Why use a Variac?

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
cinergi
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Postby cinergi » Fri Sep 12, 2008 3:36 pm

I just received the red 20 amp variac I ordered on ebay. My assumption regarding their purpose was that you dialed in the voltage you wanted (i.e. 122v or 125v) then plugged in your roaster (I have a new Hottop B) and the variac keeps the voltage at the same level throughout the roast even though the fan and element are changing levels.

This appears not to be the case, at least in my situation, because after setting voltage to 122.0v via a digital multi-meter plugged into the extra outlet on the variac, voltage dropped to 118 upon starting the roast cycle then rose a little bit as the element level was turned down.

So, other than for people who have low starting voltages, why would you ever use a variac?

Doug

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another_jim
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Postby another_jim » Fri Sep 12, 2008 4:18 pm

I'm sure you know most of this, but readers sometimes misunderstand ...

A variac is a variable transformer, it multiplies the incoming voltage by the amount dialed in. If the incoming voltage varies, the variac's output will vary as well. So it does not fix variable service voltage problems like brown outs.

Some people use it to boost chronic low voltage. But this can be quite dangerous if the voltage is normal at the service entrance and low at the roaster location. An outlet which drops 4 or 5 volts when you plug in a high amperage device is a common and very major fire hazard. If the building is more than 50 years old, it could be ancient electrical code 14 gauge wiring dropping voltage along the entire run and heating up gently and non-dangerously; but in any building less than 50 years old, it'll be a single point wiring flaw behind a wall somewhere that is seriously overheating whenever you plug in the load. For this problem, an electrician will contribute far more to your long term coffee enjoyment than a variac.

Typically, a variac is used as a profiling controller. You would hack your roaster and tie it into the heater circuit only to either boost the heat a bit or turn it down. This will allow you to manually profile the roast, similar to twiddling the valve on a commercial gas roaster.

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Randy G.
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Postby Randy G. » Fri Sep 12, 2008 4:33 pm

What was the line voltage to begin with. iirc, anything over about 112 and the Hottop (at least the more recent versions) does not really need a variac.

Interesting that you ask this just now.. The local electrical utility service person just about an hour ago. He was here because.....

I recently bought a Kill-A-Watt meter off eBay. I wanted it to be able to judge things like... coincidentally enough, the wattage drawn by the Hottop B to see how it was controlling heating element levels. I plugged it in, and was a bit surprised to see the voltage on my line. For years, every time I measured it, it read 122 volts without a load, +/- a few tenths. The Kill-A-Watt showed 128.5 +/-. "Wow! That's a lot of volts," I thought to myself. I figured that there was something wrong with the Kill-A-Watt, so I whipped out my Fluke voltmeter, and it showed the same, within abut 0.5 volts.

I was a bit concerned about my appliances, and a bit paranoid because we once we had a problem with our underground cable after the phone people drove a ground rod through on side of it. We were getting about 140 volts on one side and 90 on the other. The lights in the kitchen were really bright, though!

[and just in the middle of that last paragraph the electricity for the whole house went off! Thanks to a UPS, I saved what I had typed and shut down... I immediately called the utility's outage phone message line to report it, and the recording said they were aware of the difficulty which started one hour earlier {!}.]

As soon as the electricity came back on, I plugged in the Kill-A-Watt, and what do you know!? 121 volts!

I wonder how long it was too high, and I wonder to what extent it shortened the life of some of my appliances...?
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cinergi
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Postby cinergi » Fri Sep 12, 2008 5:03 pm

The outlet where I plug the Variac into reads 123.5v with nothing plugged into it. It reads this level day after day no matter what time I test it. When I plug in the Hottop B and start the pre-heat cycle the multi-meter which is plugged into the other outlet reads 120.5v. So, a drop of 3v. If the element gets turned down during the roast the voltage rises back closer to the 123.5v starting point.

I was trying to match the other Doug's (Fullsack) roast profile and was told that if my roasts were taking longer it might be due to not having 122v. So, I thought getting a variac and tuning it to 122v would enable me to match his roast times. It didn't. And even if the variac maintained the 122v, I'm not sure I would be able to get the same times because I suspect that there are other factors that affect roasting times like ambient temperature, the individual hottop, the filter, etc.

So, I'm confused. Is Fullsack's voltage staying at 122v via the variac no matter what level his hottop B element is at? Or, is he just using the variac to get to a 122v starting level and then it naturally drops a little due to the draw from the hottop?

Doug

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Randy G.
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Postby Randy G. » Fri Sep 12, 2008 5:49 pm

You will have to ask fullsack the details about his use.

The B model does vary its power consumption on different levels of heater element setting, roughly are as follows:
HEATER WATT
LEVEL CONSUMPTION
0 24
10 57
20 125
30 211
40 300
50 388
60 481
70 572
80 654
90 721
100 756

I have not taken voltage variation figures, but could do that... I do not own a variac and never felt the need to do so, so cannot help there either.

And as you mentioned, there are lot of factors that affect roast level other than voltage.
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cinergi
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Postby cinergi » Fri Sep 12, 2008 6:16 pm

hopefully, fullsack will respond to this thread. From what Jim is saying, it sounds like the fact that our line voltage is dropping 2-3 volts when we plug in a high voltage appliance, we may have a wiring problem somewhere. Or, is it totally normal for line voltage to drop 2-3 volts but not 4-5 volts?

It also sounds like he might be saying that a variac is designed to maintain voltage even when you plug in an appliance assuming your electrical system is modern and wired properly.

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Postby another_jim » Fri Sep 12, 2008 7:02 pm

Please reread my post: a variac simply multiplies the input voltage. If you set it at 135, it multiplies the voltage by 135/120. if you set it to 77, it multiplies it by 77/120. If the input voltage changes, so does the variac's output.

Wiring has resistance, so you will always get some voltage drop at the end of longer wiring runs. You can check if your voltage drop is normal using a voltage drop calculator. You will need to know the distance from the fusebox, the wire gauge (either 10 or 12), and the wattage or amperage of your roaster. If the drop (measuring when the appliance is off and then on) is larger than the calculation, you have a problem somewhere, and you shouldn't use that outlet for large loads.

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Randy G.
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Postby Randy G. » Fri Sep 12, 2008 8:45 pm

Image
I just completed this chart for your reference. My current line voltage is about 121 now. Figures are close but fluctuations during the process makes it hard to be precise.
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another_jim
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Postby another_jim » Fri Sep 12, 2008 9:46 pm

That calculates out at 175 feet for 12 gauge or 280 feet (one way) for 10 gauge wire. If you are closer to the fuse box then that, you may have a problem (if you are measuring at the unit, i.e. including the unit's power cord, add about three times that length, since it's probably 14 or 16 gauge)

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Postby cafeIKE » Fri Sep 12, 2008 9:57 pm

another_jim wrote:If the drop (measuring when the appliance is off and then on) is larger than the calculation, you have a problem somewhere, and you shouldn't use that outlet for large loads.
One factor missing in the calculator is the number of legs. It's typical for house wiring to daisy chain several outlets on a single circuit. Each of these connections drops a fraction of a volt. The NEC recommended maximum is 5%, half that is probably the outer limit acceptable. A HotTop is only half a 15A maximum circuit load, so under 2% should be maximum. FAQ - Voltage Drop