117VAC settings for 230VAC Variacs?

[PabloElFlamenco]

It's logical for American contributors to refer to 117VAC variac settings at various points of the roast, but I still haven't been able to draw a logical conclusion as to what that means in 230VAC terms.
To make it more comprehensible:  when a US roasting says "variac setting at 50V" (I calculate 50 volts to be 43% of maximum power, in relation to 117 volts), how does that translate to a 230 volt variac? 
Is it technically correct to assume that to be 43% of 230 volts, i.e. 99 volts?
Or, is 50V (117) the same as 99V (230)?
Thanks for clearing that up!
Paul

[edtbjon]

I'm having that "problem" too, as I use 230V fans and Variac.
On my 230V Variac (clone) I got a 0-100% scale which in my mind very roughly translates into US voltage scale (well, 60 V is at around 50%...).
Else it's like converting Fahrenheit to Celsius except that you don't need to take the 32deg deviation into account.

In my personal opinion, electric/electronic fan control is indeed needed, but a Variac is way overkill. I find myself using three settings: First, very low, just to have a "whisper" of air, supporting the flame from the start up until DE, then a medium setting in the mid/Maillard phase and a high setting from 30sec before 1C until finish. The thing IMO, is to be able to repeat these settings, where the Variac is nice, but not essential. I think any good guitar/amplifier potentiometer size knob graded from 0 to 11 would do the trick.
(If you look at most professional drum roasters, they are only equipped with dampers or even "100% cooling, 50/50, 100% drum" settings. E.g. all Diedrich IR roasters and many others in the same league.)

[PabloElFlamenco]

Braah! (borrowed from "The Bridge")
I think I get your point, and ...tak...
Indeed, my tendency towards "perfectionism" oft makes life a bit more unbearable than it needs to be, "le parfait est l'ennemi du bien", unfortunately there's far worse, at large...
Happy days ahead is my wish to you!
Paul

[SusanJoM]

If I had to do it over I wouldn't buy a Variac;  not so much because it is overkill but more because even when it's accurate it just isn't.   

What has become obvious to me of late is how long it actually takes for a change to the power to translate to a slowing down or a speeding up of the fan.  If there were a way to make those changes happen faster I think that would be a huge improvement.  Now when you decrease the power it's like throwing out an anchor, and slowly the drag reduces the fan speed;  similarly when you crank it up it takes a while for the power to translate to increased speed.  We tend to read the Artisan graph as if the changes happen when we push the buttons, but it's not quite that straightforward.

[LTB]

well, Susan, I found the fan reaction quick enought !!! And it's more precise and effective than the majority of all commercial roaster I ever saw !

[SusanJoM]

well, Susan, I found the fan reaction quick enought !!! And it's more precise and effective than the majority of all commercial roaster I ever saw !

Oh good.  I'm glad it works well for you.

[edtbjon]

The way the fan and Variac works, a bit sluggish as Susan suggests, a bit of anticipation is suggested. How long the time is from the turn of the knob to the fully intended change can be found in the RoR curves. I guess the DeltaET is the one to analyze, as it's the air temp that is changing first.
I fully agree with LTB on that we have very good control of air speed compared to most other roasters regardless of brand or cost.

If I had to replace my exhaust fan I'm really curious about how a DC fan (with e.g a 24V controller) would be compared to our AC fans. (I know that at least one member in this forum uses such a solution and wouldn't mind a report...)

[SusanJoM]

Here's an example.
You can see that it took a full minute for the fan to impact the profile.  ROR most significantly;  ET similarly and significantly; BT similarly but hardly at all.  MET?  MET just shrugged 

[LTB]

yes but the reaction time of the roast and the reaction time of the fan are two different things.... I thought that you blame the fan.

[SusanJoM]

You are right that the profile shows the reaction of the roast not specifically of the fan, however, having paid close attention to the time that it takes for the fan to adjust itself to a changed setting on the variac, I stick by my guns that it is slow.   
It may be faster than any other fan in the business, but it is still slow. 

[CGwynne]

The 115VAC fan and 230VAC fan do not have the same CFM rating (the spec I posted a long time ago and made it into the downloads section is wrong for the 230)

The 115VAC fan has a max rating of 230 CFM (3200 rpm fan speed at 60hz 115VAC)

The 230VAC fan has a max rating of 205 CFM (2700 rpm fan speed at 50hz 230VAC)

So 100V on a variac using the 230VAC fan is not the same airflow as 50V on a variac driving a 115VAC fan

Attached below are the manufacturers specs

And as there are differences in Variacs and light dimmers, the only real way to measure is profiling the Airflow. Get a cheap airmeter for $10 and see what your fan does at each voltage setting. Then compare it to my airlfow settings for 230VAC or Hankua's settings for 115VAC. He gets up to 15m/s on his 115vac fan using a light dimmer, I get up to 8 m/s on my 230vac fan using a variac but that's with the variac running at 250VAC so I'm actually over-cranking the fan motor (and to this day can't account for the difference between Hankua and my airflows, his max airflow would probably not only stall my roasts but transfer half the beans to the chaff collector).

Additionally as variacs and light dimmers are not a good way of varying airflow using an AC fan (a proper air valve with a constant fan like seen in the Mini500+ is better as is a DC fan with speed controller), my AC fan won't even start spinning until I'm up to 75v (out of 230) and the fan speed doesn't really track linearly with voltage. At 30% of voltage I get 26% airflow, at 60% voltage I get 80% airflow.

And while I've not tested this, I think there is more hysteresis changing speeds on an AC fan with voltage manipulation than a DC fan with a speed controller which could account partially for the slow reaction Susan sees.

And lastly, we all have different static pressures depending on how we setup the exhaust after the exhaust bowl. If you have the exhaust bowl venting open to the room (atmosphere) then you have no static pressure, but if you have the exhaust bowl connected to some kind of tubing with more than a few feet of travel or a few left and right turns then you are introducing back pressure that the exhaust fan has to work against. And that will affect the airflow in your roaster. You can see how much static pressure you have in such a scenario by profiling the airflow on the fan with and without the exhaust venting. I don't have the numbers handy but I think I lost about 10% with 3 meters of 2.5" flexifoil tubing running down to the floor and then out in a pretty straight line past the door. Additionally I gained between 71% at the lowest setting and 27% at the highest setting when I connected the exhaust bowl venting to a kitchen hood exhaust vent with a built-in inline fan.

Posting a profile's fan voltage settings is usually not going to be that useful relative to other peoples profiles, neither are percentages since we all have different max airflow speeds. Posting actual airflow numbers would be a big improvement in consistency. So knowing that your 70v on your setup = 6.7m/s of airflow is going to help you if you want to get better at consistency and repeat-ability and will be a better way to share and discuss profiles.

YMMV

As a side note, on a Geisen roaster, when dialing in the airflow, it shows you the actual air pressure of the exhaust. The fan speeds up or slows down to get to your pressure setting. This is not only the most accurate way of repeating a profile's airflow, but also lets you see if over time, the exhaust is getting clogged up with particulates. If you need far more fan rpms to get the same airflow, it's time for a cleaning before you get a nasty chaff fire. As this is relatively easy to add to any roaster with a Phidget 1136 and Artisan some have done it to great success.