Electrical - Fluctuation in the Input A.C. Voltage for V.F.D Application

Have you tried contacting the VFD manufacturer? That should have been your first recourse.

With the information you've supplied I doubt you'll get an answer other than in very general terms.

Dear Mr. Tony S,

Thank you for the reply and the suggestion.

The Manufacturer/Authorised Dealer were contacted and Manufacturers Engineer stayed in the Factory for 5 days and observed the working of VFD day in and day out , but no conlusion made by him. We were told in a week's time we will be informed the changes to be made.

Now it is more than 2 months but NO RESPONSE FROM THE MANUFACTURER and the AUTHORISED DEALER. The Manufacturer is multi national reputed company and I got the e-mail Id. of the Design Head at their Head Quarters, referred the problem but NO RESPONSE.

The problem resulted in a loss of Millions of Dollars.

Hence I decided to seek the advice of MEMBERS of CR4 FORUM and take their help. I should know the STANDARDS for the PERMISSIBLE LIMITS for 5th, 7th, 11th and 13th HORMONICS effect and compare with actuals.

My another doubt is if the AC Input supply voltage and frequency comes down what will be the impact on the Converter whih gives DC OUT-PUT for the Inverter to give Variable Frequency.

Thanks,

DHAYANANDHAN.S

What does it say in the VSD manufacturer's specifications?

Dear Mr. PWSlack,

Thank you for your reply. The VFD is of 600 kW, 6 pulse 690 Volts, 1000 R.P.M, continuous duty and presently the load is 330 to 340 kW only.

Thanks,

DHAYANANDHAN.S, INDIA.

For V.F.D Application what is the permissible Limits for Variation in the Input Supply Voltage and Frequency.?

These vary between VFDs (size, voltage, single or three phase, manufacturer, etc) but the ranges are generally indicated in the VFD manual, VFD application data sheets, etc.

What are the Standards for the PERMISSIBLE HEAT GENERATION in the VFD interms of Percentage of the Capacity.

I think what you are asking is how much heat will a particular VFD dissipate. If so this information is given in the VFD manual as efficiency (or similar). The lower the efficiency the higher the percentage of input power is lost within the drive as heat.

VFD datasheets and application notes relating to the specific VFD also indicate heat dissipation and design guidelines when mounting the drive within a sealed or ventilated enclosure (if you are trying to size an enclosure for a VFD), or where VFD de-rating is necessary due to high ambient temperature/humidity, etc.

In my plant the Frequency dipped to 49.21 cycles/second due to turbine governor problem.

A 0.8 cycle difference should be well within the operating tolerance of a VFD and should have no appreciable effect on its operation.

The VFD was always running very hot.

There are so many things that could cause this such as poor input or output power connections (causing overheating), incorrectly sized VFD for the load and application, incorrectly sized or ventilated VFD enclosure/cabinet, etc.

Check your VFD has been correctly chosen and sized for your application otherwise premature VFD failure will almost certainly occur if unexpected overheating is a symptom.

Dear Mr. jack of all trades,

Thank you for your reply.

Certainly you are right and I will refer the Manual / data sheet and post it here.

Yes, I refer or mean generation of heat as the quantum of heat to be DISSIPATED. The installation is VENTILATED TYPE with idividual Fan with inter lock and installed in A.C. ROOM, the Temp is 22 Deg.C. to 24 Deg.C. The Commissioning Engineer told the Ambient temp. should NOT EXCEED 35 Deg.C Hence the Ambient Temp. is well with in limits.

As regards the Capacity and Selection of Capacity, We need 450 kW but we selected 600 kW considering shock load etc. We have 3 Nos of 600 kW Drives all identical and problem of over-heating is in all the 3 drives.

Thanks,

DHAYANANDHAN.S, INDIA

You sound like you DEFIANTLY need the manual the VFD came with. I've worked with these for many years. I would suggest you address the heat issue before you worry about the incoming cycling issue. The VFD should be cycle and voltage protected already. It goes without saying the cleaner the incoming power the better. RE: Heat. You didn't provide much information on "where" the VFD is located. If you look inside the enclosure you'll see large heat sinks there to dissipate heat. That's not an accident. These generate a lot of heat and require very good air circulation. Without a picture or diagram it's impossible to say. Generally speaking you want the coolest location possible and still be within proximity of the driven motor as called for by the manufacturer. Keep that thing cool and it will keep working for you for a long time!

Dear Mr. Fredski,

Thank you for your reply.

I think I have covered your point in the reply above - in the reply to Mr. jack of all trade. Any suggestion I welcome it.

Thanks,

DHAYANANDHAN.S., INDIA.

Re: Frequency. A VFD converts the incoming AC to DC, the front end is typically a simple 6 diode bridge rectifier. Incoming frequency has NO EFFECT on the operation of the VFD, other than extremely low frequencies may increase the DC bus ripple if the capacitance cannot handle it. But may VFDs will have a tolerance well in excess of +-50% of stated frequency. I've seen them work at frequencies as low as 25Hz (there are still a few 25Hz systems in operation). Simply over sizing the VFD with respect to the load will provide the added capacitance necessary.

Re: Voltage Tolerance. Voltage tolerance is another issue. Basically a VFD cannot create voltage. So if your line voltage drops, the maximum voltage that the VFD can put out drops proportionately. So if you have 90% of line voltage, the VFD can only put out 90% of line voltage (well, maybe 89.999%) at full speed. So if that happens, and you do not have anything in the drive programmed to reduce maximum speed to match line voltage, then the output voltage goes low for full speed operation, the motor torque drops by the square of the voltage reduction, a full load will cause an increase in sip, which causes an increase in current draw, which may end up as the perceived over heating condition you describe. If on the other hand you are running at a reduced speed, the loss of input voltage may not even be noticed by the VFD. For example if ye VFD is running at 50% speed, the output voltage is also at 50%. If the input voltage drops by 35%, the VFD couldn't care less, it still has enough to supply what the motor needs at that point.

But as to the VFD operating, most will still function with up to a 30% reduction in line voltage, some are as much as 50%. A lot of that depends on where they tap off for control power. If the control power for the VFD electronics is taken from the DC bus, then the tolerance is usually greater (i.e. approaching 50%). If it has a separate SMPS that is connected to the AC Line, then the tolerance is usually less (i.e. 30%).

Re: Heat output: If a VFD has an efficiency of less than 95% it isn't going to survive in the market place for long. Most are around 97%, a few are 98%. If you assume 95% for cooling purposes you are safe.

You could save a lot of guess work by just telling us what VFD you have by the way.

Dear Mr. JRaef,

Thank you for your reply.

I have picked up some message from your reply. You may see above that the Installed capacity is 600 kW and present load is 330 to 340 kW. Next week I will try to put more load and post the data to you in this forum

Thanks,

DHAYANANDHAN.S, INDIA

I thinkit is no such problem for VFDs as these devices work on DC Link {After rectification & a well defined Capacitor Tray [bank] }.

If input voltage drops beyond limits is another thing.

So what exactly is the problem you are looking to solve, the perceived "running very hot" of the VFD? How how was hot? 600kW is a very large VFD. Even at 95% eff, the heat rejection will be 30kW. Large VFDs like that often also have some sort of harmonic mitigation scheme, and that will also have losses in it. You could easily be looking at 40kW of heat to move out of the enclosure. That will require a LOT of cool air flow to keep from creating a thermal runaway. Rough guess (very rough) is in excess of 8000CFM (4 m³/s) of 20°C air flow, more if the ambient is higher.

Dear Mr.JRaef,

THANK YOU FOR IMMEDIATE RESPONSE.

My aim is to compare with THE APPLICABLE Standards of HEAT DISSIPATION and if the present Equipment is generating more heat than the Standards and to be dissipated - the First Attempt is to take steps to reduce the additional heat generated/to be dissipated. I need some base to take up the matter with the Supplier and counter his Argument.

After prolonged and continuous attempt I came to know from a different source and got the STANDARD APPLICABLE is EMC - STANDARDS EN 55011 or EN 61000-6-3/4.

I do not have the details of the above mentioned standards and go in to the details after I get it.

In the mean time our CR4 MEMBERS can throw some light on this standard.

As the loss sufferred is in Millions of Dollars we are attempting to minimize or totally solve the problem.

Thanks,

DHAYANANDHAN.S, INDIA

Sir,

I understand your concern and going through the history in forum discussion, I get impression that you have been using above mentioned VFD for quite long time.The defect was discovered only after you noticed voltage /frequency dip.As another member already explained its not so much affected by variations and I also rule out that possibilitiy. Very important thing ignored in the sequense of events was operating conditions. I suppose your system experienced near burn out conditions during that duration. Any infrared temperature measurement will determine weak points, I am sure these are diode, SCRs and there mounting poor joints. Its not necessary to emphasise here that copper once over heated looses original conductvity and properties selected by designer. Higher the operating temperature, higher resistance and more heat, its cumulative cycle. Hope you this will eleminate simlar operational problems for remaining units as well. Since its 600kw unit and 3% power dissipation is not simple task, as the fellow member already has described.Good luck.

Dear Mr. Mubarak,

Thank you for your response.

I would like to state that the installation is BRAND NEWLY INSTALLED and tested and started on 21st Nov. 11. We had hell of time.

Certainly I will try to measure INFRARED Temp. and how to decide weak point on this basis in the sense - what are the guiding factors to decide weak, normal and O.K.

Thanks.

DHAYANANDHAN.S.,