These drives reduce mechanical stress by employing soft starters. It also improves the power factor of the whole electrical system. The other benefits include the reduction and/or elimination of motor starters, less stress on the AC motor windings and bearings, and a decrease in stress and wear on the pump or fan.
Disadvantages
The non-pure sine wave output power causes additional motor heating primarily because of harmonics and below base operation. Depending upon the manufacturer and the type of variable frequency controller, the input voltage and current waveform to the motor will be distorted by varying degrees. Harmonic currents generate no useful torque at the motor shaft. Instead, these create additional losses.
However, VFDs can be reliable and trouble-free if they are selected and installed properly. VFDs can cause damage to the motors they drive and cause a wide range of problems in the associated electrical system if they are not selected properly. Most of the problems arise from distortion of the voltage and current waveforms in the input and output of the drive, commonly called harmonic distortion.
Precautions
Never switch a motor on and off from the output of a drive without first stopping the inverter. VFDs help in stabilizing process parameters.
Quite often, VFD transistors are damaged when a motor experiences short circuit. VFDs typically have a built-in over current and other protections. The short circuit current can be very severe and its rise time can be so rapid that it could be damaged before the protections can properly react. A harmonic compensated (3% reactance is typically sufficient) will provide current limiting to safer values, and will also slow down the short circuit current rise. The drive is allowed more time to react and to safely shut the system down. Though the motor calls for repair, it still saves the drive transistors.
In conclusion, if one needs to decide on whether a VFD fits the application or not, one could ask questions like: Do I need a wide range of speed? Will I need precise control of the motor speed and/or torque? If the answer is yes to one or both of these, then consider a VFD.
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