VFD Supply Sag

[Modula2]

If a VFD's supply voltage sags for several seconds in the order of 10%-15%, and the VFD is previously at full load, rated voltage and 60 Hz speed, then what is the typical drive response. Will it shut down on low voltage or overcurrent? This is a common problem.

 

[dadfap]

Modula2

The answer to this question lies in how the drive is set up. Most drives have both time and value thresholds that have to be met before it will shut down.

These should be user programmable so that nuisance shutdowns can be avoided.

 

[Modula2]

I was expecting certain shutdowns to be fixed, for protection of the drive. And also, if the voltage only drops 10-15%, the slip should increase for the same torque, but input ac current should only increase by a comparable amount, and power stay about the same. So, I am not expecting an overcurrent nor overvoltage shutoff, unless there is something wrong with the drive.

 

[Farkel]

Sounds like a need for a voltage regulator or line conditioner. Most microprocessors which are pre programed, and that have smaller capacitors for storage will not sustain in time lower voltages for many cycles.

 

[ELECTRICALTESTING]

In most of the VFDs there is a feature called "Power Loss ride through" which usually takes care of such input voltage dips for few seconds. In this case the drive will continue to maintain the previous speed. But one should take care of the control supply part of VFD which, if provided from Input side, will again cause tripping of the drive (due to contactor/relay drop down). It is preferable to provide control supply from independent sources in such cases. In some cases, if the drive rating is almost matching the load, I mean to say that if there is not enough margin in the rating, there is a chance of drive tripping on overcurrent.