical and domestic applications like rolling mills, fans, paper machines, textile mills, pumps, robots etc. System employed for motion control is called drives. Drives employing electric motors are known as electrical drives.
OPERATION OF VFD
Understanding the basic principles behind VFD operation requires understanding the three basic sections of the VFD: the rectifier, dc bus, and inverter.
The voltage on an alternating current (ac) power supply rises and falls in the pattern of a sine wave (see Figure 1). When the voltage is positive, current flows in one direction; when the voltage is negative, the current flows in the opposite direction. This type of power system enables large amounts of energy to be efficiently transmitted over great distances.
VOLTAGE SOURCE INVERTER (VSI):
Variable frequency and variable voltage supply for IM control can be obtained from VSI. VSI allows a variable frequency to be obtained from a dc supply. It can be operated as a stepped wave inverter or pulse width modulated inverter. When supply is dc, variable dc input is obtained by connecting a chopper between dc supply and inverter. When supply is ac, dc input voltage is obtained by connecting a controlled rectifier between supply ac and inverter. A large electrolytic capacitor is connected in dc link to make inverter operation independent of rectifier or chopper and to filter out the harmonics in dc link voltage.
Harmonics are reduced, low frequency harmonics are eliminated, associated losses are reduced and smooth motion is obtained at low speeds also when inverter is operated as pulse width modulated inverter. Since output voltage can be controlled by PWM, no arrangement is required for variation of input dc voltage, inverter can be directly connected when supply is dc and through a diode rectifier when supply is ac.
CURRENT SOURCE INVERTER (CSI):
In current source inverter, inverter behaves as current source due to the presence of large inductance in dc link. For a given speed, torque is controlled by varying dc link current by changing the inverter voltage. Therefore, when supply is ac, a controlled rectifier is connected between the supply and inverter and when supply is dc, a chopper is interposed between the supply and inverter. The maximum value of dc output voltage of fully controlled rectifier and chopper are chosen so that the motor terminal voltage saturates at rated value.