Pellet controller upgrade

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TimS
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Pellet controller upgrade

Postby TimS » Fri Dec 29, 2017 7:25 pm

I have a SmokeDaddy pellet feeder that is over 6 years old and the electronic controls were not giving me the level of control customization I was needing (I'm a nerdy engineer). So instead of purchasing their new drop-in controller (which looks pretty well built) I decided to use an industrial PID auto tuning controller from Omega. During this upgrade I also took the time to service all the bearings in the motors.

Controller: https://www.omega.com/pptst/CN142.html
Relay: https://www.omega.com/pptst/SSRL240_660.html

Here is the original controller.
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Here is the new controller installed. I had to slightly widen the hole for the new controller. I designed the panel and printed on my 3D printer to hold the controller, a K-type connector and the power entry module.
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Connections were straightforward:
1. AC power comes in through the fuse-protected module and the hot lead branches from the fuse to the controller, fan and solid state relay.
2. At the controller I connect the alarm relay common to the AC power and the hot rod igniter to the alarm relay N.O.
3. The controller connects to the K-type connector for sensing smoker chamber temp, and to the low voltage trigger of the solid state relay.
4. The SSR output connects to the pellet feeder motor. This SSR is zero-switching which means after the trigger signal is received from the controller it waits until the AC power waveform reaches zero before switching on. This eliminates surge noise in my home's electrical system and increases the lifespan of the auger motor.
5. All the neutrals tie together
6. The ground from the power input module is connected solidly to the metal frame for safety. ALWAYS use a grounded service.
7. The heat sink on the SSR should also be tied to ground with a 18G or heavier wire.

Here's a picture of the underneath. I fitted the same connectors used by the original controller so I can swap back in the future.
Image

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So why do this??? For better control of the pellet feeding and hence the chamber temperature. This controller is programmable and you can set the pellet feeder cycle time, the max feed rate, the hot rod switching temperature and auto tuning settings to fit the setup. One of my biggest problems with the pellet feeder on my offset smoker was the large temperature swings and flameouts. When restarting after a flameout the smoker generates heavy WHITE smoke which I prefer to avoid as much as possible. The key to consistent results is lower fluctuation in chamber temperature and a fire that never flames out.

The hot rod control comes through the alarm relay. Simply program the controller to turn on the hot rod when the chamber temperature is below say 200F. This way it comes on quickly if you have a flameout.

The auto tuning on Omega controllers is also pretty good. You can have the auto tuning working all the time or only when manually triggered. I leave it on manual and after the smoker has been up to temperature for an hour I run the auto tune. It only takes a few minutes to complete the tuning and afterwards the system can attain +/- 2F stability. No need to repeat the tuning for future cooks unless you change the setpoint dramatically (like for poultry).

The biggest source of my issues with the original controller was the pellet feeder cycle time of 90 to 120 seconds. I have a wireless two-channel cook temperature tracker with data logging and I would see the chamber temperature fluctuate 20 degrees above and below my set point. No amount of tweaking the controller settings would remove this swing. With the new controller I have the cycle time set to 10 seconds which means at steady state it will regulate the temperature within about +/- 2 degrees F, at least until you open the door and let the heat out.

When you open the door the controller reads this as a large drop in temperature and goes full tilt on the pellet auger until the temperature comes back up which could be as long as a minute after the door is closed again. During this time the controller can put a lot of pellets in the burn cup causing a large overshoot in temperature. To avoid this, I set the maximum feed rate to about 40% for cooks at 225F and 75% for cooks at 350F. This significantly attenuates the system's overcompensation for opening the door, however it can lead to long initial heat-up times. One must learn to go slow in order to go fast. Also, you can add a handful of pellets to the cup just before you turn on the cooker to help speed up the initial warm-up.

I'm pretty happy with the results so far, and this approach should work with any old pellet machine with a burned out controller.

Image
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spacetrucker
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Re: Pellet controller upgrade

Postby spacetrucker » Fri Dec 29, 2017 7:44 pm

I would lower the hot rod turn on to say 140 to conserve the "burn out" on the rod, at 200 you could still be holding meat in a keep warm state thereby keeping the hot rod on when it is not needed. Just my $.02
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Papa Tom
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Re: Pellet controller upgrade

Postby Papa Tom » Fri Dec 29, 2017 8:39 pm

I purchased a PID controller a few years ago with pretty much the same thoughts of better control.
Then I decided that the nature of the PID is to tightly control resulting in tight temperature window at the expense of less smoke.
Because smoke cooking is what I'm after the PID is aging in the box (somewhere).
It'll be interesting to see what your experience is with the device. I know some of the commercial units use PIDs but I believe at least some of them also control the air as well.
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TimS
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Re: Pellet controller upgrade

Postby TimS » Sat Dec 30, 2017 12:31 pm

Papa Tom wrote:Then I decided that the nature of the PID is to tightly control resulting in tight temperature window at the expense of less smoke.


I'm not sure I follow your comment. PID is just a smoother way to control the pellet feeder compared to a simple on/off control like you have on you home air conditioner. The smoother control means less dramatic swings in temperature as the system attempts to maintain a set temperature.

More pellets per minute into the cup means more fuel resulting in more heat and smoke entering the chamber. In my case the pellets are providing both the heat and the smoke so I can't control them separately. Also, since there is no other heat source I am getting the most smoke I can out of burning the pellets. Adding electric heat would require a lower pellet feed rate to maintain the chamber temperature, and therefore generate less smoke. That could be advantageous in certain circumstances where you would want a long cook time with less overall smoke.

I guess I could try to get the temperature in the pellet burn pot lower to make the pellets smolder more, but that produces a different kind of smoke--white cold smoke--which is what I generally try to avoid.

/Tim
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TimS
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Re: Pellet controller upgrade

Postby TimS » Sat Dec 30, 2017 12:49 pm

spacetrucker wrote:I would lower the hot rod turn on to say 140 to conserve the "burn out" on the rod, at 200 you could still be holding meat in a keep warm state thereby keeping the hot rod on when it is not needed. Just my $.02


With this controller changing the hot rod toggle temperature is a simple one-button press on the front panel. I'll keep that in mind next time. Thanks for the tip.

/Tim
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Papa Tom
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Re: Pellet controller upgrade

Postby Papa Tom » Sat Dec 30, 2017 5:05 pm

Tim you will see what I mean by less smoke in practice.

If you have run a pellet pit at high temps you would notice less smoke compared to lower temp runs. This is because the pellet feed is running more often keeping the fire blazing.

The PID will basically do the same thing @ any temp setting, feeding fewer pellets more often to keep a consistent fire/temp and with constant (generally 100CFM) air flow you will experience a smaller hotter almost smokeless fire. .
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TimS
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Re: Pellet controller upgrade

Postby TimS » Sat Dec 30, 2017 7:30 pm

Papa Tom wrote:If you have run a pellet pit at high temps you would notice less smoke compared to lower temp runs. This is because the pellet feed is running more often keeping the fire blazing.


Yes, I see what you mean. When a fire burns hotter it more completely burns the volatiles that come out of the wood and hence less visible smoke. So when the smoker is set for higher temps it requires more fuel which then burns hotter and emits less smoke.

So by that reasoning to get more smoke at higher smoker cooking temperatures the pellets need to burn at a lower temperature like they do at low cooking temperatures. How to do this? I wonder if reducing the air flow would work. Lower air flow wouldn't fan the flames so much and therefore lower the temperature of the burning pellets.

Do any pellet smokers have variable fan speeds?
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Re: Pellet controller upgrade

Postby Papa Tom » Sat Dec 30, 2017 7:47 pm

Yes there are some that have reduced air flow but I've heard complaints of black ash (soot) when they do that. I do believe it could be done successfully though.
I have reduced the air on one of my pits by replacing the OEM fan with a 50 CFM unit with no troubles except that it will only go to about 250°F max. Of course I did it to have a low temperature smoker for sausage etc. and it works fine for that.

I have purchased a variable air setup (Amazon parts) but have not used it so I cannot comment on that approach. It would be a manual dial setting for the air.
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