Hi,
do you think a 3kW isolation transformer needs a pair of SL32 Metal Oxide Varistors on the input Line or is it superfluous?
Is there a better component than the SL32 MOV if protection during start-up is mandatory?
Thanks a lot!
Gianluca
do you think a 3kW isolation transformer needs a pair of SL32 Metal Oxide Varistors on the input Line or is it superfluous?
Is there a better component than the SL32 MOV if protection during start-up is mandatory?
Thanks a lot!
Gianluca
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Here any transformer over 500VA needs inrush protection. We agreed on this to keep mains voltage quality OK. It works.
BTW it is kiloWatt so kW.
BTW it is kiloWatt so kW.
Sorry for the capital K, I tried to edit the subject but I can't.
So a metal oxide varistor doesn't impose any dynamic bottleneck once "open". That was my only concern.
Thank you!
Gianluca
Do you think they are delicate and short-lived components?
Don't you think they're necessary for the inrush protection of the transformer during startup?
Varistor is a voltage clamp. Not "inrush".
Put a NTC resistor of the proper rating.
They are good as inrush current limiters.
But if the transformer has varying loads, read up about it.
Soft start circuits may also be considered. Similar to a Variac, but solid state.
What that does to the equipment connected to the isolation transformer is to be thought of carefully.
They are good as inrush current limiters.
But if the transformer has varying loads, read up about it.
Soft start circuits may also be considered. Similar to a Variac, but solid state.
What that does to the equipment connected to the isolation transformer is to be thought of carefully.
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You must do something or your fuses will not last long. I recently opened up a few single phase medical 2 kW isolation transformers with inrush protection but I did not make pictures. A few had electronic protection circuits ("Slow start") and some older versions just had a large NTC. None of them tripped my circuit breakers at home, this would have been impossible without inrush protection and class B 16A circuit breakers. No chance that you can get away without inrush protection in a normal home with normal breakers/fuses, even without any load. The plusses of a simple inrush protection outweigh the few (if any) serious drawbacks. If a non-linear component in the mains connection seems non audiophile to you the electronic version may be of interest. These have a relay bridging resistors/NTCs after a few seconds. My solution is to use transformers below 500VA that don't need inrush protection and these are 99% silent as well. The KISS principle always works!
As PRR indicated there is some confusion. There is line protection and there is inrush protection. The latter is mandatory in the EU above 500 VA.
Those that know these devices automatically think of inrush protection as that is standard but your question is about MOVs. Please make it clear what you want to know, line protection (overvoltage) or inrush protection (current). Or both 🙂
Before that one is suggested: never use zero crossing SSRs for this.
As PRR indicated there is some confusion. There is line protection and there is inrush protection. The latter is mandatory in the EU above 500 VA.
Those that know these devices automatically think of inrush protection as that is standard but your question is about MOVs. Please make it clear what you want to know, line protection (overvoltage) or inrush protection (current). Or both 🙂
Before that one is suggested: never use zero crossing SSRs for this.
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Actually, the manufacturer of my 3kW isolation transformer recommends two NTC SL32 in parallel on the input AC phase, immediately afterwards a 15A fuse. I therefore equated MOV and NTC probably wrongly.
The recommended NTC is the following : https://it.farnell.com/en-IT/ametherm/sl32-2r025/thermistor-series-sl32/dp/1653462
My question on the forum was about whether this solution (2x NTC SL32's) is the best for the inrush current problem at switch-on without sacrificing sound quality.
Regulations aside, I have read that the inrush current at start-up on such a large transformer can cause distortion and damage the components connected to the transformer.
Thank you!
Gianluca
Yes I think I understood correctly despite the "MOV". Please follow manufacturers recommendations. Don't worry about sacrificing sound quality and if you do please use a lower rated transformer (easier).
I don't know nor can I explain why the load would be damaged?! It won't happen as the fuses/breakers already would have melted/tripped. IF they would not melt/trip they will next time. Even without connected load the transformer will put wiring, switches at the primary side in stress (when no inrush protection is used).
I don't know nor can I explain why the load would be damaged?! It won't happen as the fuses/breakers already would have melted/tripped. IF they would not melt/trip they will next time. Even without connected load the transformer will put wiring, switches at the primary side in stress (when no inrush protection is used).
This is exceptionally clear.
I've found this on the web :
Problematic Effects of Transformer Inrush Current
Inrush current in a transformer can cause several problems. Not only does it interfere with the operation of circuits, but it could result in detrimental effects to the transformer. The distortion of the volt or current waveform, known as harmonics, is another side effect of inrush current. If not properly managed, inrush current could lead to failure of circuit components, shorten the operating life of the transformer, or even cause damage.
Let’s look at the figure below. If the AC wave is going through its zero value, the current drawn will be very high and exceed the saturation current (Figure 1). In this situation, transformer inrush current protection becomes necessary to keep the transformer functioning properly.
Transformer inrush current wave
Figure 1: A transformer draws inrush current that can exceed saturation current
Gianluca
I work in the machine tool industry and they all use control transformers, some fairly large. I have never seen any inrush protection on control transformers and it's extremely rare to have one fail. Many of these machines are turned on every morning at the start of the day, that's almost 300 cycles a year.
BillWojo
BillWojo
Control transformers actually have a Standard and it is quite conservative. Because guys like your cohorts are not going to baby their transformers.
IMHO, a well-made transformer is about the most reliable part around. (Poorly made transformers, you get what you pay for, or less.) Turning it on all day long will not hurt.
If a power authority wants reduced surge on big loads, that is understandable. In the US, motors above fractional-horsepower have agreed starting current limits. I'm supposed to tell the utility if I have an electric welder, and desist if they say I can't (nobody does either).
BTW: I have just found the worst power-factor ever. The controller for an old electric blanket draws 0.5 Watts but 5.9 VA.
A control transformer, if intended for the controller, will rarely go above 5 kVA in rating.
That apart, it is matched to the load, and has protection circuits as part of the machine.
And the makers know it will be roughly used.
So they are built rugged, and nobody will quibble for a more expensive but RELIABLE transformer, it is used in expensive machinery, CNC are not cheap.
But I was thinking, what kind of audio setup needs a 3 kVA isolation transformer, the load for most home setups is a lot less...
That apart, it is matched to the load, and has protection circuits as part of the machine.
And the makers know it will be roughly used.
So they are built rugged, and nobody will quibble for a more expensive but RELIABLE transformer, it is used in expensive machinery, CNC are not cheap.
But I was thinking, what kind of audio setup needs a 3 kVA isolation transformer, the load for most home setups is a lot less...
I've written quite a bit on the topic of soft starts and how to design a good one. You can read my thoughts here: https://neurochrome.com/pages/the-ultimate-guide-to-soft-start-design
I definitely support the use of an NTC-based soft start with a 3 kW power transformer. I'd say 200-300 VA is about where I start recommending a soft start. Some push it a bit further.
Tom
I definitely support the use of an NTC-based soft start with a 3 kW power transformer. I'd say 200-300 VA is about where I start recommending a soft start. Some push it a bit further.
Tom
Here it is forced by regulations to have a reliable grid. Even pumps for home central heating have mandatory slow start.
MOVs protect against voltage peaks, not current.
So they protect against lighting on transmission lines, or overvoltage peaks when switching OFF, the exact opposite situation of what you are asking.
Again,you got it the exact opposite way: they and normally open, all the time, until triggered by overvoltage in which case they work like an momentary short.
To be more precise: they are across the transformer input, not in series with it.
What jean-paul said in post #9 and others as early as post #6
You are confusing labels and technology 🙂
Nope, none needed.
True story: I bought this 2.4KVA 20kg monster for 10 bucks, from a barn. For over a year, it's been working fine, no buzz, no switching transient, no funny stuff required neither before nor after it. That is, I only have about 500VA of gear hooked to it.
Of course, if you feed a big iron macho amp, you need to pay some attention. But then again, it's reasonable to assume the inrush current is already dealt with, or you wouldn't be able to plug it in a regular AC socket. If you really have to use NTC's, I'd strongly suggest you add a relay to short them out after a bit.
Btw, before anyone complains, the caps at the far left came with the trafo. I replaced them with the proper polypropylene caps.
True story: I bought this 2.4KVA 20kg monster for 10 bucks, from a barn. For over a year, it's been working fine, no buzz, no switching transient, no funny stuff required neither before nor after it. That is, I only have about 500VA of gear hooked to it.
Of course, if you feed a big iron macho amp, you need to pay some attention. But then again, it's reasonable to assume the inrush current is already dealt with, or you wouldn't be able to plug it in a regular AC socket. If you really have to use NTC's, I'd strongly suggest you add a relay to short them out after a bit.
Btw, before anyone complains, the caps at the far left came with the trafo. I replaced them with the proper polypropylene caps.