VFDs and its Benefits
Variable frequency drive, a power electronics device that bought a revolution in the Industry. It controls the speed and frequency of AC Motor. Best starter for AC Motor is nothing but VFDs. It also lower KVA thus helping alleviate voltage sags and power outages
Another added advantage is the energy saving feature in drive which reduces the cost by saving electricity. It lowers the starting current of motor makes it a perfect starter. VFD provides high PF eliminates the need for external power factor(pf) correction capacitors. Reduction of thermal and mechanical stresses on motors and belts during starts, simple installation,
Harmonic distortion and industry standards
Current wave form at an integral multiple of the fundamental frequency can be called harmonics. Simply consider a 50-hertz power supplies as an example. Here, harmonics would be like at 100 hertz (2 x fundamental), 150 hertz (3 x fundamental), 200 hertz, 250 hertz, etc. The 2nd order as well as 3rd order harmonics are not harmful. The reason is they cancel each other out in each phase because the power supply is 3 phase.
If we look into the harmonics 5th, 7th, 11th, 13th and so on, we can see that the magnitude of the harmonics greatest for the lower order harmonics (5th, 7th and 11th). It drops quickly as you move into the higher order harmonics (13th and greater).
VFD, a source of harmonics!
Unfortunately VFD is a source of harmful harmonics. VFDs in fact draw current from the line only when the line voltage is greater than the DC Bus voltage inside the drive. This occurs only near the peaks of the sine wave. Thus, all the current drawn is in short intervals, which is at higher frequencies.
VFDs that are equipped with DC link inductors produce different levels of harmonics than those without DC link inductors. The VFDs with active front ends utilizing transistors in the rectifier section have much lower harmonic levels than those using diodes or silicon controlled rectifiers (SCRs). Thus we can conclude that variation in VFD design has an effect in the production of harmonics.
The effects of harmonics
The Harmonics cause disturbances in electrical systems. Higher order harmonics can interfere with sensitive electronics and communications systems. The main reason behind the overheating of motors, transformers, and conductors is the presence of lower order harmonics. The presence of harmonic sensitive equipment in an electrical system decides the opportunity of harmonics to be harmful or not. As harmonic currents flow through devices with reactance or resistance, a voltage drop is developed. These harmonic voltages cause voltage distortion of the fundamental voltage wave form.
Mitigating Harmonics
Line reactors add reactance and impedance to the circuit. Reactance and impedance act to lower the current magnitude of harmonics in the system and thereby lower the TDD.A line reactor placed between the VFD and the motor helps to protect the motor from current spikes.VFDs Using Active Front End Technology (AFE), Passive filters, active filters etc are the other methods to reduce harmful harmonics.
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