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kentj
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Posted Sat Feb 9, 2013, 1:55pm
Subject: Using a Variac to limit voltage drop
 

I realize the may already be answered but I will ask anyway.

I own a Behmore 1600 which I love but I live in a 26 year old town house.  I suspect the wiring is not very good.  I only have one outlet I can roast from due to unacceptable smoke and smell as dictated by my wife.   Using a Kill A Watt meter the voltage drops from 122V to 114V when the Behmore enters the roast cycle.  When I roast 4.0 oz. of coffee on  P3 it takes almost 11 minutes to reach first crack with the Behmore set to roast 1 lb.  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 off mode during the roast.  I am afraid to do this if it will damage my roaster.  I appreciate any advice.

Thank You.
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Prof
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Posted Sat Feb 9, 2013, 8:31pm
Subject: Re: Using a Variac to limit voltage drop
 

My Variac (Powerstat) blew its 10A fuse when I used it with the Behmor.  You would need a more powerful variac than mine.  If you use an extension cord, make sure it is 12 gauge or smaller (smaller gauge is thicker wire for some reason, probably resistance).  

Your voltage drop, without extension cord, would seem to indicate wiring that could be improved.  Perhaps an electrician could get you a dedicated 15A or 20A line.

If the Behmor stays at 117V or above under full load, then your roasts should be fine and done in sufficient time.

 
LMWDP # 010
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oldgearhead
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Posted Sun Feb 10, 2013, 7:21am
Subject: Re: Using a Variac to limit voltage drop
 

There are three types of varible auto-transformers:
1) The type that will not boost the voltage.
2) The type that will boost it only 10%.
3) The type that will boost it 17%

You will require the 2.2 KVA version of type 3. Also you should purchase one that has a voltmeter attached to the output side, so you can 'ride' the dial
during a roast.

I believe a Kill-A-watt is only rated for 1550 watts. Better check, because you might not be getting correct readings, and/or, the Kill-A-Watt may be
limiting your roaster....

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oldgearhead
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Posted Sun Feb 10, 2013, 8:26am
Subject: Re: Using a Variac to limit voltage drop
 

...in other words, if the rating on the Kill-A-Watt is, in fact, 1550 watts, then don't plug the roaster into the Kill-A-Watt.
However, you could still use the Kill-A-Watt to monitor the line voltage by plugging it into an outlet (or cord) that's on the same
circuit as the roaster...
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Posted Sun Feb 10, 2013, 9:32am
Subject: Re: Using a Variac to limit voltage drop
 

Hi kentj,

A variac is not designed to remedy the apparent issue you have with the voltage dropping to low levels when a heavy power load is applied to the outlet you've connected your roaster to.  While it may be able to boost a voltage, it cannot boost amperage, since that's what you're lacking.  If your wiring going to the outlet is only rated to supply 15 amps effectively, you can't pull another 5 amps out of that wiring either practically or safely.

If you want to look at this in a similar way using a garden hose to water your garden, if you run 500 feet of 1/2 inch hose so your wife can water her patoonie patch at the far corner of your yard, she'll only get a tiny flow out of that hose, maybe 3 gallons per minute but at a very low pressure.  If you hook up a booster pump(Variac) at the end of the hose to increase the pressure (voltage) you'll gain most of your pressure back, but only at a maximum 3 gallons per minute.  Actually much less than that since you have friction losses from the small diameter hose to consider, but you'll never get 4 or more gallons per minute that you want, since the hose can only supply 3 gallons per minute to the booster pump (Variac).

Here's a link to a fairly recent discussion on this site that might help you since it covers almost your exact same situation of low voltage at the roaster:

"Help me with my Behmor. PLEASE!"

Ken
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Posted Sun Feb 10, 2013, 1:38pm
Subject: Re: Using a Variac to limit voltage drop
 

A variac is not designed to remedy the apparent issue you have with the voltage dropping to low levels when a heavy power load is applied to the outlet you've connected your roaster to.  While it may be able to boost a voltage, it cannot boost amperage, since that's what you're lacking.  If your wiring going to the outlet is only rated to supply 15 amps effectively, you can't pull another 5 amps out of that wiring either practically or safely.

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.  

If you want to look at this in a similar way using a garden hose to water your garden, if you run 500 feet of 1/2 inch hose so your wife can water her patoonie patch at the far corner of your yard, she'll only get a tiny flow out of that hose, maybe 3 gallons per minute but at a very low pressure.  If you hook up a booster pump(Variac) at the end of the hose to increase the pressure (voltage) you'll gain most of your pressure back, but only at a maximum 3 gallons per minute.  Actually much less than that since you have friction losses from the small diameter hose to consider, but you'll never get 4 or more gallons per minute that you want, since the hose can only supply 3 gallons per minute to the booster pump (Variac).

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.
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oldgearhead
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Posted Mon Feb 11, 2013, 7:21am
Subject: Re: Using a Variac to limit voltage drop
 

What 'swines' said. I used a variable auto-transformer (Powerstat) attached to the heating element of my highly modified Z&D for two years to control the wattage from 100-600. My Powerstat is quite effective in boosting the power to anything. Also, the ability to control the bean mass temperature (turn down the voltage) during the 'drying' phase was invaluable...However, I needed bigger batches so I moved on..
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kentj
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Posted Mon Feb 11, 2013, 8:47pm
Subject: Re: Using a Variac to limit voltage drop
 

Thank you all for your replies.  I have a community club house that has an outlet next to the stove.  This line should have fairly steady voltage under load.  I will roast there.  I will record the results.  If they are better I will try a variac on my outlet.  Ebay has a 20 Amp Variac listed for $90 with a money back guarantee.  I will post my results for anyone else who has this problem.
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PJK
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Posted Tue Feb 12, 2013, 1:34pm
Subject: Re: Using a Variac to limit voltage drop
 

Be careful with all of this.  What you are planning may work and be OK.  The  20 Amp Variac should be large enough, but first lets decide if your outlet is up to the task.  The Variac does not just magically produce more voltage.  The power which your roaster uses plus the losses of the Variac (fortunately small) have to come from the outlet.  The current drawn from the outlet will be ~ the current which the roaster draws increased by the same factor which the voltage from the outlet is increased to feed the roaster.  Find out how much current the roaster draws and what the size of the breaker is.  Multiply the roaster current by the factor which you think you will be increasing the voltage. If this number isn't under the  breaker size with a decent margin get a different plan. Maybe a new wire feed with a 20 Amp service.

Phil





kentj Said:

Thank you all for your replies.  I have a community club house that has an outlet next to the stove.  This line should have fairly steady voltage under load.  I will roast there.  I will record the results.  If they are better I will try a variac on my outlet.  Ebay has a 20 Amp Variac listed for $90 with a money back guarantee.  I will post my results for anyone else who has this problem.

Posted February 11, 2013 link


 
Philip J. Keleshian
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Posted Sat Feb 16, 2013, 7:50pm
Subject: Re: Using a Variac to limit voltage drop
 

With my post reply using water through a hose as an example to compare electricity through a circuit, I'd originally tried to explain more simply what's going on with a circuit that is too small to serve a load such as a coffee roaster, since with water through a hose, that's something easy to relate to, understand and measure without too much difficulty or danger.  

With electricity, there's a little more involved, since you don't see it, you can't measure it without proper electrical meters, and determining what's going on in a circuit requires knowledge of Ohm's Law and how to apply it.  Unfortunately, with electrical theory, it's not easy for someone to look at power going into an appliance and understand why things aren't producing what they expect to see for results.  

Unfortunately too, certain points were disputed that may have confused the OP even more, enough that it sounds like he'll end up trying a band-aid approach to make his roaster work as intended instead of properly and safely correcting the deficiency of a circuit that is too small for the load.


"When the voltage goes down, the amperage goes up."

Not true.  Calculate a circuit using Ohm's Law and you'll see why this isn't correct.

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.

With a device such as a typical electric coffee roaster, coffee maker, or any other small high wattage appliance, when that's connected to a properly sized 120 volt circuit and powered up, it draws what power it needs, up to the nameplate rating.  If the circuit is adequate for the load, there will be a negligible voltage drop, usually not over 3%, and the amperage draw on the circuit and watts produced will be about correct for the nameplate rating.  If the appliance is connected to a circuit that is too small and causes the available voltage to drop even further, the watts produced is lower and so, the amperage at the appliance will be lower.  If you calculate this using Ohm's law, that's what you will find for a result.  

If you don't believe this is true, take a high wattage load like a blow dryer or heat gun for portability, and plug that into your Kill-A-Watt at a location nearest your electrical panel and record the static voltage.  Then, turn the appliance on and record the voltage, KW, and the amperage.  Then, add 100 feet of #14 or #16 extension cord and record those same readings.  If you want, add another 50 or 100 feet of extension and record those readings.  At any of those locations, the static voltage with the appliance turned off will be generally the same.  Once you turn on the load, the voltage will drop just a bit at the shortest point to the panel.  At the 100 foot location, the voltage drop will be a lot more, with the current also dropping because of the line loss from the long extension cord.

If you don't want to spend the time to do this experiment, I've already done that with a 1650 watt heat gun like a roaster might use roasting HG/DB.  My static voltage is running about 123 volts.  When I connect at my breaker panel, voltage will drop to 120 volts at 13.7 amps, watts shown is 1650.  Plugged in at the 100 foot point, voltage drops to 110 volts, current drops to 12.8 amps and watts available to the heater is down to 1425.  At 200 feet, voltage drops to 108, current is down to 12.5 amps and watts available is only 1340.  The rest of the power that my 1650 watt heat gun needs is being lost as 310 watts of heat to the extension cords and there is no way that a variac can boost voltage at the far end of the extension cord to put that lost power back into the circuit, simply because the resistance of the extension cords is acting like another heating element and those watts are being consumed and sent off as heat to the air.


"With a heating element, you are not lacking amperage - but voltage."

If you have a load on a circuit that is too small; 1600 watts on a 1200 watt circuit, or generally, trying to get 20 amps out of a 15 amp circuit; not enough amps can be supplied, so the voltage drops below an efficient level.  It is still amps that are lacking that a variac cannot supply if the branch circuit can't provide them.  You cannot pull needed amps out of a circuit using a variac if the circuit doesn't have it to give.  

Voltage is equal to pressure and amperage is equal to quantity.  If you don't have enough quantity being supplied, more pressure won't help your machines work properly, whether it is electricity, water, air, gas or even hydraulic fluid that is the power for the device.      

This is why our OP; if he wants to roast as his roaster is designed and put out some quality roasts; needs to forget about considering wasting money on a variac to solve his problem, and needs to check into upgrading the circuit or adding a new circuit to meet the requirements of the roaster.

Although your other points about 400 Hz power on an aircraft and the examples of welding machines don't apply to solving the OP's problem, some of those points need to be disputed for corrections.

"more energy is available at a given voltage level because of the higher frequency."

Volts are volts and amps are amps, and all are equal when Ohm's Law is used to calculate electric circuits.  120 volts is the same whether it is DC, AC, or any frequency of AC.  1 amp is the same whether it is DC, AC, or any frequency of AC.  The only different element is Watts or VA which may vary depending on whether the load is resistive or reactive or a combination of both.  

400 Hz equipment is used on aircraft because, with high frequencies as compared to 60Hz power distribution systems, less iron is needed to build transformer cores, motor or generator armatures, or other reactive devices, which means less weight added to the aircraft.  There is very little difference between distribution wire size ampacities on either 60 or 400 Hz systems.  If you wanted to roast coffee aboard your 747, as long as the motor in the roaster was rated for 400 Hz, you could run a load through there on a 120 volt, 20 amp circuit, with no difference if it was compared to a roaster used at home on a 120 volt, 20 amp circuit.  

As to the operation of a TIG welder, your explanation:  

"You generally have high frequency AC current momentarily inserted into the current flow to start the arc."

In TIG welding, welders designed to use high frequency for starting an arc. high amperage current doesn't flow until an arc is established. Since contact with the work is undesirable to begin an arc because of contamination, the high frequency generator causes a flow of electrons to jump the gap between the electrode and the work, which then allows current to flow.  Current can't flow until an arc is established.  


"specifically because the high frequency AC current has MORE energy available to start the arc."



The energy for welding comes from the TIG machine power transformer.  The high frequency generator in a TIG welder, causes the current to flow within the shielding gas envelope between the electrode to the work, normally around 3500 volts but at a microamp level.  Sort of similar to a coil on a gasoline engine which can produce a high voltage to jump the plug gap, but too low of a current to do much more harm than give you an annoying poke.  So similarly, the amperage from the high frequency AC source is too small to add to the electrode lead to have any effect on the heat setting of the welder for the material to be welded.  

Ken
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