How are variacs rated? - Page 1
Forum section: electronics / projects

author: najrao
author_rank: Regular Contributor
timestamp: March 02, 2019, 02:52:39 pm
content: 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"?

author: ArthurDent
author_rank: Super Contributor
timestamp: March 02, 2019, 03:50:23 pm
content: 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.

author: kony
author_rank: Regular Contributor
timestamp: March 02, 2019, 04:07:22 pm
content: 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.

author: najrao
author_rank: Regular Contributor
timestamp: March 03, 2019, 01:58:16 am
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: March 03, 2019, 04:56:14 am
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: March 03, 2019, 10:46:13 am
content: Quote from: najrao on March 03, 2019, 01:58:16 amI doubt the brush will be designed for more than the nameplate rating, so a 2A Variac will have a maximum brush contact rating of 2A, even if the windings are only carrying half that when it's set to the centre position. As with any piece of equipment, RTFM before using it.

author: najrao
author_rank: Regular Contributor
timestamp: March 03, 2019, 01:42:09 pm
content: Thank you David Hess and Zero999.Yes, the only US made one I have (Staco Energy) does show a maximum input rating, in case 0.25kVA. Also the maximum output current, 2A. Still, I must submit that 2A in any part of the winding would severely overheat that part. (I remember reading in the Staco spec that the full rating is for a unit mounted to a large aluminium plate as heat sink, but even then the single layer winding can hardly conduct all that heat along the thin wire all to the one mounting face).I would prefer to arrive at a rating based on the temperature not exceeding 125C in a local ambient of 40C, importantly on any part of the winding irrespective of brush position.The situation is worse with other units.I should think brush current density is the least of the designer's problems in any variac.

author: Wolfram
author_rank: Frequent Contributor
timestamp: March 03, 2019, 02:15:06 pm
content: Quote from: najrao on March 03, 2019, 01:42:09 pmThat is a pretty strong assertion. What do you base this statement on, thermal simulations or actual measurements?At different brush positions, the distribution of dissipation between the two halves of the winding is different, but the core still couples the whole winding together thermally. At the extremes of the adjustment range, part of the winding carries a disproportionate amount of the dissipation, but the rest of the winding (which dissipates proportionally less) helps dissipate that power into the mounting panel and the surrounding air.

author: Gyro
author_rank: Super Contributor
timestamp: March 03, 2019, 02:34:08 pm
content: Quote from: najrao on March 03, 2019, 01:42:09 pmWhy?The carbon brush bulk resistance must be high enough to bridge at least two adjacent turns of the winding without unacceptable losses. This limits it's 'wanted' current capability too. It's making spring contact with a relatively small patch of the winding wires, not the entire diameter. Resistive heating is a significant factor for the wiper (and the winding turns underneath it!)If you look at most wirewound potentiometers too, the same factor applies - the wiper current rating is less than the track current rating. That's down to contact resistance capability.For a 2A variac, 2A is the safe maximum wiper (brush) current.

author: David Hess
author_rank: Super Contributor
timestamp: March 03, 2019, 03:46:49 pm
content: Quote from: najrao on March 03, 2019, 01:42:09 pmThat is already taken into account to arrive at the specification. The output will support 2 amps at *any* wiper position with any input voltage below the maximum voltage rating.In practice as you identified, when the wiper is at 50% the current will be distributed through the winding evenly so the current rating should be higher and further, the rated current will only use 1/2 of the power rating so twice the current should be available but such a specification would just lead to confusion (and blown up variacs) so to keep it simple, a single maximum wiper current rating is given.

author: najrao
author_rank: Regular Contributor
timestamp: March 04, 2019, 02:38:45 am
content: Thanks all, I need a few days to come back. Please bear with me.

author: najrao
author_rank: Regular Contributor
timestamp: March 04, 2019, 08:05:10 am
content: I am back sooner.David Hess, that would be any user's interpretation, and indeed should be so. I agree with you entirely.Wolfram, yes sir it appears to be a strong assertion, but I do have long experience to qualify. In doing it, I admit to being guilty of overlooking heat dispersion through the core, which you have wisely brought my attention to. Since modelling for this is well nigh impossible, I have just conducted a test to see how I can correct myself for the mental lapse.This Staco 125VA 2A 120/132V variac has a winding tap at the top end, to offer a small step-up. I simply set up a a constant current of 1.50A dc in this 'overwinding' for 90 minutes. The main part of the winding was left open altogether. (I chose not to make any connection to the wiper brush, because of uncertainty of measuring resistance through it). Measuring the hot and 'cold' resistance showed a temperature rise of just over 68C over a local ambient of 27C. The unexcited part of the winding was barely warm. I did not stretch this to a heat run at 2A for fear of magic smoke.The extrapolated temperature rise for 2.0A would be 120C over 40. Much too high.The suggested aluminium plate heat sink was not attached during my test, and should give some relief, my guesstimate is 100C over 40. This may be barely acceptable if the wire, and all adjacent insulation, can be classified as F.It was never my purpose to apply higher loads while operating at mid voltages; on the contrary, it would have pleased me to be able to use the full rating in amperes over the whole range of settings. I now know what happens!'Generic' variacs may not be very different: however low one is hung, the one has to use the same copper wire; and class F insulation is not hard to come by. I will do a simulated heat run on a couple more variacs.Gyro, you are right too, the extra heat at the brush contact is highly localized and can cause a hot spot. Fortunately, the brush is gripped in a massive block of brass to heat sink it. Hopefully, heat which gets on to the winding track will spread out well enough through the copper. Please suggest a good practical way of testing what margin if any exists here.Thank you all once again.

author: Gyro
author_rank: Super Contributor
timestamp: March 04, 2019, 12:19:26 pm
content: Quote from: najrao on March 04, 2019, 08:05:10 amI know. Unfortunately the contact patch is very small and as you say, highly localised. Carbon brushes aren't great thermal conductors and thinned down at the tip to minimise the number of turns bridged. The swept area of the winding tends to be ground smooth but it is a compromise between maximising flat contact area and thinning the wire too much. Heat transfer along the length of the wire and to theinsulatedcore are probably lower than you think too.Traditional temperature measurement techniques (thermocouple etc) are impractical on such a small contact patch. A thermal camera might yield some useful results. In practical terms, the first indication of overheating will be a discolouration of the copper surface under the brush (oxidised copper will further increase contact resistance).

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 04, 2019, 01:20:05 pm
content: Go to the people that invented them! There are various articles in the GR Experimenters, but December '55 is a good one-https://www.ietlabs.com/pdf/GR_Experimenters/1955/GenRad_Experimenter_Dec_1955.pdf