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Author: Subject: Variac Heating Up
BaFuxa
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[*] posted on 1-12-2019 at 13:24
Variac Heating Up


Hi,

My variac is heating very quickly when I use it. I set it up to about 1KW and the control panels goes up to 100°C in about 5 minutes. What could be the cause of this ?

I assumed it was a difference in current, the variac outputs 8A from 0 to 250V but the current in AC systems is " infinite" so I ruled that out.

Any idea ?







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G-Coupled
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[*] posted on 1-12-2019 at 13:31


AC current is what now? o_0

What is the Variac rated at, and what are you powering with it?
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BaFuxa
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[*] posted on 2-12-2019 at 08:20


Ha. Yes I have read that on an electronics forum. I could not find any info as to how many amps flow from the mains.

Here the mains are 230V at 50 Hz. The variac output is 8A from 0 to 250V.
I am suspecting a resistance somewhere on the load.




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RedDwarf
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[*] posted on 2-12-2019 at 08:44


I would stop using your variac or any non-standard use of mains power until you have a better understanding of what you are doing. Very little of what you have said made any sense and you are likely to damage yourself, others or property if you continue.

Variacs control only voltage NOT current or power.
8A is the maximum current that your variac can safely handle.
The current is determined by the load (the device that you are powering).

I'm not trying to be rude, but you are high risk of serious injury or death if you continue without a better understanding of what you are doing.
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[*] posted on 2-12-2019 at 08:52


RedDwarf speaks sense - I would listen.

What is the load you're running with the Variac?
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BaFuxa
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[*] posted on 2-12-2019 at 10:31


@ RedDwarf : Thanks I appreciate the concern.
Variac used

I have used this variac for over a year to power small lab furnaces and I always had this heating problem but I lowered the output to keep it to a reasonable level.

I never had any accidents or problems with it because I am very concerned with safety. In fact I stopped immediately when I noticed the overheating.

The load is about 7 meters of 1mm thick nichrome wire. It glows yellow at around 1 Kw.







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[*] posted on 2-12-2019 at 11:19


So, how many volts are you hitting the NiChrome wire with to get it to glow yellow?
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[*] posted on 2-12-2019 at 11:20


What voltage do you have it set at when using the 7 m of 1 mm nichrome wire?
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BaFuxa
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[*] posted on 2-12-2019 at 13:12


125V



[Edited on 2-12-2019 by BaFuxa]




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[*] posted on 2-12-2019 at 14:06


I would put an ammeter in line and check the current. You may be pulling more than 8A.

Given the Chinese origins, its probably only barely capable of 8A, and it will get hot. In order to get the price down it is usual to use the least iron and copper possible, which means zero margin on the ratings.

[Edited on 2-12-2019 by Twospoons]




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[*] posted on 2-12-2019 at 14:14


Quote: Originally posted by BaFuxa  
125V


The VA scale on the variac indicates the maximum VA rating at that voltage. Its is not necessarily the power dissipated in the load connected to the variac.

As has already been said the variac principaly controls the voltage to what you connect it to, not the VA/power.

A quick look at a table of resistances of nichrome wire suggests your wire is about 10ohm in which case the current would be 12.5A which would about double the dissipation in the 8A variac and eventually over heat it.

You need to measure the current to the wire and keep it below the 8A maximum current of the variac. Yes it does look like you need to swot up on electrical theory.

You can buy a clamp on ammeter at ebay for about £7. Be careful using it too close to the variac as the magnetic field from the variac can make the reading very inaccurate.





clamps-l1600.jpg - 83kB

[Edited on 12/2/2019 by wg48temp9]




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[*] posted on 3-12-2019 at 09:45





[Edited on 3-12-2019 by BaFuxa]




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[*] posted on 3-12-2019 at 14:27


My (admittedly haphazard) calculations indicate that you'll be hitting the (optimistic) 8A rating of your Chineseum Variac when you dial it to around 75V.

It looks like you need a more powerful, higher rated Variac for the heater you're using at the moment to get the temperatures you want, and you should also brush up on Electronics 101 - Current, voltage, resistance, Ohm's Law, what is power etc. as well IMO. It's not that hard (easier than chemistry IME!), and it's well worth it.
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[*] posted on 3-12-2019 at 15:56


Or you could use thinner nichrome in your heater. This would increase the resistance, raising the voltage and lowering the current needed to get the same 1kW. With lower current the losses in the variac will be considerably lower.

Going from 1mm wire to 0.9mm wire will raise the resistance by ~23%, drop the current by 10%, and should drop the variac losses by about 20% (possibly more as the wiper will be in a different position).

[Edited on 4-12-2019 by Twospoons]




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