Basic Operation of Variable Frequency Drive


A Variable Frequency Drive controls the speed of an ac motor, which offers flexibility towards the procedure because speed may be changed effortlessly for procedure optimization. It requires the fixed energy supplied to it and converts it into a variable frequency and variable voltage supply which then feeds a motor. This enables the drive to manage the speed and torque the motor produces.

VFD Operation

Understanding the basic principles behind VFD operation requires understanding the three  basic sections of the VFD: the rectifier, dc bus, and inverter.

VFD Operation

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.

A typical 6-pulse VFD has six diodes as a front-end bridge rectifier that converts AC to DC. VFDs can also have 12 diodes — two sets per phase (2 × 2 × 3 = 12 pulse) — or 18 diodes — three sets per phase (3 × 2 × 3 = 18 pulse) — and so on (click here to see Table). One set of diodes is supplied by a Delta-Y transformer to create a phase shift on the AC side between the two rectifiers to reduce harmonics reflected back to the source.

6-pulse VFD

A 6-pulse VFD develops the output DC voltage by taking every phase from the AC supply and installing 1 set of diodes to gate on and off (click right here to determine Fig. two). A 6-pulse VFD is most generally utilized within the developing method. Common present total harmonic distortion (THD) back towards the supply may be as higher as 35% in the input terminals from the VFD. You are able to set up an inline inductor to decrease the reflected harmonics back towards the point of coupling, as defined by IEEE 519. The inductor reduces the current distortion — and thus the voltage distortion — at the source. The input line inductors are typically 3% to 5% impedance. Base the selection of the inductor on harmonic evaluation of the electrical system at the building, impact of voltage drops across the inductor, and impact of power factor to the building electrical system.

The impact of harmonics should take into account available fault current (i.e., the stiffness of the electrical system). See IEEE Standard 519 for more details. However, if there are significant VFDs and other harmonic-producing devices in a building’s electrical system, such as electronic ballasts, uninterruptible power supplies, electronic switching devices, etc., think about using 12- or 18-pulse VFDs to reduce the harmonics reflected back to the source.

12-pulse VFD

A 12-pulse VFD has phase shift transformers ahead of 6-pulse VFD to cancel the harmonics reflected back to the source. The phase shift transformers can be tuned to reduce harmonic distortion to less than 10% at the input terminals of the VFD (click here to see Fig. 3). The windings of the transformers are offset to cancel the largest harmonics from the VFD. By reducing the harmonics at the input terminals, the intent is to reduce voltage distortion at the source because the current THD at each piece of equipment is reduced. Conduct a harmonic evaluation to validate the findings. Disadvantages in providing a 12-pulse VFD are the cost (i.e., it can be up to 50% more) and the physical footprint required. A 6-pulse VFD for a typical 25-hp motor can be installed on the wall next to the motor. A 12-pulse VFD will be a free-standing “switchboard” type unit.

18-pulse VFD

An 18-pulse VFD provides low harmonic distortion through phased cancellation of primary harmonics (5th and 7th) and the higher order harmonics that could cause resonance on capacitive and inductive loads (such as filters, transformer, etc.). Because cost can be prohibitive for most building applications, only consider this approach for facilities that have significant harmonics on the electrical system.

Recommend 220V/380V Delta VFD Drives

This low voltage (single phase 220V, three phase 380V) variable frequency drive manufactured by Delta has compact design and integrated advanced technology for new features, like tracking the motor speed in real time, automatic running in instantaneous power off, it’s a very useful function in unstable power supply environment. Dynamic current control and output high torque under very low frequency.

VFD Features:

High-performance optimized voltage space vector V/F algorithm and high efficiency VFD, low noise and low electromagnetic interference.
15kW and below VFDs are equipped with build-in braking unit.
High-performance bipolar PID with correction function is convenient for closed-loop control.
Motor speed monitor function allows smooth start for various loads such as centrifuges and dewatering machines anytime.
Built-in software special for constant pressure water supply system enables one VFD to control two pumps simultaneously without adding extra expansion unit.
English LED control panel with friendly human-computer interface (Chinese LED control panel available as well)
Real time clock timing control (easily set the variable frequency drive running time)

VFD applications:

Widely used in textile, cable, printing& dying, washing, fans & pumps, package, Plastic extruding machines, machinery, ceramic, constant pressure water supply, constant temperature control and OEM.

In summary, whenever a load has either a variable torque or a variable speed, a VFD should be considered. A VFD might be considered if a large motor has a problem with voltage drop, torque, or inrush current during start-up. Even though VFDs undoubtedly solve their fair amount of problems and provide substantial energy savings, the heat they generate must be dissipated — and the harmonics they produce must be mitigated.




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