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Snaxx
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Posted Sat Feb 16, 2013, 8:07pm
Subject: Re: Using a Variac to limit voltage drop
 

PJK Said:

Find out how much current the roaster draws and what the size of the breaker is.

Be careful there with the installed breaker size as a determining factor for safe or adequate installation.  There are no guarantees that the wire size for a 20 amp circuit (#12) is what is installed.  No way to tell that without determining the wire size all the way from the panel to the outlet, since over the years, anyone could have installed or added wiring to a house which may or may not meet code for the work done.  In the case of the OP, since he's dropping 8 volts to 114 Volt from 122 V, that's an excellent sign that the circuit is only a 15 amp. and correcting his installation to feed his roaster with a 20 amp circuit is the only safe or recommended solution.  

Ken
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PJK
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Posted Sun Feb 17, 2013, 1:26am
Subject: Re: Using a Variac to limit voltage drop
 

What Ken is saying is absolutely true and I am embarrassed that I was sloppy and did not mention to verify that the correct wire size is in place as well as the 20 amp breaker.

Phil


Snaxx Said:

PJK Said:



Be careful there with the installed breaker size as a determining factor for safe or adequate installation.  There are no guarantees that the wire size for a 20 amp circuit (#12) is what is installed.  No way to tell that without determining the wire size all the way from the panel to the outlet, since over the years, anyone could have installed or added wiring to a house which may or may not meet code for the work done.  In the case of the OP, since he's dropping 8 volts to 114 Volt from 122 V, that's an excellent sign that the circuit is only a 15 amp. and correcting his installation to feed his roaster with a 20 amp circuit is the only safe or recommended solution.  

Ken

Posted February 16, 2013 link


 
Philip J. Keleshian
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swines
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Posted Sun Feb 17, 2013, 8:40pm
Subject: Re: Using a Variac to limit voltage drop
 

That's only true if the design parameters are met by adequately supplied energy.  If you have a 2 KW heating device which has two 120 volt, 1 KW elements and can be multi-connected and operate on 120 volts if it's connected for 120 volts or 240 volts if it's connected for 240 volts, then the current draw for that heater is 16.7 amps at 120 volts and 8.3 amps at 240 volts.

You've proven my point.   As the voltage goes up 120 - 240 - the amperage draw goes down - making the reverse true also.  Voltage goes down - amperage goes up.  Same things happens at 120VAC outlet with a roaster.  As the voltage goes down - 120 to, let's say 114 VAC the amperage is going to go up.  I don't use a Kil-A-Watt meter, as I find them inaccurate.  

What I have with my roaster is a GenRad 0-140 VAC, 18 Amp, dual-metered Variac.  The roaster is a Diedrich HR1 that uses three, independently controlled IR heating elements.  On "Low,"  one element is turned on.  On "Medium: two elements are turned on; and on "High" three elements are turned on.

Using a Fluke multimeter on the second outlet of the duplex outlet to monitor what happens to the voltage, there is a nominal 4 volt drop between no heating elements and three heating elements (119 VAC to 115 VAC).  This has a significant effect on the efficency of the roaster as there is less energy going to the heating elements as the voltage drops.  

You can watch this on the dual-meter Variac as there is an Amp meter and a Volt meter.  As you switch from L to M to H, you can see the voltage output of the Variac drop - and, you can see the amperage draw increase proportionally.  As you bring the voltage back to 120 VAC using the Variac you can see the amperage going back down.  

Without the Variac, the roaster is difficult to control as the heater efficiency is changing in a non-linear response due to the voltage drop - and the resulting roaster heat control becomes unpredictable.

With the Variac, and the ability to bring the voltage back to 120 VAC, the roaster is highly controllable as the results are repeatable.

You can argue electrical theory all you want - but, the simple fact is that heating elements are unpredictable because they are a resistive load that is specific to the type of heating element.   Empirical data from 7 years of roasting with the HR1 shows that the ability to control the voltage to the roaster is a critical element in predictable results from roast-to-roast.

The original question was whether a variable transformer could help with and electrically powered roaster - and, my answer is yes based upon my experience with the HR1, my Variac, and my house wiring.

At my house, I know exactly the wire size going to all of the outlets.  The 15 Amp outlets are wired with 12 gage and the 20 Amp outlets are wired with 10 gage.  I had this done when the house was built, so that there was a built-in safety factor in the wiring.

The HR1 runs from a 15 Amp circuit, and there is one other appliance on that circuit - the refrigerator.  To date, we have had no problems with running both simultaneously as the amperage draw never exceeds the 15 amp circuit breaker.
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PJK
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Posted Mon Feb 18, 2013, 11:16am
Subject: Re: Using a Variac to limit voltage drop
 

Has someone informed Mr. Ohm about this?

Poor fellow died thinking :  Current = Voltage / Resistance  

Phil

You've proven my point.   As the voltage goes up 120 - 240 - the amperage draw goes down - making the reverse true also.  Voltage goes down - amperage goes up.  Same things happens at 120VAC outlet with a roaster.  As the voltage goes down - 120 to, let's say 114 VAC the amperage is going to go up.

 
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kentj
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Posted Sat Mar 9, 2013, 12:32pm
Subject: Re: Using a Variac to limit voltage drop
 

Hi All,
I have a friend that has a dedicated 20 amp line so I wound up taking my Behmore there.   I roasted ╝ pound with settings of Ż lb profile P3 A.  The line voltage varied from 121.7 volts to 116 volts while the Behmore was cycling.  The roast time went from 11:45 min down to 9:30 min.  The roast tasted much better.  Roasting on my line with the voltage drop down to 111.2 volts gave the beans a baked flavor.

My friend actually read the manual more thoroughly than I did.  (Reading manuals is not my strong point.)  He showed me where Behmore suggests preheating the roaster.  I took my roaster home and got a 1st crack time of 10:10min.  The roast tasted much better and for now I am very happy.  It seems that in my case with a bigger voltage drop it is very important to preheat the roaster.

Based on the replies I got will probably buy a variac latter but due to $ constraints I will wait awhile.  I now have more information to make an informed decision when I do purchase a variac.

Oh well live and learn.  Next time I hope I read the manual carefully.  I hope this thread helps others with low voltage line problems.

Thank You for all the input.
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Posted Sat Mar 9, 2013, 12:51pm
Subject: Re: Using a Variac to limit voltage drop
 

I direct wired a dedicated 20a breaker with 10/g wire instead of 12guage wire to a 15a duplex outlet to run 2 behmors back to back. my voltage on either doesnt drop below 119v. Idle state is 121v. power will fux in waves , thicker wire will help reduce flux, runnung 2 appliances off 20a breaker to 1 15a receptical is up to code because it runs 2 behmors. cheaper to run a direct line than find and buy a good variac

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rgrosz
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Posted Sat Mar 16, 2013, 7:38am
Subject: Re: Using a Variac to limit voltage drop
 

kentj Said:

Can I use a variac to adjust the voltage up to 118V AFTER  the roast starts?  I assume this means the voltage will jump to about  124 volts when the roster is idle but drop to 118V when the roast cycle starts or is in on ľ off mode during the roast.  I am afraid to do this if it will damage my roaster.  I appreciate any advice.

Posted February 9, 2013 link

I also had low voltage problems, which I struggled with for 2 years. In the summer, I had to roast in the morning, before all the air conditioners started up.

I finally broke down and bought a variac last year. I have used it to roast with both a Behmor and a Hottop. It definitely improved the quality of my roasts. The main difference is that I can roast any time of the day or night.

As you mention, if you dial up the voltage during the roast, you MUST dial it back down before the roast ends. Most people reduce power when first crack starts - this works fine for me.

 
Life is too short to drink bad wine (or bad coffee)
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Posted Mon Mar 18, 2013, 1:41pm
Subject: Re: Using a Variac to limit voltage drop
 

Electrical theory and Mr. Ohm's Law aside, I've been using an 18 amp Variac (actual trademarked General Radio Corporation variable AC transformer) for the past 4+ years with my Behmor 1600. As has been stated, have a way of monitoring voltage (my Kill-a-Watt has been sufficient for roasting) and ride the control as the roast progresses and the heaters and afterburner kick on and off. I tend to aim for 118v as operating average and it's been working fineŚno fires, no overheated wiring, no sparks or arcing.

I would go of the safety cushion of a transformer that's rated somewhat higher than the current you're expecting to draw. I like a 20% fudge factor in such things, hence the 18 amp unit.

Since a 15 amp circuit can, in theory, carry 1800 watts (watts=volts x amps according to Ohm and 120x15=1800) and 12 gauge wire is conservatively rated at 18 amps, I figure I'm safe.
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PJK
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Posted Tue Mar 19, 2013, 12:06pm
Subject: Re: Using a Variac to limit voltage drop
 

If you attempt to put electrical theory and Mr. Ohm's law aside, you may be doing so at your peril.

Phil


GVDub Said:

Electrical theory and Mr. Ohm's Law aside, I've been using an 18 amp Variac (actual trademarked General Radio Corporation variableatte AC transformer) for the past 4+ years with my Behmor 1600. As has been stated, have a way of monitoring voltage (my Kill-a-Watt has been sufficient for roasting) and ride the control as the roast progresses and the heaters and afterburner kick on and off. I tend to aim for 118v as operating average and it's been working fineŚno fires, no overheated wiring, no sparks or arcing.

I would go of the safety cushion of a transformer that's rated somewhat higher than the current you're expecting to draw. I like a 20% fudge factor in such things, hence the 18 amp unit.

Since a 15 amp circuit can, in theory, carry 1800 watts (watts=volts x amps according to Ohm and 120x15=1800) and 12 gauge wire is conservatively rated at 18 amps, I figure I'm safe.

Posted March 18, 2013 link


 
Philip J. Keleshian
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BLrdFX
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Posted Sun Mar 31, 2013, 6:34am
Subject: Re: Using a Variac to limit voltage drop
 

swines Said:

Not quite accurate.  When the voltage goes down, the amperage goes up.  So by boosting the voltage, you can lower that amp draw.  Heating elements are reactive to voltage drop as several interelated issues are going on.  When the voltage drops, the electrical power factor goes down (a bit complicated - but, basically frequency and votage level).  

If you think of AC current as a series of sine waves, the wave peak is smaller at a lower voltage so there is less electrical energy available.  Some of the electrical energy is momentarily converted and made into a magnetic field around the heating element, while some of the energy is converted to heat because of resistance and the remainder of the energy passes through the heating element (completes the circuit).  You lose electrical energy through lower voltage as the power factor goes down causing less heat to be generated - NOT because of amperage draw.  The major factor for energy loss that affects a heating element is NOT amperage (current flow of electricity), but voltage (volume / potential energy) of electricity.

With a heating element, you are not lacking amperage - but voltage (sine wave height / volume / potential energy).    The increase in voltage from a high quality variable transformer will actually lower the amperage draw to an electrical heating element.  



A generalized metaphor for voltage versus amperage - but, totally inaplicable to how a heating element interacts with electricity.

As an example, the voltage onboard a commercial aircraft is 400 Hz, specifically because there is more energy available at a given voltage level because of the higher frequency, giving a greater power factor because of the higher freqency.

The opposite of that is an arc welder that uses relatively low voltage, but high current to create the energy needed to heat the metal and melt the metal and filler material.   In this case, you need enough current flow to continuously jump a gap between the electrode and metal.  Yet, in tungsten inert gas (TIG) welding you generally have high frequency AC current momentarily inserted into the current flow to start the arc and eliminate the need to scratch start (lift TIG) the electrode to start it - specfically because the high frequency AC current has MORE energy available to start the arc.

The point is that electricity usage and requirements are highly variable depending upon the device being powered and the water hose analogy can't be applied accurately to all circumstances.

Posted February 10, 2013 link

Excellent discussion!  Since I have the opportunity to get an HR-1 and you are using the variac method of voltage control you have caught my attention!!

BTW, please email me with your thoughts on the HR-1, I would really appreciate it.
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