help with trasformer

I have a transformer that sometimes trips its primary circuit breaker on power up. If you reset it it runs fine until the next time the power to the cabinet is cycled.

Has anyone got any idea what can cause this.

I am using a sola Hevi Duty transformer E1000 1kva
480 primary 120 secondary.

sprek said:

I have a transformer that sometimes trips its primary circuit breaker on power up. If you reset it it runs fine until the next time the power to the cabinet is cycled.

Has anyone got any idea what can cause this.

I am using a sola Hevi Duty transformer E1000 1kva
480 primary 120 secondary.

Click to expand...

More than likely it's an "inrush" situation. Transformers have inrush issues similar to motors, and in a motor the inrush can be up to SIX times the full load amps. You have to compensate with over-sized or dual element fuses or a circuit breaker that is rated Higer than the transformer primary. You should ALSO up-size the wires for safety's sake.

What size is the breaker and wiring now?

Stationmaster

Thats what my first guess was so I bought a breaker that they call a D-curve for transformers

http://www.grainger.com/Grainger/ww...=Go!&QueryString=6b120&submit.x=43&submit.y=2


maybe I didn't go up enough

Jamey

My wire on the primary side is 14awg MTW wire


Here is a page out of my print to show how it is wired in

http://www.sprektech.com/grinder.pdf

This may help:

http://www.sola-hevi-duty.com/products/transformers/industrial/FuseRecommend.htm

I'm sure they have one for wire size and ckt breakers as well. I just googled it.

Stationmaster

I would condider the load you have on the transformer
Receptacle 6 A
FU 7 at 2A
Fu 8 at 3A
and who knows what on p 3. Now I realize the fuse sizes are not what the load draws. I also know you said nothing about blowing fuse 8 so steady state or average long term load is 960 W. Total of fuses is 11A which on a 120 line is about 1320 watt which I think would somewhat overload. a 1000 W transformer. I would check out how much current this draws under normal operation and if you canon startup.

I would check transformer short term overload capacity and possibly get a new transformer. YOu could also shift some load off the transformer. I would start with the receptacle lest someone plug a heat gun or power tool in it and fry the transformer.
Dan Bentler

Try to have only the transformer on line
this will give you the inrush current of the transformer.
the drawing and use of the D looks good.
however if the power taken from transformer is very high in the beginning.
like more then half the rating the transformer will look shorted and the fuse will blow.
especially electronic cicuits like power supplys etc have high inrush currents.
Bigger trafo and even bigger fuse is only thing for this.

I used an amp clamp to get these readings at system startup I got 1.5 amps on the secondary side the primary side has about .5 amps on it



My complete print

http://www.sprektech.com/grinder2.pdf


Jamey

Inrush happens very quickly, right when you turn on the power. You may need an analog meter to see it, or if you want to be EXTREMELY scientific you could capture it on a scope.

That really shouldn't be necessary though, SOMEwhere there HAS to be a table/chart......

Stationmaster,
Not to be picky, but wouldn't you want to use a digital clamp meter, preferably with an "inrush peak" recording function, to get an accurate idea of what the inrush current is? I'd think that an analog meter would not show peak values well, due to the fast transient time of the peak, and the inertia of the analog meter movement.

Thanks,
rpoet

sprek said:

...maybe I didn't go up enough
Jamey

Click to expand...

That's my guess. Transformer isn't hot or melting down, so there doesn't appear to be any other issues. It's either inrush, or back EMF on the primary line from the grinder motor.
You said startup only, so I don't see the grinder motor coming into play.
I don't like the idea of increasing the size of the breaker. Is this a new design, or something that has been running for years and now has a problem? Maybe the transformer is starting to have insulation breakdown. The high voltage at startup causes it to arc once.

He did not say he was having any problems blowing the 8 A fuse on the secondary and he says steady state current is 1.5 amp. However he has one transformer feeding multiple transformers ie 24VDC power supply and the transformers in each of the PLCs (?).

Technically if he has the secondary protected with an 8 amp fuse he should be good enough but I like the idea of protecting primary also.

I think solution is to find a breaker that has a longer delay for peak current OR put in a slow blow fuse on primary.
Dan Bentler

rpoet said:

Stationmaster,
Not to be picky, but wouldn't you want to use a digital clamp meter, preferably with an "inrush peak" recording function, to get an accurate idea of what the inrush current is? I'd think that an analog meter would not show peak values well, due to the fast transient time of the peak, and the inertia of the analog meter movement.

Thanks,
rpoet

Click to expand...

Yes! If you put the "digital meter with an inrush peak recording function" in the list of choices, then I would select it above an analog. It's also less COMMON than an analog amp clamp IMHO, so I surmised that the op would be less likely to have access to one. I still say an analog will demonstrate the inrush better than an ordinary digital amp-clamp, though I never claimed it would do so flawlessly. There IS a time lag due to the real-world inertia of the meter movement, but the reaction is STILL typically faster than an ordinary digital amp-clamp because of the built-in interval between display scans.

Good observation. Good suggestion on the "inrush peak capture".

I STILL say that there must be a chart or guide for selecting wire size and ckt-breaker for this transformer.....

Stationmaster

Thanks Stationmaster. I guess I fell into the trap of thinking everyone had the same toys. The place I used to work had two; those Fluke meters would even do peak inrush and average current with DC - pretty handy.

I agree - a call to Sola would be a good idea. They should be able to give some advice, and maybe even a chart.

-rpoet

D-curve breakers are set to trip instantaneously between 10 to 20 times In.

The full load primary current of a 1kVA is only 2.1A at 480V: An inrush current of up to 25 times In = 52A would still be less than the worst case scenario of a D6 breaker.

So the breaker should be able to hold the inrush current, UNLESS the secondary is momentarily shorted (by a large cap in a DC power supply or a VFD) OR by severe harmonic distortion.

You mentioned that the problem is gone by resetting the breaker; I'm assuming you instantly reset the breaker once it's tripped. So it sounds more like a capacitive secondary problem. What happens if you wait several minutes before resetting the breaker? Anyhow, if you can't change the secondary load, there's no other option than to install a D10 breaker and to change the wiring accordingly.

BTW, not being picky either, don't know what you guys mean by "inrush peak recording" function but the "peak hold" function on a clamp meter isn't really any good either to get a correct readout of the inrush current. Since the current is sampled in a digital meter, the measurement represents nothing but A sample at A given time (at X samples/second). The peak hold function will hold nothing but the peak value in a series of samples: Chances are very slim a sample is taken at the exact moment of the inrush current. So Stationmaster does have a point, in many cases the relative slow but continuous reading on an analog clamp could give you a better indication of the inrush.