3000VA UPS

 

 A few weeks ago (September 2018) an Uninterruptible Power Supply was dropped off at the Low Cost Repair Centre. Although I'd suggested that most problems could be fixed with a new set of batteries these hadn't arrived with the equipment. I checked over the UPS but didn't see much wrong. I did notice a discoloured choke inside the box but what can go wrong with a choke, so left it and returned the equipment., suggesting new batteries were probably needed.

It arrived back a few days later, again sans batteries, so I removed the darkened choke and fitted a stout copper link in its place. Whilst the main board was detached I decided to test all the electrolytic capacitors. To do this meant removing about a dozen small circuit boards. This was no easy task as all the boards were soldered into position. Hours later, and much to my surprise I'd given scores and scores of the electrolytics a clean bill of health. The designers had apparently done an excellent job.

Despite all my efforts I'd been unable to find any information about the equipment so I was still on a learning curve.

After reassembling everything I returned the thing for the second time and awaited results.
 

 I heard back and there was still a problem, so I spoke to the site engineer who told me it was still troublesome. I suggested buying a new one but then I discovered a massive jump in price from a 1.5KVA to a 3KVA so agreed to see the UPS for a third time, but insisted on getting the battery pack so I could run some proper tests.

A clue to the problem was that he'd reported that smoke had come from the UPS case after a short time of running on batteries.
 

 Above is the main circuit board which I removed from the chassis. Dotted around the board perimeter are the daughter boards, some of which carry extremely complex circuitry. The main board carries two major areas of high-power circuitry.

I've already removed a blackened choke and replaced this with copper wire (centre right) and below you can see where solder connections have degraded from heat. I've also removed a component adjacent to the bad choke which I'll come to later.
 

 Here's a better view. You can see that the missing part is a capacitor. When looking over the board I hadn't noticed this, instead vaguely thinking it was a large relay.
 

 Below is another circuit board which is fitted on a metal cover under the lid. That other, small board, was screwed to the lid and carries a battery condition display and some LEDs
 

 Because I'd now got the battery pack, I could do some proper tests. On the right (above) are all the UPS connections which are brought out to a simple choc block. Not having any documentation I was unsure of two of the connections. I noticed track visible through the board that appeared to connect the brown/blue wires (bottom right) to a relay so initially made the wrong assumption that these wires indicated the state of the equipment ie. closed meant ON and open meant OFF, however the wires are marked "Control Switch" so I checked to see if there was a voltage across them and found 60 volts DC. To prevent a mistake causing any damage I found a 270 ohm resistor and discovered only a small current between the connections,so shorted the wires with a 15 ohm resistor and found it was an on/off switch.

 

 Below is the battery pack. It uses eight 12 volt 7Ah batteries connected in series. I noted the fastons don't fit the battery terminals so the battery pack is not the original one fitted by the manufacturer. The can at the back is from another project dating from 1917...

 

 

Powering the unit from 240 volt mains produced a start-up routine which apparently is a battery test. Most types of UPS do this and the nature of the test will reveal any problem with the battery pack. After a short period an alarm sounded and clearly, figuring out the display which now showed a red LED, the batteries had been determined to be not up to scratch. I've found in the past, with other UPS equipments, that a battery can be obviously bad, looking bloated or split, or looking quite normal, but presumably not reaching the correct level of voltage with a specified current. Whatever the test, bad batteries seem to be reliably diagnosable but are those above really bad? The report of smoke made me think a fault was present that somehow resulted in a false bad battery warning..

Although a simple test isn't going to prove this one way or another it may give me a clue. I measured the voltage of each battery with the UPS off, then on. Each battery looked pretty well nominal with none showing more than a fraction of a volt lower or higher than average so I wondered how the UPS would operate if the mains was turned off. I connected a 60 watt lamp to the mains output and poked my 15 ohm resistor into the choc block. Much to my suprise the lamp immediately came on.

I tried a few times and each time I got 220 volts AC from the unit so I decided to leave it on and see what happened. At this point I'll mention the purpose of this UPS. It's to provide a mains voltage for a short period, if the local mains fails, in order to allow lift passengers to get out of a lift which would otherwise be stuck. Hence the large 3KVA output, which is necessary to provide a lot of power for a short period to get the lift to a floor and safety.

By 20 minutes I'd noticed that a slight hot smell had grown to a definite burning smell and my XYL suggested I find out where it was coming from. I already knew the answer so I turned off the UPS, lifted the lid and gingerly felt around for something hot. There are a couple of large heatsinks and to my suprise, both were cold.... but I could feel heat emanating from something. It was one of the two large chokes. In fact, it was incredibly hot so time to do some calculations.

First, a 60 watt bulb consumes 60 watts from the mains which works out, given 220 volt mains, 270mA of current. I hadn't yet noticed how the choke was connected, but clearly 270mA through a pair of 1mm wires in parallel shouldn't worry it. I'd already measured its resistance (not easy) and found it was roughly 35milliohms. Losses at 270mA should be around 2mWatts so the choke surely couldn't be in a mains feed delivering 60 watts. So what about the battery connections? Well, the batteries are connected in series to provide around 96 volts so equating the loss at 220 volts to 96 volts gives me 618mA. The loss increases by a whopping factor of 6.5 to 13mWatts. No way can the choke get to be that hot from 13mWatts.

A UPS takes in mains, rectifies and chops it at a figure usually between 20 and 30KHz then transforms this to generate a lower AC voltage. This is rectified to produce a supply for an inverter and a voltage for charging the batteries which are kept a trickle charged until required.

When mains fails, the UPS uses its batteries to produce a mains supply. Various relays perform the switchover from the public mains supply to that generated by the UPS. With luck, and good design of course, any mains powered equipment will see a loss of mains power for only a very short time.

Why on earth would a choke running such a low current get so hot then? Clearly the UPS is working, it's not as if it wasn't, and the output surely wouldn't heat up a choke as much as that observed. It took 20 minutes for the choke to cool sufficiently to be able to comfortably touch it and therein lies a clue.

Below, overheated solder connections to the choke.

 

 It's possible that, because the UPS can handle 3KVA, somewhere in the unit there's a fault which is drawing a huge current, but not enough to cause the equipment to stop functioning? The contradiction to this is the absence of heat anywhere else in the equipment. Let's say the choke is drawing enough current to make it get very hot... let's say the dissipation is 40 watts. The current necessary to produce 40 watts in 35mohm is about 34 amps. 20 watts would be produced by 24 amps, but surely those sort of currents would raise the temperature of a heatsink, yet both are stone cold.

The answer of course is nothing to do with DC current (see the capacitor's rating below). The two chokes are connected in series and intially one was burnt, then removing this caused to second one to burn. A DC current through two chokes in series would produce about the same heating effect in both, but it was only one that appeared to suffer. Adjacent to the chokes is a capacitor (see below) which from the size and shape I'd assumed was a relay. I checked it with my ESR meter and found it measured 1.44uF and zero ohms ESR. Removing it revealed it's previously hidden markings which showed it should have been 10uF.
 
   

  Above, the fried choke and the pristine-looking duff 1.44uF capacitor.

 

The two chokes and the capacitor represent a filter. At this point I hadn't traced exactly where these fit into the UPS circuitry but what's happening is a ripple is present which is being inadequately filtered by the duff capacitor. The ripple component of the current passing through the first choke is heating, not the copper coil, but the ferrite core of the choke. The core is getting hotter and hotter to the extent the copper coil is reaching a temperature sufficient to burn the enamel insulation. The second coil might also be suffering the same fate, but by the time the current leaves the first choke the ripple has been absorbed by the core and isn't bad enough to cause excessive heating. Removing the first choke and replacing this with a jumper wire allowed the ripple to reach the second choke and this was the one I'd found to be incredibly hot.

 

 Above, from the position of blue and brown wires which are mains in and mains out, the filter may be in the internally generated mains output and it's function would to eliminate roughness in the 50Hz output so a decent sine-wave results? I traced the circuit and sure enough the filter is in the mains output.

My guess is the missing 8uF or so of capacitance is in some way associated with a measured reduction of the battery charging voltage which is confusing the battery test routine. Once the capacitor has been replaced, hopefully the batteries will pass their test.

Going back to some theory (for the purists: leaving out "j"). What's the impedance of a perfect new 10uF capacitor at 50Hz ? Answer 318 ohms.

Current drawn at the point where the mains voltage reaches 220 volts RMS will be 0.7A RMS but as the mains goes periodically from +220 volts to -220 volts the current will vary in sympathy from +0.7A to -0.7A. In practice the output from the UPS will not be a perfect symetrical sinewave so there may be a residual current. However, the purpose of the capacitor is to help filter noise and this noise will be related to the method used by the equipment to generate the mains output. I imagine to get the best efficiency from the equipment the transistors fitted to the giant heatsink will be turned on and off and this means that harmonics (mainly odd) of the 50Hz output voltage will be present. The 10uF capacitor will present a lower impedance to these harmonics than for 50Hz, for example at 150Hz, 106 ohms, 1KHz, 16 ohms, 10KHz 1.6 ohms. Presumably the UPS designers calculated the effect of the 10uF capacitor, together with the pair of chokes to eliminate as much noise as possible from the output and make this look like a nice clean sinewave within commercial budget constraints.

As a capacitor degrades not only does its capacitance reduce but its internal resistance rises also. A degraded capacitor in many stressful applications will rapidly expire and usually turns into a resistor, goes open circuit or sometimes just explodes. The example below didn't get that far but I've seen the remains of many that have ostensibly vanished leaving shredded aluminium and packing material plastered around the equipment.

I'm afraid that capacitor reliability is going to suffer more and more due to pressures to reduce their physical size. Bearing in mind that internally generated heat and ambient temperature are the key factors determining the life of a capacitor, the smaller the package the less the reliability. Maybe this explains the size of the replacement I used for this repair? It's 50% larger, maybe because the suppliers only deal in components that don't result in customer's complaints? Nowhere could I find a replacement that matched the (smaller) size of the one I removed.

After reassembling the UPS I didn't test it. Instead I waited until my customer arrived to collect it. I turned it on and after a short time switched on the battery pack. The battery test routine ran and declared 50% capacity (which is about right as I'd run the batteries for at least 20 minutes a few days back), then it gave green lights (no red alarm LED and no annoying bleep). Success... so I unplugged the mains supply and the 60 watt lamp immediately lit up. After a minute all was OK so I turned off the equipment and loaded it on the wheelbarrow.

I understand there a a few more of these equipments on site. I wonder how long these will last?
 

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