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Help, Big Problem With Vsd Installed


altis

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Hi, currently facing a major problem with my ACMV system mainly from VSD failure.Heres an outline of wat happened :-

 

My ACMV system consist of 2 exhaust fan, 1 running and 1 standby, 2 ahu supplying to a lab. 100% exhaust system. Exhaust is controlled by static pressure and ahu is in turn controlled by exhaust by airflow to maintain negativity in the lab.

There are two labs with exactly the same system, but lab 1 has higher capacity, thus higher airflow required.

 

fan size : 30 HP

VSD : 30HP, 22Kw

 

During T&C, found that lab 1's exhaust fan, is under-capacity. after checks with fan supplier decided to upsize pulley, then tested at 50 hz, vsd overloads. But system deemed able to achieve at 47hz, thus limit vsd upper limits to 47hz. System in operations for about two months when suddenly, VSD suddenly failed(totally no display everything else in the control panel intact), the standby unit kicked in and soon after the second unit failed too(same syptoms).

Thus, VSD was changed immediately due to operations need, one week later, the new VSD failed.

 

Question

 

1. Has anyone met with a total failure of the vsd before, and what could be the cause?

2. With the frequency limited, it should not overload even if it did, the vsd should be protected.Am I right?

3. The other lab has exactly the same system, only difference is the pulley size(smaller) for the fan motors

would this be a critical factor? The fan supplier recommends that even with the pulley changed, its still within the current ratings.

4. The VSD is controlled by start/stop(24vAC), frequency (0-10v), would the controls have any contributing factors to the VSD failure?

5. Checked all the power input and panels, everything seems to be in place and functioning. What else could be the critical factor that I might have overlooked?

 

Currently, waiting for the VSD supplier to trouble shoot the failed unit, but I am trying to find out if any external source is the culprit, any help is appreciated from anyone who might have experience this or know the probable cause. Thanks and regards!

 

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Hello Altis

 

Provided that the rated current of the VSD is not exceeded at 47 Hz, there should not be a problem.

To loose everything completely, suggests that something in the DC bus circuit has failed. Modern VSDs use a switchmode supply of the DC bus to power the drive.

If a soft charge resistor failed open circuit, this could cause the problem you describe. Alternaively, if there is a fuse failure on the DC bus, it could also cause the problem.

 

Some drives are not designed to be switched ON and OFF on the input side. If you fit a line contactor and operate this each time the VSD is run, this can cause the soft charge resistor to fail. - just a thought.

 

We really need to know what has failed to comment any further.

 

Best regards,

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Hello Altis

 

Provided that the rated current of the VSD is not exceeded at 47 Hz, there should not be a problem.

To loose everything completely, suggests that something in the DC bus circuit has failed. Modern VSDs use a switchmode supply of the DC bus to power the drive.

If a soft charge resistor failed open circuit, this could cause the problem you describe. Alternaively, if there is a fuse failure on the DC bus, it could also cause the problem.

 

Some drives are not designed to be switched ON and OFF on the input side. If you fit a line contactor and operate this each time the VSD is run, this can cause the soft charge resistor to fail. - just a thought.

 

We really need to know what has failed to comment any further.

 

Best regards,

 

Hi Marke

 

Thanks for the prompt reply. I've ckecked during operations that the vsd at 47hz has not exceeded the rated current. Iam also suspecting that its the drive that is giving the problem. but to lose three drives in a row prompt me to think that it could be the system instead of the drive. because the drive cannot be powered on, thats no way to find out what has failed, i'm still awaiting the report from the supplier from france who has got the faulty drive now to find out.

mainly I need to know if based on the system, is the anything external source that could trigger the failure?

 

Thanks and regards

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altis,

using a simple ohmmeter you could check the damaged unit to see where is located the damage.

Input bridge? DC bus? Output IGBTs ?

 

Without this info, a reply is very hard!

 

Regards

Mario

Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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When I have had symptoms such as those, it has always been one of the things Marke has mentioned; a DC bus fuse (although they are getting rare now), a charging resistor failure, or a power supply failure. Given the 3 identical failures, I'd think it was load or installation related, possibly something such as windmilling that is overcharging your DC bus and opening a component, or some other system that is creating severe line noise or spikes. Yaskawa drives used to have a DC bus fuse and excessive regeneration would cause the precharge resistor to fail, which would cause the bus fuse to blow on energization. Since the control power supply for the drive electronics was tapped off of the DC bus, when that fuse blew the drive and display died. PDL drives (now part of Schneider) had a separate switch mode power supply for control power that took it's line power from the AC source instead of the DC bus. Grounding problems and/or line noise would often take out that power supply, which of course left you with the same conditions, no electronics or display.
"He's not dead, he's just pinin' for the fjords!"
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What is the altitude and ambient temperature of the VSD location? Does the VSD have an feature that automatically attempts to restart if the unit shuts down? If so, how many attempts are allowed? Is there a feature that is supposed to allow the VSD to start (or restart) a coasting motor?
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Hi Altis, it sounds like an interesting problem. You did not say whether the input fuses were blowing at the time of the fault. It certainly sounds lke an input side issue as most reputable modern drives can protect themselves against output side faults. I am wondering how clean your power supply is on that paticular system.Is there anything that is different on that supply compared to the other lab? It may pay to check the low voltage start/stop and 0-10V input circuit as well as the control card may be getting zapped by stray voltage from some where. It seems too much of a coincedence that three drives would fail in the same application without some outside influence.

 

Kens

An expert is one who knows more and more about less and less until he knows absolutely everything about nothing
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Hello Altis.

 

I also agree with Mr.Kens. It is a general rule that if two equipments fail at a same place one by one, it means that an external source causes the units to fail. This external source can be the bad input supply, problem with control circuit, out put cable or out put device.

I recommend you to check these output devices first before installing another drive.

 

I have faced many such problems during my service. Once, the components of one of the electronic control cards of a cutting machine started blow out. When I changed the ICs/opto-couplers of card, it again blow out after working 1 to 4 hours. At last fault found in the Encoder’s cable which insulation was damaged only from 5mm of size. Any time cable touched with machine’s body, the components blown out.

 

"Don't assume any thing, always check/ask and clear yourself".

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Hi All,

 

Thanks for the replies, the drives in question was replace within the hour it failed and the supplier took the unit back for analysis. Moreover being a technical idiot myself, i wouldn't be able to analysis the drive.

 

As for the ambient temp. it's about 34 deg. thereabout, as the drive is outdoor on the roof but within a panel.

The drive will not be able to restart if there is a fault as the control will switch to the standby fan once the master fan trips.

 

On the control panel, nothing has blown nor has any input fuse been blown. We have done our checks to make sure the supply for both lab don't differ, and it's stable. As for the control inputs, we've checked during operation on the new drive and everything seems normal.

 

The supplier has just informed me that, it could most likely be a batch issue as they have got a few other drives failing in the same manner from other sites but not able to conclude on the exact problem. I'm keeping my fingers crossed and hope schneider can come back with a report soon.

 

One more question, if the acceleration and decceleration time for the drive is set to 5 secs. with there be any issues? The lab needs quicker response from the drive as it needs to be maintain negative at all times.

The default setting is 30 secs. Thus at 5 secs the drivew would be constantly hunting, as doors are open or closing, fume cupboards opening/closing.

 

Thanks and regards

Daniel

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If you set the acceleration too fast, the motor will try to produce too much torque and draw too much current. With detailed information about the torque required to drive the fan and the fan't inertia, you could calculate what rate is ok, but it would probably be easier to watch the current with an ammeter as the fan operates. The VFD likely has protection features that will not let the drive accelerate at a rate that causes too much overload. If rapid acceleration causes a momentary overload that is within safe limits for occasional operation but the overload is repeated too frequently, the drive might shut down.

 

Something similar happens with deceleration. If the deceleration is too fast, the DC bus voltage could increase due to braking energy regenerated by the motor. The drive likely has protection features that will limit the actual deceleration rate to prevent the drive from shutting down due to overvoltage on the DC bus.

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Hi Daniel,

 

 

 

1) Drive size I always recommend drive one size bigger than the motor size.( for a 30 hp motor i always recommend a 40 hp drive). The reason i do that is, if you read the fine print of VSD manual you will see that there is a 2.2% deration (in amps) of drive for every 1 deg C rise of ambient temp inside the panel above 40 deg C.

 

2) 5 secs accn time is too low for a Fan using 30 hp motor. I would suggest it to be minimum of 15 secs. Keep decn time also at 15 secs. If kept aacn time is kept too low the drive will draw larger current at startup and for a high inertia load it is not advisable.

 

3) The most ignored fact about the VSd installation is the temp of the panel in which it is mounted. Mind you, panel (internal) temperature is much higher than ambient ( room) temperature.

 

As you have already confirmed the power quality is good, just check the OCR, and also set the switching frequency to minimum.

 

 

 

Chaterpilar

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