17-12-2011, 09:40 AM
I'd want to measure the magnetising current at a range of different input voltages - for this I use a variac. I'm less experienced with transformers as big as this, but certainly with the smaller transformers, they are designed "on the limit". In other words, at their rated input voltage, the core is at the cusp of saturation, and just a few volts above this causes a rise in magnetising current (often accompanied by a lot of mechanical noise). This is less of an issue for a small transformer (say, <100VA) because of the resistance of the primary windings, but when you get to around 500VA, this effect is very pronounced. Especially with torroidal transformers.
A while back I had a torroid custom-made for me by Antrim, and because it was for an audio amplifier, I wanted to avoid all of this. So, to avoid saturation with normal mains voltages, they upped the size of the core. It meant that the 400VA rated device was the same physical size as a normal 500VA transformer. There are no free lunches...
Another issue. Just because our mains is nominally 230V, it doesn't mean it is. It's more like 245V around here. As you no-doubt know, so-called "harmonisation" just changed the tolerances so that our 240V and continental 220V all met the "230V" spec. No-one changed any actual voltages...
So, given that saturation is fairly abrupt, especially with torroidal transformers (I have much more experience with these), I would be cautious about a 220V transformer - at least around here. So, if you have access to a variac, measuring the magnetising current at a range of input voltages would be my recommendation. A good 500VA torroid would be 10s of milliamps when not saturating. This, BTW, is why people are suggesting you monitor the operating temperature, which is the next best thing in the absence of a variac.
Depending on the construction of the transformer, you might be able to use that 22V winding to make a 242V primary. It means the transformer will step down slightly, but will run cooler and more quietly as a result, and to be honest, few bits of gear will mind running at 220V... If it really bothered you, the second transformer could be used to step back up. And being "double-isolated" in this way would half any leakage currents.
In practice, such big transformers are of limited use in an electronic workshop. For safety reasons, only the DUT (device under test) should be powered from an isolation transformer. Note the use of singular - only one item should be powered from the transformer. For that reason, I have a number of 500VA units (some "fixed", some portable) - these are big enough for most things.
Also, don't carry the earth through to the isolated outlet. And always keep the test gear earthed.
Sorry if I'm stating the obvious, but whenever the topic of isolation transformers comes up on forums, there is a lot of "mixed" advice offered. My advice aligns with that of my employer - and I should add that the current advice is to not use isolation transformers unless you have a really good reason to do so - from a safety point of view, an RCD-protected outlet is better. Isolation transformers are really only needed when working on the live side of switched-mode power supplies (or TV sets that have a live chassis - the pre-Scart sets). Indeed, isolation transformers often endow the user with a false sense of security, making them almost as hazardous as not having one at all
All the best,
Mark
A while back I had a torroid custom-made for me by Antrim, and because it was for an audio amplifier, I wanted to avoid all of this. So, to avoid saturation with normal mains voltages, they upped the size of the core. It meant that the 400VA rated device was the same physical size as a normal 500VA transformer. There are no free lunches...
Another issue. Just because our mains is nominally 230V, it doesn't mean it is. It's more like 245V around here. As you no-doubt know, so-called "harmonisation" just changed the tolerances so that our 240V and continental 220V all met the "230V" spec. No-one changed any actual voltages...
So, given that saturation is fairly abrupt, especially with torroidal transformers (I have much more experience with these), I would be cautious about a 220V transformer - at least around here. So, if you have access to a variac, measuring the magnetising current at a range of input voltages would be my recommendation. A good 500VA torroid would be 10s of milliamps when not saturating. This, BTW, is why people are suggesting you monitor the operating temperature, which is the next best thing in the absence of a variac.
Depending on the construction of the transformer, you might be able to use that 22V winding to make a 242V primary. It means the transformer will step down slightly, but will run cooler and more quietly as a result, and to be honest, few bits of gear will mind running at 220V... If it really bothered you, the second transformer could be used to step back up. And being "double-isolated" in this way would half any leakage currents.
In practice, such big transformers are of limited use in an electronic workshop. For safety reasons, only the DUT (device under test) should be powered from an isolation transformer. Note the use of singular - only one item should be powered from the transformer. For that reason, I have a number of 500VA units (some "fixed", some portable) - these are big enough for most things.
Also, don't carry the earth through to the isolated outlet. And always keep the test gear earthed.
Sorry if I'm stating the obvious, but whenever the topic of isolation transformers comes up on forums, there is a lot of "mixed" advice offered. My advice aligns with that of my employer - and I should add that the current advice is to not use isolation transformers unless you have a really good reason to do so - from a safety point of view, an RCD-protected outlet is better. Isolation transformers are really only needed when working on the live side of switched-mode power supplies (or TV sets that have a live chassis - the pre-Scart sets). Indeed, isolation transformers often endow the user with a false sense of security, making them almost as hazardous as not having one at all
All the best,
Mark
