3 phase isolating transformer - shorted? - Page 1
Forum section: electronics / beginners

author: sebdehne
author_rank: Contributor
timestamp: September 13, 2015, 10:30:45 am
content: Hi guys,I bought a used 3 phase isolating transformer (230V -> 400V) like this one:http://www.noratel.com/produkter/p/3lt-250-1/detail/and when I connect it, then the fuse goes at once.I measured 0,1 Ohm across all three primary terminals. That means that there is a fault somewhere, right? I also measured around 0,1 Ohm on the secondary terminals. The specs says it should have around 300W idle consumption, so there should be some resistance.This is not normal, right?Thanks!

author: Andy Watson
author_rank: Super Contributor
timestamp: September 13, 2015, 10:41:25 am
content: Quote from: sebdehne on September 13, 2015, 10:30:45 amMeasured with a DC meter? A 36A, 25kVa transformer winding will have very low DC resistance.QuoteYes - but measured with AC.What size is the fuse - have you allowed for inrush/magnetisation current?

author: rob77
author_rank: Super Contributor
timestamp: September 13, 2015, 10:42:08 am
content: for bigger transformers the DC resistance of 0.1ohm might be pretty normal.what is the size of the transormer ? power rating ? how did you wire it up ?you say idle consumption 300W ? that would mean it's a multi-kilowatt one if the loses are 300W alone.

author: dmills
author_rank: Super Contributor
timestamp: September 13, 2015, 10:44:35 am
content: DC winding resistance on a 25KVA transformer should actually be pretty negligible, 0.1 ohms sounds reasonable to me.What fuse values are you using, and are they slow blow or motor rated?Transformers tend to have massive inrush currents, and a 25KVA unit could well draw 150A or so for a cycle at startup.Regards, Dan.

author: sebdehne
author_rank: Contributor
timestamp: September 13, 2015, 10:55:47 am
content: It is a 25kVA transformer. Currently there is a 40A type C fuse in front of it. The seller of the transformer said that it is in working condition. So it sounds like I would need a fuse which allows for higher inrush current. Which type? My main fuse for the entire house is of type G.

author: Andy Watson
author_rank: Super Contributor
timestamp: September 13, 2015, 10:59:22 am
content: Download the "3-Phase installation" pdf from the page that you originally linked. I believe it recommends an 80A fuse for 230V operation.

author: all_repair
author_rank: Frequent Contributor
timestamp: September 13, 2015, 11:03:21 am
content: You probably need to startup through resistors and after the inrush period when the coil is energised, then the fuses are cut in.

author: thefamilyman
author_rank: Regular Contributor
timestamp: September 13, 2015, 11:34:15 am
content: you'll be looking at using a D curve MCB.depending if you have the secondary side also protected will determine what primary side rating you need.if secondary side is protected you'll need a 150A D Curve MCB on the Primary and a 25A C or D (depending on what your load is) on the Secondary side.if just the primary side is protected you'll need a 70A D curve MCB with no protection on the secondary.

author: sebdehne
author_rank: Contributor
timestamp: September 13, 2015, 11:50:21 am
content: I am not planning to use the transformer to its full capacity. The main fuse for the entire house is 63A and the plan is to use a 50A (or 63A) fuse for the transformer. At startup, there is no load on the secondary side.Would it be enough to use a 50A (or 63A) fuse with D curve or should I be looking at limiting the current during startup, like all_repair suggested?

author: SeanB
author_rank: Super Contributor
timestamp: September 13, 2015, 12:05:54 pm
content: You will need inrush current limiting for sure, a set of power resistors ( 3 2kw electric heater elements in the line) and a bypass contactor with a 1 second power on delay closing the contacts will do.

author: Circlotron
author_rank: Super Contributor
timestamp: September 13, 2015, 12:38:08 pm
content: Just for the sake of a test, power up the 400V side with 230V. Then there should be almost no inrush current to speak of, depending on where in the AC cycle you hit the switch. You should get about 132 volts out the other side.

author: Zero999
author_rank: Super Contributor
timestamp: September 13, 2015, 12:39:01 pm
content: If anything a DC resistance of 0.1R is probably a bit on the high side. The datasheet says the copper losses are 496W which will be 165.3W per phase, so 82.67W per primary and secondary. Use Ohm's law and you'll find the DC resistance should be around 0.064R for that loss at 36A.

author: rob77
author_rank: Super Contributor
timestamp: September 13, 2015, 12:50:05 pm
content: Quote from: Circlotron on September 13, 2015, 12:38:08 pmexactly, and that would also verify whether or not your wiring is correct...furthermore... are you sure you have 230V between phases (delta)? the transformer you linked expects that. just asking because 3 phase outlets in houses are always star configured over here (400V between phases and 230V between phase and neutral).

author: mij59
author_rank: Frequent Contributor
timestamp: September 13, 2015, 01:09:03 pm
content: According to Wiki the mains in Norway is 230V/400V, so you are feeding the 230V transformer winding with 400V.Why do you need a isolation transformer for the entire house ?Electricity in Norway must be really cheap, in the Netherlands the idle power consumption of 300W cost about 600 Euro per year.

author: sebdehne
author_rank: Contributor
timestamp: September 13, 2015, 01:13:40 pm
content: Thanks for all your replies. I connected it on the secondary side and it worksI measured 136 Volts between all three phases on the primary side. I guess that confirms it is in working condition. Thanks for the great tip!I guess I will try a D-type fuse (don't have any available right now) and if that doesn't work then I will have to use 3 heaters with a contactor which bypasses the heaters after 1 sec.(BTW: yes, in Norway you get 230 between the phases and there is no neutral but I want 400V with neutral for faster charging of an EV)

author: rob77
author_rank: Super Contributor
timestamp: September 13, 2015, 01:19:15 pm
content: Quote from: mij59 on September 13, 2015, 01:09:03 pmi think that will be the problem as well - feeding the 3x230 with 400V... - hence my question in the post abovebtw.. those prices in Netherlands are crazy ! over here it's 0.0575 Eur per kWh - so the 300W idle would cost something like 150Eur per year.

author: mij59
author_rank: Frequent Contributor
timestamp: September 13, 2015, 01:34:40 pm
content: Quote from: rob77 on September 13, 2015, 01:19:15 pmYes good observation, still wondering about the need for a isolation transformer, bit concerned about earthing.Just wondering, do have very very long extension cord ?

author: SeanB
author_rank: Super Contributor
timestamp: September 13, 2015, 04:06:46 pm
content: He does not really need the isolation, but needs the 400VAC output, though the isolation is a bonus.

author: Zero999
author_rank: Super Contributor
timestamp: September 13, 2015, 04:46:04 pm
content: Quote from: mij59 on September 13, 2015, 01:09:03 pmA primary side switch is essential.

author: sebdehne
author_rank: Contributor
timestamp: September 13, 2015, 06:52:32 pm
content: Quote from: Hero999 on September 13, 2015, 04:46:04 pmIts maybe around 200 Euro per year. So switch is a good investment :-)Anyway, I will try to draw the circuit with the resistors for the startup time in a simulator and see how that works... LTSpice?

author: sebdehne
author_rank: Contributor
timestamp: September 19, 2015, 04:50:03 pm
content: Thanks for all the good advice I got here. I will use a timer so that the transformator is disabled automatically and is not on permanently.I have attached the schematic of how I want to wire it all up. The second contactor's reaction time is around 10ms, which is all I need to filter out the initial peak of inrush current I think.My question is which 4.0 Ohm resistor I could use that would withstand 60A for 10-100ms.I tested one of thesehttp://www.arcolresistors.com/resistors/hs100/(25Watt version) by manually toggling a switch (maybe it was on for 300ms), but the magic smoke got out :-)I found those:http://www.digikey.no/product-detail/en/LPS1100H47R0JB/LPS-47.0D-ND/3125517but they are quite expensive.Any recommendations regarding the soft-start resistors?

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 19, 2015, 05:42:08 pm
content: Quote from: sebdehne on September 19, 2015, 04:50:03 pmA 200-250W wirewound resistor should take this kind of abuse, otherwise use a "motor starter" contactor (probably the easiest and most robust, if not the cheapest, solution).

author: johansen
author_rank: Super Contributor
timestamp: September 19, 2015, 05:43:36 pm
content: I would use 10 ohm resistors or higher, and the cheapest wirewound resistors are appropriate.stove or oven elements are just fine.I'd use at least 1 second for soft start.

author: SeanB
author_rank: Super Contributor
timestamp: September 19, 2015, 06:18:15 pm
content: 3 2kW kettle elements instead of the resistors, just in free air. That way even if the contactor fails the elements will survive a few seconds, providing you use a non resettable thermal cutout per element to disconnect the main contactor coil if the elements overheat. You can use a 210C cutout, because the kettle elements are going to survive that easily till the trip.You only really need resistors in 2 of the 3 phases, as that will still limit current in any case. If the 2kW elements do not work use some 6kW immersion heater elements, available at plumbing suppliers or electrical wholesalers. Just buy 2 of the cheapest ones at 6kW they have, it really does not matter the mounting or shape provided you can fit it in a box of sorts.

author: sebdehne
author_rank: Contributor
timestamp: October 12, 2015, 01:18:41 pm
content: Thanks for all the good help I got here. Here is the result of the project:http://dehnes.com/electronics/2015/10/11/22kW_charger_and_transformer_soft_start.html

author: SeanB
author_rank: Super Contributor
timestamp: October 12, 2015, 06:11:00 pm
content: Nice to see it all working.