HomeDefinitionVFD input Line reactors

VFD input Line reactors

Input Line reactors provide increased input section Inductance and reactance, causing any (especially sudden) input current to be elongated and therefore lower peak transients as well as the average value levels, lowering the possibility of damage from input disturbances. This helps to protect the front end of a VFD (Variable Frequency Drive) from typically unintended input transient damage from any number of sources to include unintended grounding, Capacitor bank switching, storms, unstable loading and unstable power supplies. They are not however as complete a protection solution as transformers, which also provide raised input impedance for current transient protection. Reactors are sometimes used as a lower cost substitutes for Input Step down or isolation transformers, but really should be carefully evaluated as Voltage issues can quite often be better handled by input transformers lowering (for instance) voltage transients by ten to 1, and as well giving you isolation from the supply offering opportunities for better grounding with say a Delta to Wye transformer.

To try to describe this simply, Output reactors smooth the output current as the VFD output is a Pulse Width Modulated supply and is "inherently Noisy" .The complex waveform generated is in the neighborhood of 30MHZ in bandwidth. You have the Switching Frequencies, Output Carrier frequency, the Base Frequency, etc. The sudden rate of rise from Off to On of the IGBT is high frequency (Insulated Gate Bi-Polar Transistors) (in the neighborhood of 1 to 20uSec, (Lightning is in the neighborhood of 50 to 100uSecs). With all this manipulation of the output power, the use of Inductors/Reactors can enable a kinder gentler output waveform reach the motor, much closer resembling a sine wave rather than a PWM (resembling more of a square wave) waveform.

On the bearing protection, that is usually a combination of ceramic bearings and a stationary conductive brush ring that grounds the motor shaft. Some VFD's will produce harmonics that cause a flux current flow through the shaft and will pit steel bearings causing premature motor failure. This is not unusual on surface motors and a lot depends upon the size and loads of the motor.

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