Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

VFD Under Voltage Fault

VFD Under Voltage Fault

VFD Under Voltage Fault

Hi all,
I've had 2 single phase to 3 phase 2.2kw 10A VFDs for several years now with no issues. One on my lathe, one on a mill.
Recently got an INTERMITTENT error of UU undervoltage (on both VFDs). It can run fine for ages then suddenly trip. Sometimes under load, sometimes not. Tried with different motors, makes no difference. I wired up a new (3rd) vfd of same model to the lathe and works fine. Sometimes before it trips you can hear the relay clicking randomly. I believe this relay controls the input single phase voltage charging the capacitors with rectified DC voltage and shorts the high power resistor to stop the charging when full.
Mains voltage is fine and on 32amp breaker.
Unplugged all other equipment on the same ring. No difference.
Opened the VFD and checked the capacitance of the 2 820uf caps which gives approx. 1476uf for both which is low but within 10%.
Unsoldered relay and this seems fine. Coil resistance is ok I think and switches with 24v appied over the coil.
Unsoldered the rectifier and seems to test ok with multimeter.
Checked the resistor on both VFDs, one about 152ohm and the other 158ohm. I assume this should be 150ohm???
I replaced the resister (158ohm reading) with the (152ohm) resistor and put the VFD back together and it worked flawlessly last night. Tried this morning and tripped again. I think this may be due to the motor struggling because I had the lathe setup with the steady a bit too tight. Now it tries to run the motor and just trips again.

Is this because the resistor is getting burnt out and increasing in resistance when hot preventing the capacitors from charging fully (quick enough)?

Model of the VFD is Ecogoo 9100. Manual attached.



Please excuse Mrs moaning in the background :)

RE: VFD Under Voltage Fault

Cant find the fault code for UU undervoltages, but with all your extensive troubleshooting without finding the problem, maybe the simplest and most likely cause that is left, is that your incoming main feed is dropping the voltage under the set limit.
That would also explain why both drives trips at the same time.

Best Regards A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

RE: VFD Under Voltage Fault

Given the UV trip, check for loose connections or improper terminations/frayed cables/iffy fuses or breakers upstream of the VFD right up to the main supply source.


RE: VFD Under Voltage Fault

Check voltage on charging resistor when start inverter.
I think relay is dead (contacts burnt) and charging resistor reamin in circuit so voltage at inverter input drop below UV limit and trip.

RE: VFD Under Voltage Fault

If the above ideas are not conclusive get a recording voltmeter or power quality analyzer, and let it run until the drive trips. If you observe the voltage disturbance on the motor circuit, move it to different locations, that should help figure out the location of the problem, if the disturbance is from an upstream location (loose connection or off site).

RE: VFD Under Voltage Fault

@ RedSnake, Thanks yes I should have said code UV. I'm not use both VFDs at the same time. The Lathe VFD started playing up so then replaced it with the mill VFD. Both are playing up now so bought a new one which I switched over last night and works fine.

@ edison123, I can't find any loose connections.

@ iop95, I posted a video on youtube showing the voltage over the caps.

@ FacEngrPE, I don't think I have access to such equipment other than rigging something up on an ardruino? Not sure what voltage they can take :)

RE: VFD Under Voltage Fault

Most small drives do not actually measure the line voltage, they detect an Under Voltage by monitoring the DC bus ripple. DC bus ripple is increased when feeding single phase power into the rectifier, so drives that are DESIGNED for single phase input will typically have more capacitance than one designed for 3 phase input. For this reason any dip in line voltage, even temporary, can result in a UV trip.

If your drive specifically states it is rated for a single phase input, then the OTHER thing that affects the DC bus ripple is load. So if, even for brief moments, your load is demanding more torque (ergo more current) and the VFD is attempting to deliver it, then that can result in an under voltage trip due to it causing high DC bus ripple. If that increase in demand current from the motor also corresponds with a dip in line voltage, which can easily happen on a single phase source, then the UV trip is all but guaranteed.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: VFD Under Voltage Fault

Now I see AC input voltage ... 249V and there is about 323Vdc (with motor not started) instead of about 350Vdc.
Seems that DC bus capacitors are at very low value... check them.
But also check if contacts relay are realy closed (may measure voltage at these contacts - must be near zero after relay close).

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close