[RE-wrenches] Transformer inrush

[boB Gudgel]

Richard L Ratico wrote:
> Interesting question, Mick. It may depend on the point in the cycle when the
> transformer lost power and  the duration of the blip.
>
> Dick Ratico
>
> --- You wrote:
> Before I present a suggestion, here's a question:
>
> If by chance William is able to energize the transformer without tripping
> the breakers, what if there is a brief break in the power...such as a
> half-second?
>
> I don't suppose there's capacitance here...more like magnetic flux...but
> would there be a brief window before the transformer bleeds down and
> requires another equally strong inrush current?
>   


One thing that can (and does) happen is that, depending on where in the 
AC cycle the AC source
went away, it can leave the transformer magnetized in one polarity or 
another.  Then, when
the AC source comes back on, it ~may~  return in a polarity to further 
magnetize the transformer
core in the SAME direction it was magnetized in, and that can saturate 
the transformer core
giving you basically, a short across the AC source.  That's where the 
majority of unknown and
intermittent surges come from  IMHO and experience.    If the 
transformer is way bigger than it
needs to be for a particular application, or if the wire it is wound 
with is small enough and has high
enough resistance, then the current is more limited.

This can also be a problem just turning an inverter off and back on 
again.  Capacitors charging up
is also a reason for surging of course.

2.5 cents

boB





> Or...would a tiny blip in the input power result in another big surge
> requirement?
>
> Mick Abraham, Proprietor
> www.abrahamsolar.com
>
> Voice: 970-731-4675
>
>
> 2010/5/13 Richard L Ratico <Richard.L.Ratico at valley.net>
>
>   
>> Sorry for the additional post here, but it includes a typo correction:
>>
>> Bill,
>>
>> Not a lot of personal experience with this issue. The one time I had to
>> deal
>> with it, we were able to leave the circuit on, once we got it energized.
>> As you know, the crux of the problem is the nature of the voltage relative
>> to
>> it's cycle when connection is attempted.
>>
>> An internet search resulted in this explanation that was very helpful. See:
>>
>> http://www.opamp-electronics.com/tutorials/inrush_current_2_09_12.htm
>>
>> As I understand it now, counterintuitively, you don't want to switch at the
>> zero
>> crossing. SS relays aren't my strong suit either, but if they connect at
>> that
>> moment.......
>>
>> Is your step up transformer actually designed by the manufacturer to be a
>> step
>> up unit, or are you using a step down unit in reverse?
>> Again, according to information found in my search, not personal
>> experience, the
>> latter can contribute to higher than expected inrush.
>>
>> Dick Ratico
>>
>>
>> --- You wrote:
>> Thanks for the reply.  To clarify, we have tried his with one transformer
>> and with two transformers.  Our success rate is lower powering one
>> transformer.  My hunch is that the capacitance of the feeder is
>> buffering...
>>
>> I wonder if I use a large solid state relay with zero crossing switching I
>> might have better luck.
>>
>>
>> William Miller
>>
>>
>> At 06:23 PM 5/12/2010, Richard L Ratico wrote:
>>     
>>> Bill,
>>> Your branch circuit breaker is pretty small.
>>> Are you energizing both transformers simultaneously? If so, it might help
>>>       
>> to
>>     
>>> energize them individually.
>>>
>>> The branch breaker can be up to 250% of the step up transformer's primary
>>> rating. 10kVA/240V=41A.
>>> 41A x 2.5=100A. (round numbers). To do this you would also need
>>>       
>> overcurrent
>>     
>>> protection on the transformer secondary.
>>>
>>> Of course, you've already got the 100A main tripping. That's why I asked
>>>       
>> if
>>     
>>> you're
>>> energizing them simultaneously.
>>>
>>> Transformer inrush can be a real problem.
>>>
>>> Hope this helps.
>>>
>>> Dick Ratico
>>> Solarwind Electric
>>>
>>> --- You wrote:
>>> Friends:
>>>
>>> I have on off-grid client that is trying to eliminate winter generator run
>>> time.  He has a rental on another corner of the property that has grid
>>> power.  When it was installed, he put in an extra meter, hoping to use it
>>> to run a utility back up feeder to charge batteries.
>>>
>>> We are now installing this feeder.  We purchased two 10KVA 240/600 volt
>>> transformers to bump the voltage to 600 and then back down to 120/240.
>>>
>>> Here's the problem:  when we energize the circuit breaker that feeds the
>>> transformer, the circuit breaker AND the main breaker trip.  The
>>> transformer is 10KVA, the main is 100 amps and the branch breaker is
>>> 60A.  The breakers hold sometimes but trip other times.  This is obviously
>>> an inrush problem.  How do I correct this problem?
>>>
>>> Thanks in advance,
>>>       
> --- end of quote ---
> _______________________________________________
> List sponsored by Home Power magazine
>
> List Address: RE-wrenches at lists.re-wrenches.org
>
> Options & settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
>
> List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
>
> List rules & etiquette:
> www.re-wrenches.org/etiquette.htm
>
> Check out participant bios:
> www.members.re-wrenches.org
>
>
>