Help Calculating resistors needed to limit inrush current - Page 1
Forum section: electronics / beginners

author: CST
author_rank: Newbie
timestamp: April 19, 2022, 04:12:07 am
content: So I have a 100amp 208v 3 phase panel. The transformer is a 3 phase 75kVA step down transformer from 480v to 208v. However I have it wired in reverse to act as a step up transformer from 208 to 480. I understand when doing this it can create a much larger inrush current than normal. The inrush current is blowing past the 100 amp breaker and blowing the 100amp fuse at the main panel service disconnect. This is happening with no load on the secondary side I know my panel is too small for the transformer, but I will only be running one machine off of the transformer and the current the machine pulls is within the limits of the 100amp service. I'm trying to avoid bringing in larger service as it's a bit of a run.A little more info. When the transformer is energized with no load on the secondary side the magnetizing (steady state) currents are as follows:L1 = 3.7 apmsL2 = 2.3 ampsL3 = 3.6 ampsI was hoping someone might be able to help me determine what size resistors I could put in line with each phase to use to limit the current for the brief inrush of current. I've heard everything from a 10 watt 5-10ohm resistor that drops out after about a second to 5000 watt light bulbs. Any input would be greatly appreciated.

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 19, 2022, 06:17:47 am
content: If the 100 amp breaker doesn’t trip, it sounds more like a case of the fuses being low peak type being cleared by the transformer saturating at turn on. Swapping them out for dual element time delay (motor type fuses) should solve the issue.ddPost a picture of the fuses, we can cross them over to a time delay type.

author: CST
author_rank: Newbie
timestamp: April 19, 2022, 02:21:43 pm
content: The fuse type are jlln100 amp fuses. I have attached a picture here.Thanks so much

author: rstofer
author_rank: Super Contributor
timestamp: April 19, 2022, 03:40:10 pm
content: 75 kVA at 480V 3 ph => 90A per phase75 kVA at 208V 3 ph => 208A per phaseMinimum fuse size is 125% of full load current and, even then, it might need to be time delay because the inrush is probably at least 6 times full load (wild guess, could be higher, maybe 10x). For circuit breakers, the instantaneous trip is usually 10x nameplate which, given a 25% cushion, means the breaker won't trip instantaneously due to inrush. You need to check the fuse curves to see where they clear.https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/fuses/solution-center/bus-ele-tech-lib-fuses-time-delay.pdfhttps://m.littelfuse.com/~/media/electrical/white-papers/littelfuse-transformer-protection-white-paper.pdfI would expect a time delay fuse to handle the inrush which could be as high as 10 x 208A or 2080AGiven that your 480V output is delta connected, you have no place to connect a grounding conductor as you would when a proper transformer is installed. Ungrounded systems are problematic. The first ground fault is free but the second fault has a good deal of fault current (phase to phase versus phase to ground). In the very old days, we would put neon indicating lamps on the phases to ground to show when there was a first ground fault. Bottom line: I wouldn't do this on a bet.There are proper delta-wye step-up transformers. They aren't cheap at nearly $3400!https://store.maddoxtransformer.com/products/copy-of-3-phase-480v-delta-208-y-120?variant=2102327345161The link misstates what is going to be presented, it will be a 208V Delta -> 480V Y 3 ph 75 KVA transformer

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 19, 2022, 06:05:50 pm
content: Quote from: CST on April 19, 2022, 04:12:07 amI guess you have to look at something like 0.1 R or 0.05 R and 500-1000 W rated. (having resistance value similar to current primary winding resistance). You have to short-circuit it after the start.

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 19, 2022, 07:15:15 pm
content: Power resistors (>100W) sized for 100% time use are going to be big and expensive. My advice is to use a power resistor that limits the in rush current with a 3 phase contractor in parallel activated 1sec after power up. Overall this is easy and cheap

author: CST
author_rank: Newbie
timestamp: April 19, 2022, 10:03:25 pm
content: Thanks everyone for the info.@PartialDischarge - Given my setup, what size resistor do you think would be effective in limiting the inrush?Thanks

author: CST
author_rank: Newbie
timestamp: April 19, 2022, 10:14:15 pm
content: I had tried the following, but it didn't seem to work.I set up a separate 20amp three phase circuit with a 1500watt light bulb in series on each leg (so 3 bulbs total) to act as resistors.Turn on the 20amp circuit to limit the inrush current with the light bulbs and then switch on the 100amp circuit to fully energize the transformer and then turn off the 20amp circuit.Both the 20amp and 100amp circuit are feeding the H1 H2 H3(wired in reverse) on the transformerWhen the 100 amp circuit is turned on the light bulbs should essentially dim because the electricity will follow the path of least resistance and then I can shut down the 20amp circuit.When I turned on the 20amp circuit the bulbs were very dim (I thought they would be very bright, not sure why they didn't), but did get warm/hot. Then I turned on the 100 amp circuit and all 3 of the bulbs got very bright. When I switched off the 20amp breaker the bulbs slowly lost their light, but apparently it didn't work, because I shut everything down and tried again and the fuse was blown. So alas that's why I am here. Thanks again everyone.

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 20, 2022, 07:09:50 am
content: The JLLN fuses are fast blow. For them to blow in less than 1sec, a current of more than 350A is passing through according to their datasheet.So I would use a 100Ohm resistor 75W or 100W, but the delay for the 3ph contactor to close must be short, 0.5..1sec max. otherwise you will burn the resistor.https://www.digikey.es/es/products/detail/arcol/NHS100-100R-J/16163084https://www.digikey.es/es/products/detail/arcol/NHS75-100R-J/16162864

author: CST
author_rank: Newbie
timestamp: April 21, 2022, 04:01:02 am
content: Thank you!! I really appreciate everyones help!