VFD crazy high amps

[RANGER1]

Has anyone ever had unexplained very high amps on VFD drives.
In this case it's a Danfoss.
Also in these cases very experienced electrical engineers & Danfoss reps witness it, with no ideas
on what can cause it.
(Usually mechanical problems are mentioned of course, but both new compressors & turn over freely by hand).

On several occasions at different times the 2 seperate motor/s have been started & compressor fully unloaded, 25Hz.
The amps are so high the drive wants motors to back off even more but cannot due to minimum setting of 25 Hz.
We are talking about 465 & 510kw motors.
The compressors are definitely unloaded, not that it should matter, Voltage to all phases checks OK.
Off the same transformer 1 is OK the other out of control.
Turn the troubled machine off, come back tomorrow, all normal no worries.

Plant does not have power factor correction, but we are told not the problem in this case with the Danfoss drives.

[Glenn Moore]

Hi Ranger
Which models of VFDs and what model and type of compressors are they . Are the compressors on the same fridge circuit using a common condenser .what alarms on the drives are showing have you run an MCT10 trend log to monitor the motor start up conditions. What refrigerant is the plant using

[RANGER1]

Glenn,
Ammonia compressors, common plant,
Danfoss guy hooked up laptop to monitor start up.
Drive model not sure but was for HVAC .
Will try to find out models.

[Grizzly]

Hi Ranger.
I have not seen your issue with Danfoss drive Yet!
But I have witnessed High Amps and control system unloads where CT coils are used to provide a signal to tell the VSD what actual amps are being drawn.

Meaning if the CT coil ratio is set incorrectly or even if the CT coil is fitted on the wrong side of the Line contactor. The controller sees the wrong Amps and can read ridiculous values.
This is becoming quite a common issue as shall we say "Less Experienced" but cheaper Electrical companies are being used.
The Old Boy tricks are not being passed on. (Thank God for the forum )
Sadly using these, you cannot just check with a clamp meter as the actual reading bears no resemblance to what you are actually reading on the clamp meter.

My guess is however the Controller reaches it's decision of how many Amps are actually being Drawn is corrupted.
Seeing as you mention 2 comps of a new install, I would check for a common Input / Wiring issue.
On others the run amps are a internal software calculation. Maybe a setting incorrect, however you have had a Tech out so not so likely!

Just thought How new is the install? I was assuming the Motors where malfunctioning, however there are plenty of outside issues that would cause an actual High amperage.
Is that what you are actually asking?
Grizzly


We use Danfoss as a preference on Industrial Ammonia Plants and we do have the occasional issue like anyone else. Their back up and Tech assistance so far has been brilliant.

[RANGER1]

The trouble is it's almost a one off problem.
The plant is old & a few new compressors installed with VSD operation.
THe problem occurred on start up of both of them, but start up was a month apart.
BEcause problem is severe, we tried what we could, no good so turned off.
NExt day turn on & running perfectly as you would expect.
3 months later on first machine it reappears one day.
Next day problem gone.
We have used a lot of these drives as well as others, but this appears to be a new one on everyone.
Because here today & gone tomorrow makes it even harder to explain.
We will be working towards remote monitoring of compressors & drives, as client in remote location, more than 4-5 hours away.
HAve heard of things like harmonics, but know nothing about them & what they can cause.

[Grizzly]

Hi Ranger.
It's an ABOL Fault.
In other words, you need A BIT OF LUCK!
Well a lot of luck really, we have all been there.
Lets hope you can pin this one down and please let us in on the secret if you find it.
Nice One!
Grizzzly

[mbc]

As you know we have current control in screw compressor especial in high horse power .
CT we use for them should be very good ( I forgot the class name at the moment )and should not be lose or near other cable and should have shield cable .
in CT direction and connection of K_L and also position of CT and fixing of that in cable is important.
sound of your case for me is lose wiring or connection .

[RANGER1]

As you know we have current control in screw compressor especial in high horse power .
CT we use for them should be very good ( I forgot the class name at the moment )and should not be lose or near other cable and should have shield cable .
in CT direction and connection of K_L and also position of CT and fixing of that in cable is important.
sound of your case for me is lose wiring or connection .

Pretty sure its built into the drive so thanks anyway.

[Magoo]

Hi Ranger1
had similar problems with a screw compressor at low hertz.
Resolved by changing set up settings for motor, when it asks for motor voltage eg 400 vac, change that setting to 480 vac. At lower hertz will increase voltage and torque to motor and lower the amps drawn.
magoo

[Grizzly]

Hi Ranger1
had similar problems with a screw compressor at low hertz.
Resolved by changing set up settings for motor, when it asks for motor voltage eg 400 vac, change that setting to 480 vac. At lower hertz will increase voltage and torque to motor and lower the amps drawn.
magoo

Clever answer Magoo.
You may be onto something?
CT coil ratio set-ups if incorrect can give similar.
What has thrown me so far is the randomness of the problem.
Grizzly

[kiwi]

Hi Ranger1
had similar problems with a screw compressor at low hertz.
Resolved by changing set up settings for motor, when it asks for motor voltage eg 400 vac, change that setting to 480 vac. At lower hertz will increase voltage and torque to motor and lower the amps drawn.
magoo
Problem there though is you will run out of torque at increased speed, especially if you're overspeeding. Our local voltage at 400V will not maintain Volts to Hz ratio at above nominal line frequency (runs out of volts) and will give problems in the upper frequencies.

[Magoo]

Hello Kiwi.
that is correct, but I was applying system to 40Hz and 50Hz options not over speeding. The screw remains unloaded 'till up to select speed. Ramp time 5 seconds,using a danfoss HVAC 5000 drive. System works a treat has done for 5 +years .

[Tycho]

I've seen this problem many times, especially on systems where the motor has been sized "perfectly", or with no power to spare compared to the compressor size.

the issue would however be constant.

What is the application? freezing or cooling?

I've had a motor this size stall and cause the VFD to reboot (danfoss) when running on low speed.

We have stopped using the Amps as a safety because they are not "real" amps on the VFD, instead, after consulting with danfoss, we are now using the motor KW as the limiter since this will be a real value no matter what the voltage or amps are changed to to give it the correct HZ.

Another solution is to start with the suction valve closed, but that's not an option when the system is running in auto.

I'm in sydney now, want me to pop over and have a look at it? ;)

[RANGER1]

I've seen this problem many times, especially on systems where the motor has been sized "perfectly", or with no power to spare compared to the compressor size.

the issue would however be constant.

What is the application? freezing or cooling?

I've had a motor this size stall and cause the VFD to reboot (danfoss) when running on low speed.

We have stopped using the Amps as a safety because they are not "real" amps on the VFD, instead, after consulting with danfoss, we are now using the motor KW as the limiter since this will be a real value no matter what the voltage or amps are changed to to give it the correct HZ.

Another solution is to start with the suction valve closed, but that's not an option when the system is running in auto.

I'm in sydney now, want me to pop over and have a look at it? ;)

It's a 8 hour drive from Sydney, what time can I meet you there:)
Plant is a two stage ammonia plant, both compressors on second stage.
Has not happened again as far as I am aware since commissioning.

[RANGER1]

Tyco thanks for the information on Danfoss set up.

[RANGER1]

Problem solved, Danfoss "guru" used wrong setting in drive, set it for pump, not compressor.
Needs to be constant torque, otherwise it tries to optimise causing problems.
Only found out very recently.
On other drives we installed all "guru" settings copied making problems with all new drives.

[Tycho]

so param 1-03 was set to Auto Energy Optim. VT not to compressor torque?
or did they change any of the other parameters that has to do with pump characteristics?

[Magoo]

Hi Ranger 1
thanks for feed back and resolve.
Can imagine the " Guru " status has been reviewed down to **** head with Danfoss tech., any recourse for problems generated.
magoo

[Grizzly]

Problem solved, Danfoss "guru" used wrong setting in drive, set it for pump, not compressor.
Needs to be constant torque, otherwise it tries to optimise causing problems.
Only found out very recently.
On other drives we installed all "guru" settings copied making problems with all new drives.

I agree with Magoo!
A very useful update and one I am sure I may run into at some stage.
Thanks again.
Grizzly

[RANGER1]

He had laptops, start up trends, all types
of fancy gear.
Interestingly I had been warned about this from drives years ago.
It didn't cross my mind with all the gurus there.
Everything easy when you find answer later on of course.
No one perfect of course.
New drive settings I don't really know, but think it's in menu, pump, compressor, that's it from what I am told.
Amazing didnt appear again till next new compressor installed.

[weagle]

Great thread

[Tycho]

He had laptops, start up trends, all types
of fancy gear.
Interestingly I had been warned about this from drives years ago.
It didn't cross my mind with all the gurus there.
Everything easy when you find answer later on of course.
No one perfect of course.
New drive settings I don't really know, but think it's in menu, pump, compressor, that's it from what I am told.
Amazing didnt appear again till next new compressor installed.

You should have just set it up with the settings I sent you and become the hero :)

During all my dealings with danfoss, I feel that we as users know more about the drive and application than their service technicians do.

I've had a few calls to danfoss's VFD service line where I feel like I know more about their product than the guy trying to help me. their service manual is quite extensive, so after the first few times of not getting any help, I have used that to find the symptom and cause of my problem.

[engrp]

thanks for the thread on danfoss vfd's. I am planning to use also a danfoss vfd. I plant to install it on a 110 kw motor , belt driven recip compressor mycom nwbh8. were your discussions on the vfd only for screw compressors ? has this vfd been used on recip compressors ? thank you for any inputs/advise.

[RANGER1]

thanks for the thread on danfoss vfd's. I am planning to use also a danfoss vfd. I plant to install it on a 110 kw motor , belt driven recip compressor mycom nwbh8. were your discussions on the vfd only for screw compressors ? has this vfd been used on recip compressors ? thank you for any inputs/advise.

Vfd on recips no problem.
Mycom 8wb 800 to 1200 rpm

[engrp]

ok ranger1, thank you for the input. will consider use of vfd, if we can afford the cost.

[RANGER1]

ok ranger1, thank you for the input. will consider use of vfd, if we can afford the cost.
Cost depends on why you want to do it.
We had one client thinking they would ramp it up/down, but set to balance plant, so compressor not to big or small in different conditions throughout the year.
Recip efficient unloaded anyway.
Mid you want very smooth suction pressure it is ideal.