What's wrong with the step down transformer?

Hi,

We have a customer using a 500VA step down transformer, step down from 240VAC to 110VAC. The transformer will be connected to a 500VA UPS. Once we switch ON the transformer, the building circucit breaker also trip! We tried with 2 step down transformer and some same model of UPS, it's the same!

Could be the transformer not enough VA for the UPS?

Do you have a switch after the transformer for the UPS?

A UPS is basically an inverter running off of batteries instead of capacitors. Perhaps the batteries are dead, and therefore, the UPS is drawing more current: Load current (4ish Amps @120) and battery charge current (XX amps @120V).

That's just a guess, though. I'd put some ammeters in the circuit to see where the problem is... as well as disconnecting loads as I mentioned above.

The circuit breaker probably trips on instantanous overcurrent (short circuit protection).

You must take into account that transformers have very high inrush current when connected to power, with or without a secondary load connected. The current peak may be 10-20 times the rated current (!).

There are circuit breakers with special characteristics intended to take the extra high inrush current into account. These may be called either with "transformer characteristic" or "D characteristic"
For example Siemens circuit breakers of the 3RV10 series is for motors, whereas the 3RV14 series is for transformers.

Good point, Jesper.

I once had fits using instantaneous fuses when I should've been using slo-blo's.... I wasted half a day until I realized why my 250 mA circuit was blowing 500 mA fuses every time I turned it on.

Try putting another circuit breaker between the transformer and the UPS. Turn it off and then power up the transformer.

If successful, then power up the UPS.

Both these devices can have high inrush currents and by separating the 2 inrushes, it may help to get the system on line, or determine which device is causing the problem.

I am not clear on your description. Is the UPS supplying power to the transformer, or is the transformer supplying the UPS? It could be either way, you know.

I am not clear on your description. Is the UPS supplying power to the transformer, or is the transformer supplying the UPS? It could be either way, you know.

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In this case using VA I doubt I would ever have the UPS before the transformer, that would be roughly the equivalent of a 1/3HP (or larger) single phase motor.

As Jesper mentioned a transformer (just like a motor) can have inrush current 10, or 20 times its full load capacity, even with no load.

I think the key was in this statement:

Once we switch ON the transformer, the building circucit breaker also trip!

Click to expand...

I am not sure what your standard breaker size is in the panel but even if its a 20A standard "lighting" (non time delay) breaker the inrush current would probably trip it....ie 2.25A (xfrmr )x 10 (inrush factor) is 22.5A....just a simple example. As stated replace that with a time delay breaker.

Hi,

Thanks for everyone's reply, sorry if my question is not clear.

The step down transformer (240 to 110VAC) is supplying power to the 110VAC UPS. The UPS will then use to backup power the PC & some field devices.

The transformer 110VAC output only has 2 pin (0V & 110VAC), the transformer incoming 240VAC has 3 pins (Life, Earth & Neutral). Life & Neutral will be going into the coil, the Earth will be grounded to the transformer casing (body).

Our UPS (110VAC) has 3 pins, so obviously it cannot directly plug into the transformer 110VAC output. We drill a hole on the transformer 110VAC output socket, on top of the 0V & 110VAC pins, so that the UPS 3 pins can go into the 110VAC socket & the UPS Earth pin is shorted with the transformer incoming 240VAC Earth.

1) I've tried connecting the step down transformer (500VA) into the UPS without any load, once I press the ON button on the UPS, my office CB also tripped. Same thing happen in my customer office.

2) We also tried connecting an extension (no load) into the transformer, it also tripped our CB.

3) We also tried to wrap the Earth pin of the UPS with insulator tape and plug in into the transformer, it's fine, no tripping, but all the devices & PC powered by the UPS are not grounded, so we got electricity short when we touch them!!

4) We tried to power up the UPS with another transformer at my office. This transformer 110VAC comes with 0V, 110VAV & Earth pin. It fitted the UPS socket nicely! When we power up the UPS, everythin's fine! No tripping the building CB. It's a 750VA step down transformer.

So, is it possible the low VA (500VA) of the transformer that cause the tripping problem?

o.k...to wire a transformer..

your 220v goes to the primary side..(If it has 3 prongs make sure one of them is earth..The secondary side is as follows..

110v to load (Hot)
0V to neautral AND ground (This gives the secondary side a refence point

Then wire a plug(Receptical) from the transformer..Hot to 110 and neutral to 0v..ground to ground..This should work..

(sorry if this is what you are doing i still cant quite understand why you are drilling anypart of the transformer)..As for inrush..everyone is right the inrush is high but i have never seen a 500va trip a normal circuit breaker constantly..


O.K...re read your last post..Could you have damaged the transformer when you drilled it??..can you posta pic of what you did..I think i understand what you did but want to be sure..

Thanks

D

ckchew666 said:

We drill a hole on the transformer 110VAC output socket, on top of the 0V & 110VAC pins, so that the UPS 3 pins can go into the 110VAC socket & the UPS Earth pin is shorted with the transformer incoming 240VAC Earth.

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Not exactly the 'soultion' I would recommend, but an interesting approach nonetheless...

Here's my guess...

Since the UPS has a 3-prong plug, the hot and neutral blades are the same width. It relies on the ground prong to achieve correct polarity. On a ungrounded plug, polarity is accomplished by having the wider neutral blade.

In other words, if you chopped off your ground prong, you could insert the plug either way into your transformer. I'm betting that the transformer has a polarized receptacle, and that the wide slot is tied to the incoming 240V ground (creating the neutral reference).

You drilled the new hole ABOVE the receptacle slots. Now the UPS plug can only fit in one direction, but it just so happens to be the WRONG direction. The hot and neutral are now swapped (reverse polarity). Had you chosen to drill that hole BELOW the slots, the polarity would have been correct (and this thread would not exist).

But wait... Under normal circumstances, even though the polarity is reversed, I wouldn't expect the circuit breaker to trip. It would only trip if the load's neutral (which, thanks to the reverse polarity, is now the 'hot' side) has a path to ground. I'm guessing that the incoming neutral is tied to ground within the UPS? Maybe on purpose?...

The 'moral' here is to always use matching plugs and receptacles. If something doesn't fit, don't just 'modify' it...



-Eric

ckchew666,
I agree with Eric. (That is a masterful diagonosis, Eric. I think you nailed it exactly.)

Your symptoms fit the description of a short circuit. Somewhere Life and Earth are connected together. The most likely place is where you changed the receptacle by drilling a new hole. Life and Neutral cannot be swapped without consequences.

Could you replace the transformer receptacle with a 3-hole factory-made version? Would that not be safer than building your own?

I aggree that it is an incredibly clever deduction by Eric there.

chchew666 can verify this be removing the 240V supply, connect the UPS to the transformer, and then measure with a meter if there is a short circuit.

However, the following two statements makes me think that the problem is even simpler:

2) We also tried connecting an extension (no load) into the transformer, it also tripped our CB.

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With a simple extension chord there is no connection between the wires in the connection cable, then why is there are short ? Investigate with a meter.

3) We also tried to wrap the Earth pin of the UPS with insulator tape and plug in into the transformer, it's fine, no tripping, but all the devices & PC powered by the UPS are not grounded, so we got electricity short when we touch them!!

Click to expand...

If the earth-pin is isolated, then there is no short.

Could it be that the earth-pin makes a short directly inside the transformer somewhere ?

Very good observation there, Jesper. I mis-read (or simply mis-sed) item 2. If simply plugging in an extension cord (I assume 3-pin) trips the breaker, then the ground prong could be contacting the winding, etc... All the more reason NOT to be drilling holes in electrical items...

I'd like a clarification on point #2 though. Is the 'earth pin' of this extension cord being externally grounded similarly to "the UPS Earth pin is shorted with the transformer incoming 240VAC Earth"? I'm also interested in how you accomplished this connection. On second thought, I'm probably better off NOT knowing...

One more clarification please...

ckchew666 said:

3) We also tried to wrap the Earth pin of the UPS with insulator tape and plug in into the transformer, it's fine, no tripping, but all the devices & PC powered by the UPS are not grounded, so we got electricity short when we touch them!!

Click to expand...

You GOT electricity short (I assume you mean 'shock', i.e. ) when you touch them? Or, you were simply worried that you COULD get a shock if you touch them? If you GOT a shock, then the neutral is DEFINITELY connected to ground within the UPS.



-Eric

Lancie1 said:

Could you replace the transformer receptacle with a 3-hole factory-made version? Would that not be safer than building your own?

Click to expand...

Yes, please do! I'll even point you to a SOURCE for these...



-Eric

Something is fishy here.
From the 1st post:

We tried with 2 step down transformer and some same model of UPS, it's the same!

Click to expand...

Does chchew666 mean that he drilled holes in two similar transformers and he can observe the "funny" things on both ?
It was the above statement that made me think that the problem was with the inrush current. Normally when you exchange a component with a simlar one, and the problem stays, then it is not defective but it may be used in a wrong way.

edit:
If you have a meter, and you are just a little bit proficient in "electrics" then you will locate the source of the problem in seconds, rather than having us guess at it.