Power Supply 60v 250A Repair

[edpalmer42]

Onics web site appears to be https://powerresistor.in/ , but there's more info at http://resistoronline.com/ .  There's no 'AH' resistors, but there are 'AHR' resistors http://resistoronline.com/pdf/new%20design%20AHR.pdf .  This states that overload spec is 5X rating for 5 sec.  The Vishay datasheet for the resistors you ordered says the same thing.  So even with those surge ratings, those resistors are too small for the job unless Lambda talked to the factory and got their signoff on the design.  But perhaps the replacement Chinese resistor wasn't rated for a 5X overload.

This topic has been debated before.  If you insist upon specific datasheet approval for something like a 100X (power) surge for 1ms, you'll have a much harder time finding a part.  But typical wirewound power resistors will shrug that off, and if you need proof you can characterize it yourself or consult the manufacturer.  5X (power) for 5 seconds would be similar to 25X (power) for 1 second and so on, but of course you might want a bit more margin as you go down that route.  And of course 25,000,000,000X (power) for one nanosecond might be problematic even if you weren't over the voltage limits, so there will be limits to the principle.  5X 25W x 5 seconds is 625J.  Charging that capacitor to 350V will dissipate about 280J in the resistor.  Some WW resistors could experience limited life or resistance change as a result of that, but I wouldn't expect them to blow up on the spot.

For you and me in our workshop - absolutely!     That's exactly what I did for the soft-start circuit I used.  For a company like TDK-Lambda with military contracts and liability insurance - not a chance!    If anyone suggested running beyond specifications without a signoff from the manufacturer, the corporate lawyers and accountants would beat them to death on the spot!  You can't characterize it yourself because the manufacturer could change their processes and totally negate your testing.  Since they don't know what you've done, they won't know that their change will sabotage your design.

But I think we can agree that these resistors were being used in a manner that was not specified in the datasheet.  It worked well enough that it lasted for 18 years so it wasn't a horrible decision.  At this point, it's an open question whether the replacement resistor failed because of the current surge or some other fault that is yet to be discovered.

[bdunham7]

http://htr-india.com/design-notes/pulse-surge-capability-resistors/

Many mfrs will have published a supplement like this so that you don't have to bother them.  If this company was the mfr of a 35R 25W WW power resistor, I think this document would show that it was sufficient in this case.

[edpalmer42]

http://htr-india.com/design-notes/pulse-surge-capability-resistors/

Many mfrs will have published a supplement like this so that you don't have to bother them.  If this company was the mfr of a 35R 25W WW power resistor, I think this document would show that it was sufficient in this case.

 

Wouldn't it be nice if every manufacturer did this?

[Jay_Diddy_B]

Hi,
If the soft-start is working correctly and there is no load on the capacitor during starting the Energy dissipated in the soft-start resistor is always:

E = 1/2 CV²

In this design the energy is

E = 1/2 x 4700u x 340 x 340 = 271 Joules

Jay_Diddy_B

[CrazyTiger]

Hi All,

Here are some pictures I have taken of the Inverter, Rectifier, Resistor section. I will take more picture later today. I am working on this power supply in my other house, the house is empty, and comes with 3 phase, but i am traveling daily to visit the PSU.



all the markings, is not made by me, so I believe someone tried repairing before



I have removed the 33ohm resistor here, you can see the other resistor at the edge







Good news is I managed to find the entire circuit diagram yesterday night, will scan and post it later. Regarding Inverter 3 and 4, it was isolated prior to turning on after resistor change.

The inverter does have a fuse, which is tested good. I have also fed 60 V directly to each inverter to see if there is any short and it has none. 60V is the maximum my PSU can deliver.

[Jay_Diddy_B]

Hi,
Can you check the capacitor in a circuit like this:



The Incandescent lamp will limit the current. The capacitor may be bad.

The capacitor should charge up to around 320V and there should be be no light from the lamp if the capacitor is good.

You can check them first with your 60V power supply.

Regards,

Jay_Diddy_B

[CrazyTiger]

Hi,
I will do the test as Jay_Diddy, suggested for the caps. I have also pulled out all the 4 inverters out for further checking. I will be testing the IGBT's off circuit to see if it actually switches, but nothing is shorted for sure. At least I can eliminate the inverters once and for all. Here are more pictures.

The power supply front look


The resistor in question, the red box is where the resistors belong, I have taken out the defective one.


The control board


Inverters:

[Jay_Diddy_B]

Hi,
 You should try and find a wire-wound resistor that is this style:



What did the original resistor look like?

It is described as having a Silicone coating. It is not the vitreous enamel coating. The vitreous enamel, glass, coating will crack if there is a high power surge, as there is in this application.
The aluminum resistors are not very good. They tend to blow out of the ends !!

It looks like there is a fuse on each of the inverter boards. I think the fuse would be blown if the IGBTs are bad. I think the soft-start burns without the inverters connected? This would mean it is not the inverters.



Jay_Diddy_B

[Doctorandus_P]

With the little experience I have, all wirewound resistors on a ceramic form are very capable of handling significant overloads during a short time.
Those cemented resistors in a rectangular ceramic block also.

I've had very bad experiences with the gold colored aluminium resistors which are made to fit on a heatsink. These tend to blow at the slightest overload. Even to a point I suspect that they fuse by design.

[CrazyTiger]

Hi,
 You should try and find a wire-wound resistor that is this style:



What did the original resistor look like?

It is described as having a Silicone coating. It is not the vitreous enamel coating. The vitreous enamel, glass, coating will crack if there is a high power surge, as there is in this application.
The aluminum resistors are not very good. They tend to blow out of the ends !!

It looks like there is a fuse on each of the inverter boards. I think the fuse would be blown if the IGBTs are bad. I think the soft-start burns without the inverters connected? This would mean it is not the inverters.



Jay_Diddy_B

The resistor you are suggesting  makes sense, I have had this doubt too wondering if it was replaced. Reason is I have basically opened the PSU to bare body, any high powered resistor inside the unit looks like the one you are suggesting except the two resistors for the soft-start circuit which came with Aluminum Casing. It might have been replaced before, but the heat shrink that came with the resistor feels and looks the same with the existing heat shrink on other cables and component, so we will never know unless someone have the same PSU. I am finding the type you suggested so far no matches.

[CrazyTiger]

This looks good, rated 30W, close enough, not sure about minimum order quantity

http://htr-india.com/wp-content/uploads/2021/01/HFOM.pdf

Sorry Link fixed

[SeanB]

Using a local link to the datasheet is kind of going to be hard to see though, perhaps better to link to the manufacturer website instead?

This looks good, rated 30W, close enough, not sure about minimum order quantity

file:///C:/Users/amith/Downloads/HFOM.pdf

http://htr-india.com/wp-content/uploads/2021/01/HFOM.pdf

And yes the 10x short term overload capacity will work, though probably better to use the 40W versions instead of the 30W, though it may be a bit hard to fit in the available space, which the 80W definitely will not do.

[bdunham7]

I've had very bad experiences with the gold colored aluminium resistors which are made to fit on a heatsink. These tend to blow at the slightest overload. Even to a point I suspect that they fuse by design.

With that design, I suppose they can put a smaller element in it and just let it run quite hot internally.  I would have thought that this would be the case with no-name eBay parts.  A look at a few datasheets for 'good' versions shows that some state 'high pulse capability' and so on without much specificity--these are probably as robust as I claimed WW resistors to be generally.  However, there are some name-brands that quite clearly give much lower pulse/overload limitations and  would be greatly in excess of them if used in the OPs PSU.  So I'll have to withdraw my statement of WW being universally robust when it comes to this style.

https://www.mouser.com/datasheet/2/414/TTRB_S_A0012496756_1-2565697.pdf

[strawberry]

https://eu.mouser.com/ProductDetail/Vishay-Dale/FSOT5509E30R00JE?qs=JsD2jqDDQCxJ79D6QFrdTQ%3D%3D
FSOT5509E30R00JE

[CrazyTiger]

Been searching internet for an inside picture of another PSU made by lambda, found a video, its a different model but architecture seems the same.



At 0:10 we can get a glimpse of the resistor, its definitely not a Aluminum housing, seems more like a cement type or something radial.

[Doctorandus_P]

Sometimes it works to put in a DC current of a few amps and then measure where the current goes by measuring voltage drops over tracks.

You can also connect a function generator and generate a sinewave current a a few amps and then pick it up. Maybe with a small inductor, or with a head from a cassete recorder.

[edpalmer42]

You can also get high power resistors in a case that looks like a TO-247 or TO-220 transistor.  That would be an example of my earlier comment where you don't actually have to dissipate the rated power.  A small heatsink or just a block of metal would be all that's required.  Actually, if you mounted it to almost anything, that would provide enough of a heatsink for this application.  This particular one is 39R @ 100W, but there are others at different values and power ratings.

https://www.mouser.ca/ProductDetail/TE-Connectivity-Holsworthy/MPT100C39RF?qs=sGAEpiMZZMtlubZbdhIBINNjBjh6UlIP%252B9ccrjePZE4%3D

I didn't see any info on surge ratings in the datasheet, but if you go with the 100W version it might not matter.

[bdunham7]

I didn't see any info on surge ratings in the datasheet, but if you go with the 100W version it might not matter.

The peak power might be as high as 3500W and TF resistors aren't the best for that.  I'm a bit surprised to see that there are more than a few WW lines that seem to have diminished pulse withstanding, presumably because they have used thinner, shorter wires and thus less mass.  They seem to have high joule ratings only for very low resistance values.  Here's an example of a line where the 10W version would barely work but the 25W aluminum-housing version would not.

https://www.ohmite.com/assets/docs/res_30.pdf

Metal oxide or ceramic composition resistors may actually be better for this application if they've cheaped out on WW models.  Here is a series where the 2 watt (!) version can withstand 275kilowatts for 1 ms.  Now that is a surge rating!

https://www.ohmite.com/assets/docs/res_oc.pdf

I'm starting to think that the OP might in fact just need some different resistors.

[edpalmer42]

I'm starting to think that the OP might in fact just need some different resistors.

I'm concerned that he said that the relay has never operated.  That almost guarantees that the resistor will fry - both the original and the replacement.  I'd suggest replacing both resistors rather than just replacing the blown one.

Tiger, if I were you, I'd be collecting incandescent (or halogen) bulbs and sockets to use as inline surge limiters.  That will allow you to power the unit up and continue troubleshooting.  With the bulbs in place, you can jumper out the contacts on relay K1 to bypass that part of the circuit.  If there are problems further down the line, the bulbs will glow bright.  If you have one available, you could also consider using a small 3-phase breaker (maybe 5 amps or less) in the power feed in addition to the bulbs.

Since you're dealing with 400V AC 3-phase, don't just throw something together.  Mount everything on a piece of plywood or similar, use good quality wire, etc.  100W bulbs might not be enough to allow the unit to start, so plan for bigger bulbs, if necessary.

[Jay_Diddy_B]

Hi,

The energy dissipated in the soft-start resistor is equal to the energy stored in the capacitor during starting.
It is also equal to the energy dissipated in the resistor if you discharge the capacitor.
Charging takes longer, because the line voltage has to be more positive than the capacitor voltage, so it less severe.
If you can discharge the capacitor without breaking the resistor, it is probably good enough.

Summary for CrazyTiger:

Theory 1

The resistor is weak and doesn't survive the inrush current.

Theory 2

The capacitor is bad and the dissipation in the resistor is high because the capacitor is faulty.

Jay_Diddy_B

[Ground_Loop]

Maybe I missed it somewhere above, but have you confirmed the relay contacts across that resistor are clean and functioning?

[CrazyTiger]

Maybe I missed it somewhere above, but have you confirmed the relay contacts across that resistor are clean and functioning?

The relay contacts are good was tested with 34401A to check for any significant resistance drop. The relay was also tested with a insulation tester to check for any contact leakage between NO and NC. Everything tested ok.

I will be getting few 100W and 50W bulbs and sockets today. I will be also borrowing a 3 phase variac, at least probing can be done in a safe manner with a variac.

[edpalmer42]

Maybe I missed it somewhere above, but have you confirmed the relay contacts across that resistor are clean and functioning?

The relay contacts are good was tested with 34401A to check for any significant resistance drop. The relay was also tested with a insulation tester to check for any contact leakage between NO and NC. Everything tested ok.

Did you check the coil to make sure it isn't open?  In a perfect world, put 24V on the coil and confirm that the relay operates and the contacts close.  There's a diode across the coil so mind the polarity.

I will be getting few 100W and 50W bulbs and sockets today. I will be also borrowing a 3 phase variac, at least probing can be done in a safe manner with a variac.

Careful!  A variac does **NOT** provide isolation from the line.  It only reduces the voltage.

For the bulbs, remember that you'll need two 220V bulbs in series for each of the three phases.  So a total of 6 bulbs & sockets.

By the way, you said that you were working in your other house that has 3-phase power.  Is it common for houses in Malaysia to have 400V 3-phase electrical service? 

[bdunham7]

For the bulbs, remember that you'll need two 220V bulbs in series for each of the three phases.  So a total of 6 bulbs & sockets.

Why can't he just put one bulb in place of the resistor?  It will be a bit over voltage, but unless there is a short it should just blink at the most.

[edpalmer42]

Hi,

The energy dissipated in the soft-start resistor is equal to the energy stored in the capacitor during starting.
It is also equal to the energy dissipated in the resistor if you discharge the capacitor.
Charging takes longer, because the line voltage has to be more positive than the capacitor voltage, so it less severe.
If you can discharge the capacitor without breaking the resistor, it is probably good enough.

Summary for CrazyTiger:

Theory 1

The resistor is weak and doesn't survive the inrush current.

Theory 2

The capacitor is bad and the dissipation in the resistor is high because the capacitor is faulty.

Jay_Diddy_B

But that doesn't include the effect of relay K1.  We don't know when K1 is supposed to operate, but as soon as it does, the resistor is shorted out so energy dissipation stops at that point.  If the capacitor voltage at that point happens to be 100V then that determines the energy dissipated in the resistor, not the peak charged voltage.

I just checked the schematic of my Xantrex XKW33-33 power supply (33V @ 33A).  It uses a similar soft start circuit.  The capacitor bank is only 1500 uf @ 400 V.  The soft-start resistor is 39R @ 5W.  I tested it and the relay operates about 1 sec. after powerup.  There's also a 0.5A fuse in series with the 39R resistor.  This fuse is also shorted out when the relay operates.  But that shows that in my unit, the surge is no more than 0.5A. 

I also see in the instruction manual for the ESS that inductor L1 limits the peak charging current.  Unfortunately, they don't specify the maximum surge.  Bottom line here is that we don't know what the surge current is or how long the resistor has to withstand it.  All we know is that it finally gave up and we're trying to figure out why.