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Preventing the inrush current saturation in a toroidal/EI transformer
Preventing the inrush current saturation in a toroidal/EI transformer
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Old 24th August 2012, 12:12 PM   #101
zigzagflux is offline zigzagflux  United States
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Quote:
Originally Posted by nigel pearson View Post
Anyone like to say why auto-transformers have less inrush ?
They don't have less inrush; there is nothing inherent to shared windings that reduces inrush.

Take a 96VA 120/24V transformer connected as an isolation transformer. Measure inrush, say that comes out to 8A when the 120V winding is energized. Now, connect it as a step-up auto transformer. It will still exhibit 8A of inrush.

It is possible it appears inrush to be smaller, since one could consider the auto transformer connection to be rated for 576VA instead of 96VA, therefore instead of inrush being 10x rated, it is now 1.67x rated. In other words, the auto transformer is designed to only require enough core and winding to accomodate the effective VA transformation difference, not the entire primary or secondary VA.

At the end of the day, inrush still comes down to air core inductance, DC resistance, the degree to which the core is operated near saturation, and remanence. The fact there is a shared winding is irrelevant. The amount of copper is really no different with an auto rated for transformation difference; copper is based on desired winding losses.

Autos do have special construction to accomodate unique short circuit forces and BIL requirements, so that will cause leakage inductance (and therefore air core inductance) to be different from a standard two winding xfmr. The degree to which inrush is changed based on this becomes fairly trivial.
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Old 24th August 2012, 12:26 PM   #102
nigel pearson is offline nigel pearson  United Kingdom
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OK then here is a better possibility . When I get my auto transformers built I always say be generous with the core as the weight is dramatically less than an isolating type ( 230 % greater power possibility is typical if 2: 1 ratio and in the KVA ratings ) . I think that is why . Also the simple design is reasonably cheap for labour so if absolute size is not a problem cost is reasonable . A 4 KVA auto costs less than a 300 B transformer . The latter has 1.5 hours of work to build .
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Old 24th August 2012, 02:44 PM   #103
AndrewT is offline AndrewT  Scotland
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A 4kVA auto former could be built from a 400VA 230:10+10Vac .
The 10Vac could be split into 4 windings of 5Vac each.
This, gives an autotransformer with nominal input tappings of 220, 225, 230, 235, & 240Vac and the 4kVA output could be taken from any of the input tappings to suit the downstream equipment.

What does that do wrong?
Post101 says that autoformers are made different. What is the difference and how do these affect performance?
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Old 24th August 2012, 04:29 PM   #104
nigel pearson is offline nigel pearson  United Kingdom
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Funny you should say that as I have one much like that I use . It ranges from + 20 and - 10 % . That compensates for 160 V to 270 V from memory .

A recent design at 2 : 1 is 1000 VA core gives 2300 VA in auto ( 230 : 115 ) .
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Old 24th August 2012, 06:04 PM   #105
zigzagflux is offline zigzagflux  United States
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Post 101 is talking about autotransformers sized in MVA and voltages in kV. Look up BIL and surge testing with 8/20 wavefronts and you will start to appreciate some of the challenges with autotransformers, and why they are treated with special consideration; it's really out of scope of this thread.

I doubt there is anything unique about little popcorn autotransformers measured in a few kVA. And the real point is that when hooking up an isolation xfmr as an auto, inrush doesn't change.
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Old 14th September 2012, 01:53 PM   #106
nigel pearson is offline nigel pearson  United Kingdom
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If anyone is still following this link a question ( Andrew T )

I have 2.3 KVA transformer which I will start on a 16 A 10 R inrush thermistor made by Amathermic . Simply putting a relay on the output side ( to short the thermistor ) seems to be to be a possible solution against the usual 555 timer . The reaction time of the relay is 7 mS and has an AC operated coil ( 115 V ) . I would imagine 7 mS already helpful ? The transformer I have in mind will only infrequently be switched off . It would allow a 10AT fuse . The only issue is the fuse . I have even considered fitting a switch with LED reminder of status .
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Old 14th September 2012, 02:18 PM   #107
AudioSan is offline AudioSan  Norway
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7mS may be too fast. you want the resistence drop as close to 0ohm as possible.
if not, you may weld the relay as it kicks in.
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Old 14th September 2012, 02:36 PM   #108
nigel pearson is offline nigel pearson  United Kingdom
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Hi Audio San . It might just be OK as it is a 16 A relay . I did think the same so value any suggestions . I will try it as it is cheap to find out . >40 mS would be better .

I was researching the arcing of relays . I was using the same 16 A relays in a similar application which has proved to be reliable . I took the top off of the relays and noticed reasonable levels of arcing . Like the old contacts of a motorcycle points system ( HT coil LT side ) . On a motorcycle these were usually damped with a 0.1 uF paper capacitor if memory is correct ? Reading up suggested the reason for arcing is air being ionized . Solution was to use oil to exclude air . I only had cooking oil at hand , sparking seemed identical .
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Old 14th September 2012, 02:53 PM   #109
RJM1 is offline RJM1  United States
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At 50Hz a full cycle is 20mS, so 7mS is not even a half cycle.I'd go with at least 10 cycles; 200mS.

See figure 2-G.
http://relays.te.com/appnotes/app_pdfs/13c3206.pdf

Last edited by RJM1; 14th September 2012 at 03:03 PM.
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Old 14th September 2012, 03:36 PM   #110
nigel pearson is offline nigel pearson  United Kingdom
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Thanks for the link . As it points out it could hit at excatly the worst time .

A NE 555 timer is fine for the job and cheap . About 1 uF and > 200 K should do fine for the timing ( 1 M ? ) . Use a polyester 1 uF as they are cheap and reliable . Nice thing about 555 is the internal crowbar resets it very quickly if the mains has a brown or blackout . Although most will know , fit a 1N4001 or whatever reverse biased across the relay coil to protect the 555 from back EMF of the relay coil .
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