Isolation Transformer Help Needed - Page 1
Forum section: electronics / projects

author: DevManiac
author_rank: Newbie
timestamp: June 13, 2020, 02:03:43 am
content: Hi All,I have an issue that is really difficult for me to solve.I have an isolation transformer on my bench. The primary is connected to 120 VAC mains. It has a fuse in series (5 amp slow blow) that I added to the primary. The transformer is a 3 kva with 1:1 isolation, that drives a variac on the secondary (0 to 140 vac). The issue is it constantly blows fuses. Today I even blew a 7 amp slow blow fuse! IT IS NOT A CURRENT INRUSH ISSUE! Here is the possible conditions:SWITCH POWER ON:1. Turn variac to 0 v while main power is off, then turn on isolation transformer (turn main power on), no problems! (with load or no load on secondary, still no problems!) - so not inrush.2. Turn variac up to 120 v while main power is off, turn on isolation transformer, no problems! (with load or no load on secondary, still no problems!) - so not inrush.SWITCH POWER OFF:3. Turn variac to 0 v, then turn isolation transformer off, no problems! (with load or no load on secondary, still no problems!)4. Leave variac at 120 v (or so), then turn isolation transformer off, FUSE BLOWS about 4 out 10 times! (with load or no load on secondary, SAME Problem!) - so it is outrush.Since I tend to be absent minded while powering down, I hit #4 above quite often!Pleas help! My patience, my fuse supplies and my wallet is suffering!Thanks for your reply,DM

author: jmelson
author_rank: Super Contributor
timestamp: June 13, 2020, 02:15:49 am
content: Why not put the Variac first? It is known that some Variacs draw a huge inrush due to magnetizing the iron at random line phase when the switch closes. (Funny, the Variac dial setting should have no effect on that.)Anyway, using a small fuse on a 3 KVA transformer can be a problem, again due to the magnetizing at random phase issue.OR, are you saying the fuse blows when you turn the switch to the off position, or when you turn it on NEXT time?If it actually blows on turn-off, it probably has to do with arcing of the switch contacts and the huge inductance of the transformers.Jon

author: bdunham7
author_rank: Super Contributor
timestamp: June 13, 2020, 02:21:10 am
content: Please post a drawing of how you have that wired and a photo, especially of the variac connections. Just take some pics with your phone if you don't want to draw. I'd like to see what a 3 kVA isolation transformer looks like, that sounds large and I'm not sure where you could use it except maybe on an RV.If the switch and the fuse are both on the primary side of the transformer, then you would have to have some pretty serious arcing going on to blow a 7A Slo-Blo fuse. I just don't see how that is possible. If the variac is wired correctly, it should make absolutely no difference if it is set low if there is no load on the output. So I think something is not right.It is typical to put the variac first in an isolation setup, but that should wait until we see what you have now.

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 14, 2020, 07:54:06 pm
content: It is really unusual for variac to blow fuse while turning off. Usually it is vice versa (while switch on).Maybe a load with those transformers make some resonating LC-circuit?If it is connected with turning off then maybe some overvoltage arises (and then it overlaps isolation and so on). Magnetizing currents have very inductive nature.Maybe some portion of powerful varistors, placed between transformer and variac can help?Or an arc-damping RC-circuits? (resistor must be very powerful otherwise it will blow like a fuse)And you can put that small (I mean 5-7 A) fuse before variac but not before 3kVA transformer because such a transformer has to have about 25 A rated fuse or larger. I mean if the transformer and variac have (too) different rated power it could be better to fuse then separately (every with it's own fuse).

author: bin_liu
author_rank: Contributor
timestamp: June 15, 2020, 01:12:12 am
content: This is a very strange thing, I do not know how your variac is wired? It would be nice if I could see the circuit diagram of this system.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 15, 2020, 02:20:55 am
content: What is the size of the variac?Also:3kVA / 120V = 25ASo 7A is a pretty small fuse for this setup.An easy way out could be to use a TRIAC to switch the transformer. The triac only switches the transformer off when there is no current through it, which will reduce inductive phenomena during switch-off.Another simple solution would be to use a switch (relay) with dual pole contacts. Then, when you turn the (primary) circuit off, you can short circuit the variac with a series resistor (with the same switch / relay) A resistor of 10Ohm 10W should be plenty for this.I assume you have an oscilloscope. How about putting a 1Ohm series shunt resistor between the Isolation transformer and the Variac, and measure voltage & current during these switch-off events? This gives you a much better Idea of what is really happening. It is likely that you can not get consistent results with a manual switch. I suspect that your turn-off event is related to the phase during switch-off, and consitent measurement would then need some zero crossing circuit and a timer.