Power Offset Calculation

[rweiley]

Hi All

I am creating a custom tweedy mixer (Bread dough mixer) for a client and there is a requirement to calculate the total energy (Wh) that goes into a dough mix. I am achieving this through a Allen Bradley compactlogix controller and a Allen Bradley PowerMonitor1000. My only issue is the calculation, i need to work out the total energy going into the mix minus the no load energy so that it is only calculating the energy actually working the dough and not to spin the pulleys etc.

Has anybody done a similar calculation?

 

[Epy]

Interested to see others' responses to this. My thoughts:

When you say total energy, is your client talking about the electrical input (full load - no load), or the mechanical energy (mech will obviously be less)? In either case, I think my course of action would be to contact the manufacturer or try to get some efficiency numbers from a data sheet, etc.

 

[rweiley]

I believe it could only be determined by electrical input full load - no load, assuming the rest of the energy is going into the dough (Not entirely accurate but perhaps close enough).
I recently found this calculation elsewhere


EXAMPLE 12.5. Mixing time for bread dough
In a particular mixer used for mixing flour dough for breadmaking, it has been found that mixing can be characterized by the total energy consumed in the mixing process and that sufficient mixing has been accomplished when 8 watt hours of energy have been consumed by each kg of dough.
Such a mixer, handling 2000 kg of dough, is observed just after starting to be consuming 80 amperes per phase, which rises steadily over 10s to 400 amperes at which level it then remains effectively constant. The mixer is driven by a 440-volt, 3-phase electric motor with a power factor (cos f)) of 0.89, and the overall mechanical efficiency between motor and mixing blades is 75%. Estimate the necessary mixing time.

Power to the motor = Ö3 EI cosf where I is the current per phase and E and cosf in this case are 440 volts and 0.89, respectively.
In first 10 s, Iaverage = ½(80 + 400) = 240 amperes.

Energy consumed in first 10 s
= power x time = Ö3 EI cosf
= Ö3 x 440 x 0.75 x 240 x 0.89 x 10/3600
= 339 watt h
= 339/2000
= 0.17 W h kg-1.

Energy still needed after first 10 s

= (8 - 0.17)
= 7.83 W h kg-1.

Additional time needed, at steady 400 ampere current

= (7.83 x 2000 x 3600)/(1.73 x 440 x 0.75 x 400 x 0.89)
= 277 s.

Total time = 277 + 10 s = 4.8 min.

In actual mixing of doughs the power consumed would decrease slowly and more or less uniformly, but only by around 10-15% over the mixing process.


This however does not factor in any no load offsets but counts all input power to the mixer.

 

[Saffa]

Is it possible to run the mixer with no ingredients in it at the start of each batch?

If so, you could run it for 10s or whatever, measure the amount of energy consumed, and calculate the average. Then during the mix, read the instantaneous Watts and subtract your average. Totalize that once a second and you'll get Watt seconds. Divide by 3600 to get Watt hours.

That assumes that mechanical losses don't change once the mixer is loaded. This may or may not be significant, hopefully the manufacturer can provide some data.

 

[Chris Taylor]

OT, in the 80s, I worked in a bakery for a couple of years where there were two tweedy mixers, that had Star Delta 400KW motors on slip rings, forever indexing the rings to move the burnt sections out of the static positions. the control system used weird MTE stackable Logic blocks, it was always a bottom one the failed. but boy could they mix dough.

 

[shooter]

well measure the empty power and subtract it from the running power. both of them you can find in the monitor.
please do use the real power thus the kWatts and not the current as the cos phi changes.

 

[rdrast]

As usual, I'd just go ahead and toss in a LoadControls power module to measure my load. Take that into a controller so you can essentially "Tare" it at no load, and read the mixing load directly. Totalize that over time for actual kWh.

Load Controls may also have dedicated controllers that can do what you are looking for.

* I am in no way associated with Load Controls, just a very satisfied user of their products for about 20 years.

 

[redbarron55]

If you were to use the intelligent overloads then you could control the motor and get power information from it.
Either way you can set a limit power for unloaded and store the actual power when in that condition and then when it is exceeded you could start to integrate the power differential over time to measure the net power consumed by the mixer.
Using just numbers pulled out of the air
less than 10 watts read the power into a register update the register every minute or what ever works for you.
When the power exceeds 10 watts for 30 seconds subtract the idle power and store the difference in an array and add the array.
Probably better you could calculate the number of watt seconds (or whatever) and totalize them for power consumed mixing.
When the desired amount of power had been put into the dough stop the process.