Welding Shop in Automobile Industry

It might be as simple as adding more capacitors to the PF correction bank for the welding shop. (But don't overdo it and get into leading PF.)

Thanks for the prompt reply. The problem being face by the industry is that the installed capacitors are frequently damage due to high torque in the welding shop resulting in unbalancing of the current which keeps on continuously varying between 50 to 200 amps. The industry keeps on replacing the damage capacitors which is not the solution to improve the power factor.

I don't know anything about this sort of thing so please ignore me if I'm being stupid, but, surely the load from a welding shop is essentially resistive: so you should remove all the power factor correction. Advancing the current in front of the voltage will reduce the power factor just as much as delaying it.

Hi Randall,

Most welding plant involves heavy duty transformers 3Ph and 1PH so heavy inductive loading would drag down the PF.

In large machines it is worthwhile fitting individual PF correction as the loading is intermittent but heavy it can give a single power factor correction unit a hard time. PF correction at the machines is the best way.

Hi,

Addtion of the capacitor banks depends on the load and load pf..

if you specified above i ill tell you the rating of the capacitor banks required to raise the pf..

Sathish

Mr. Zia,

This is a well known problem with few solutions. You are correct that capacitor banks do not live long in this impulsive environment.

We have a semi-standard product to solve this problem. Contact me directly by clicking on my name and send me a mail with the sizing details, and penalty value. We will have to evaluate if we can solve this problem for your customer.

Regards.

Dear Scholar

Thanks for your comments tride my best to sent email through your instruction cant find link to sent you mail hence I am enclosing the required data. will be great ful if can kindly email your email address. my email address is salahuddinzia@gmil.com.

Total Electric Bill amount Pak Rs. : 9662600/=

Total Electric Bill Amount kWh : 990467/=

P.F. Penalty amount Pak Rs. : 46866/=

Total Number of Spot Weldding Trafo : 300 No. Some of the are of 150 kVA, 125 kVA and 100 kVA.

Regards

Salahuddin Zia.

Sir,

I replied to your email.

To send an email on this site, simply click on the commenter's name to see their profile page, then use the email link in the top right corner of the profile page. This is safer than giving the world your private email address.

You mentioned a penalty of Pak RS = 46866 or approximately $5600. I assume that it is per month. Is this correct? This means an solution Bidget of $100K-140K to payback within 1-2 years. This is feasible with fast capacitor switches and controllers using some of the hardware you have in place (capacitor banks and cabinets). If you are an electrical contractor, you could produce the electrical cabinet and do the installation using our product and specifications.

I will contact you directly with details but here is a general presentation of our solution.

The PF problems in this type of application are often aggravated by the slow PF controller, the use of contactors, and untuned capacitor banks.

1- We produce Fast Capacitor Switches that activate the tuned capacitor banks at the exact time needed to minimise (eliminate) turn on and off transients.

2- The Fast Capacitor Switches are controlled by a Fast Controller that calculates the required power factor correction within half a line cycle and brings in or out the banks within the next half cycle. This keeps the line voltage better regulated and avoids PF penalties when properly sized and maintained.

3-The usual slow controllers that you are probably using with electro-mechanical contactors takes at least 30 seconds and even many minutes to react because you don't want those contactors to switch to often anyway as they stress the capacitors every time. Contactor switching produces current spikes many times the rated bank current stressing the capacitors which eventually fail. In a rapidly changing KVARs demand environment such as yours, the slow response usually means that the penalty limits are triggered. Also the power factor oscillates between lagging and leading which invites potentially destructive resonance problems.

4- Harmonics are often an issue in these plants. The capacitor banks need to be tuned properly to avoid resonances.

I hope that this description will be useful and explain why standard capacitor banks and controllers don't perform very well in this type of extreme applications.

Regards,

futhre to my reply would like to point out that there are 400 spot weldding Trafo instead of 300 spot weldding Trafo, 200 Trafo 100 kva and 100 trafo 150 kva. main source is from 1000kVA setpdown trafo 11000V/400V. Capacitor Bank rating is 18 X 50 kVAr.

Regards

Salahuddin Zia

¿¿ 200x150kva + 100x150kva = 45,000 kva. I realize these won't be fully loaded all the time, but how does a 1000kva transformer supply all this??

I don't know what the largest size of common PF correction capacitors is, but one way to design a bank for this would be one each of 100, 200, 400, 800, 1600, 3200, 6400, and maybe 12,800 kvar capacitors, switching them in and out by binary logic according to plant PF. I don't readily visualize 300 or 400 individual capacitors for each welding transformer.

Just out of curiosity, is this plant Tata Motors by any chance?

OT, I would like to advocate the abbreviation XF (rather than xfmr or trafo) for transformer. I hope this catches on.

Tornado,

The scheme you presented might sort of work for a while but the impulsive nature of spot welding quickly destroy capacitor banks. I have cleaned some of those "simple solutions" in the past years. I give you an GA because you are trying hard to help them with the simplest solution possible. They might try standard capacitor banks but eventually, we are likely to get called in.

With their large number of welders, some "statistical filtering" would help but dramatic load changes when all the welding stops every minute or two to let the chain advance produces a wild variation of the KVARs needed. This translates into early capacitor banks and contactor failure. Only the correct type of "intelligent" semiconductor switches and capacitor banks can switch rapidly and minimize the resulting transients.

This variable load often has a large effect on the line voltage and produces various annoyance trips and even failures in nearby equipment.

The choices are expensive harmonics filters (not all of them created equal...) or fast capacitor switches and controllers that handle sub-cycle switching. Then the systems have to be sub-divided in functional sections with sizes depends on the plant layout.