| Electronics > Projects, Designs, and Technical Stuff |
| How are variacs rated? |
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| najrao:
Basically autotransformer, the output current of a variac is just the sum of the currents in the two sections as divided by the brush position. With the brush at the exact mid point, each of the upper and lower sections would carry 1.0A for a 2A out. At lower or higher output voltage settings, the same 2A out will entail higher current than the 1.0A in the corresponding part of the winding. 2A into a short circuit would thus need to come out of the first turn or two, with the rest of the turns carrying little current. Almost all the heat comes from ohmic loss, so a "2A variac" if supplying rated output will need to be wound to take the full 2A throughout. Such a unit would happily source 4A at half voltage. None of the many variacs I use seem to be capable of the rated load away from the centre. Even the more reputed ones will exceed a current density of 15A/mm2, and will need to dissipate some 50W/m2K by surface cooling alone! Simple testing at low output voltage shows extreme heating of the end turns. So all variacs need to derate 50% straight! About the only way of mitigating this is to wind with wire of gradually reducing diameter towards the middle -- suggestion bordering on the preposterous. Wait a minute: the Japanese may yet find a way! Seen the almost perfect circular section of the tape wound "R-core"? |
| ArthurDent:
If a variac is rated 2A then no lead should see more than 2A. The common failure mode is to have the lower turns of the winding burn out because when you only have a 2A fuse on the input and the wiper is set toward the low end, a output short will allow a huge current to flow through the lower end of the winding. The way I've prevented this for years is to fuse both the input and the output wiper leads for 2A. Simple and cheap solution. |
| kony:
My recommendation is using motor overload relay on the output, breaking both live and neutral on fault (so using 3F model and looping the live trough twice). Fuse might react too slow and is not resetable, circuit breaker thermal trigger is way way too slow for this. |
| najrao:
The replies so far do not address the point raised: I did not seek a method of protecting the variac. I strongly urge a considered reply, on how to establish the true rating as against "commercial" rating assigned by makers. Or at least define under what conditions such given rating will apply. In practical usage, one can not work to reducing ratings as he twiddles the knob away from the centre. Thanks to kony and ArthurDent. |
| David Hess:
All of my variacs are unambiguously marked with two ratings. One is the maximum output current for any output voltage and the other is the maximum output power for each input configuration. The later is because most variacs can be configured for a maximum 1:1 ratio or 1.17:1 ratio to produce a 140 volt output from a 120 volt input. As pointed out, variacs must be protected using current limiting on the wiper lead. Otherwise a short at low output voltages will burn the windings before saturating the transformer or activating any current limiting on the primary side. |
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