Now the question is of course how your home installation is fused/protected. In normal EU homes often 16A class B breakers are used. These will definitely trip with such a transformer even without load. That is why recent (that is young than 30 years) transformers above 500VA and sold in the EU usually have NTCs. They are simply needed and inrush current protection is required by regulations. Exceptions confirm the rule as they say. Transformer manufacturers are aware of this and mention this as well. It helps that your transformer is an EI type and not a toroid type.
Assumption is the mother of f**kups. Still: a reason it may work OK is that the home has fuses and not breakers and possibly higher than normal impedance grid. Fuses also withstand abuse somewhat better. It could also be that class C or even class D breakers were used. My own home has 80A knife type fuses which don't panic with such loads but that sure is not standard.
It is awkward to state the "none is needed" and "no funny stuff is required" while it is mandatory for inductive loads in many countries for good reasons. Add to this that almost any isolation transformer is of the toroid type these days.
Assumption is the mother of f**kups. Still: a reason it may work OK is that the home has fuses and not breakers and possibly higher than normal impedance grid. Fuses also withstand abuse somewhat better. It could also be that class C or even class D breakers were used. My own home has 80A knife type fuses which don't panic with such loads but that sure is not standard.
It is awkward to state the "none is needed" and "no funny stuff is required" while it is mandatory for inductive loads in many countries for good reasons. Add to this that almost any isolation transformer is of the toroid type these days.
Last edited:
I'm tempted to agree with the toroid part. However, such devices will not be as good for EMI isolation as the good ol' EI.
Just to be complete, mine is wired to a 10A circuit breaker, which is the norm here in (non-EU) Switzerland. It never tripped, not once.
I've got nothing against NTC's, but I think it's a good idea to short them after they've done their part: just picture yourself turning on, then turn off after a while, then on again while the NTC is still hot.
Just to be complete, mine is wired to a 10A circuit breaker, which is the norm here in (non-EU) Switzerland. It never tripped, not once.
I've got nothing against NTC's, but I think it's a good idea to short them after they've done their part: just picture yourself turning on, then turn off after a while, then on again while the NTC is still hot.
Yeah like opening and closing and opening the refrigerator door again to check if the lamp goes out 🙂
The "smart" slow start circuits do just that, they short the NTC/resistor after a few seconds. They contain more parts including a relay. So what is more reliable? I noticed recent versions of medical isolation transformers to have the electronic type of protection.
The "smart" slow start circuits do just that, they short the NTC/resistor after a few seconds. They contain more parts including a relay. So what is more reliable? I noticed recent versions of medical isolation transformers to have the electronic type of protection.
Last edited:
... more like "Oh, I wanna hear that song again" 🙂
Another thing is, I have the feeling the NTC's may not like being cooked for too long (decades). Or maybe I'm wrong.
Another thing is, I have the feeling the NTC's may not like being cooked for too long (decades). Or maybe I'm wrong.
NTC go to nearly zero resistance at working conditions, do they not?
Just buy a good brand, and slightly higher rated than needed.
And PTC and NTC are both used in HVAC circuits, also refrigerators, not considered a high maintenance item, failure rates are low, provided they are mounted properly with ventilation.
Just buy a good brand, and slightly higher rated than needed.
And PTC and NTC are both used in HVAC circuits, also refrigerators, not considered a high maintenance item, failure rates are low, provided they are mounted properly with ventilation.
Last edited:
They do not: CL-80: R = 1.2 Ohms at 1.5A, 2.7W.
Power ratings and resistances are mutually dependent.
HVAC's use (used?) NTC's as temperature sensors, nothing to do with current carrying duty.
You're perfectly right, I wrote MOV but I meant NTC.
Toroid or EI core : my 3kVA isolation transformer is a balanced (two output phases with half input voltage each) Noratel "XQ" (Extra Quiet) 40kg toroid.
What's the difference between toroid and EI about inrush current?
However I see the matter of "inrush protection yes or no" is really controversial!
Gianluca - OP
The transformer inrush should be 2x the idle current, and that should not be a problem, depending on the transformer quality/margin. The idle current should be ~5% of the operating current. A real problem is turning the transformer OFF, when it could generate a "flyback" voltage that arcs the switch, and hence the need to MOV the transformer (not the line input).
I measured far more than that. See the Soft Start Design Guide I mentioned earlier. I measured the inrush current of a handful of toroidal transformers ranging from 5 VA to 1 kVA and found several transformers had inrush currents in the 100-200 A region. That's why power resistors tend to burn out in resistor-based soft starts.
Ametherm has a good application note on this as well: https://www.ametherm.com/inrush-current/transformer-inrush-current.html
EI core transformers may be slightly gentler due to their higher losses, but the magnetic flux still has to build within the transformer when it's turned on. That's where the inrush current comes from.
Tom
Minor error in my Post #28 above, it should have been 'compressor start circuits', not 'Starr'.
Sorry for the confusion.
Sorry for the confusion.
My personal interpretation,others may differ, is that:
1) mains line "sees" a quite lower toroidal transformer impedance at turn-on
Which obviously causes a higher "starting/inrush" current.
2) because having a lager window, thicker wire can be used in Toroids; good for regulation, bad for inrush.(lower DCR)
3) also EI have an unavoidable built-in gap because they are assembled out stacked stamped steel EI pieces, while toroid cores are built out of a gap-less looong strip of silicon steel, wound in many continuous layers around a mandrel.
So beyond the blanket statement "toroids have higher inrush current than EI", there is also physical reasons behind that.
In fact, if somebody knowledgeable wants to sit down and write equations describing that behaviour, he can.
No "magic" or "mystery" involved 😉
4) fwiw I wind my own EIs with less turns of thicker wire , in fact I have to somewhat compress bobbins in a hydraulic press (built out of a hydraulic car "lifter") to make them perfect rectangles (otherwise thick wire tends to lay out roundish) to fit in available windows, I always fill available space with copper .... and guess what? ... my EIs have very good regulation (which was my goal) but also nuisance blow regular fuses , so I need slightly larger ones, and T/SB type too.
Finally I've decided to use a pair of NTC Thermistors Ametherm AS32 5R020 (5ohms 20A, last generation) in series with the AC phase (right after the fuse) and soldered to the primary of the trafo. The two thermistors are in parallel with each other. Plus, I'll use a switch between the fuse and the trafo's primary so that AC and transformer are coupled directly after the thermistors have done their job (will 1 minute be enough?). Switch and thermistors are in parallel and I suppose the electrons' flow will choose the direct path in place of the thermistors.
Thanks!
Gianluca
Thanks!
Gianluca
Might be, it all depends on applied/dropped power and thermistor mass/time constant.
It might even be stated in datasheet, in some more or less cryptic way 😉
That said, and just a hunch, 20 milliseconds is the time constant of a single 50Hz cycle, in principle looks too optimistic to me.
Do inrush limiters reach steady low resistance value so fast?
That said, it´s easy to measure, and you don´t need AC, just apply sudden 5A (or whatever the nominal/suggested current value is) and scope voltage drop across it, image will clearly tell what we want.
You are both welcome to test and post results 😉
Again:wouldn´t be surprised if it´s already on a (full) datasheet, maybe stated as: steady value reached in X milliseconds/cycles or similar.
It might even be stated in datasheet, in some more or less cryptic way 😉
That said, and just a hunch, 20 milliseconds is the time constant of a single 50Hz cycle, in principle looks too optimistic to me.
Do inrush limiters reach steady low resistance value so fast?
That said, it´s easy to measure, and you don´t need AC, just apply sudden 5A (or whatever the nominal/suggested current value is) and scope voltage drop across it, image will clearly tell what we want.
You are both welcome to test and post results 😉
Again:wouldn´t be surprised if it´s already on a (full) datasheet, maybe stated as: steady value reached in X milliseconds/cycles or similar.
The question is why transformers have an inrush current since they are inductors. It seems to me that the first half cycle can potentially saturate the core? Normally a transformer idle current lags the voltage by 90 degrees, so current zero crossing happens at the voltage peak, i.e., 90 and 270 degrees. But the first half cycle has no previous current to offset the integration of the voltage, so it will build for a full half cycle and not return to zero until sometime after 270 degrees. It seems to me that such transformers do not have enough inductance margin, but that would not stop manufacturers rating their product as close to the line as they dare. This is consistent with the idea the EI cores have a bit of a gap that prevents saturation. A device that connects the transformer synchronously at the voltage peak may the best solution, although this is the opposite for power supply bulk capacitor charging.