I made a mill

[Ben1]

Out of curiosity, what was your eff before matplat?

 

[Weizguy]

Wow, 1.6mm! I think that the optimum gap size depends alot on the size of the knurl on the rollers. Looks like off-the-shelf mills have a much coarser knurl. If I were to set the gap at 1mm as it seems alot of people do, I would end up with flour.

Mostly agree. The optimum gap size depends on both the knurl size and, more importantly, the roller diameter - which determines surface speed.

It's great that you have 75% efficiency. I'd be happy if I had that as a consistent result.

 

[Ducatiboy stu]

My 50mm rollers need about 0.9mm and they have a fairly decent knurl.

I also dampen the grain first which aslo make a fifference

I try to aim for about 20-30 % flour

 

[Lyrebird_Cycles]

As I understand it, you can't slow down AC motors with a speed controller as the current draw goes up and it overheats.

Is there anyone here that can confirm or reject my claim above?

A bit belated but here goes:

Power = torque x speed, so for any motor if you reduce speed at constant power you must increase torque. For most motors increasing torque increases current draw BUT in this case the torque requirement should go down with the speed so that doesn't apply.

Next we have the problem of back EMF: any motor also acts as a generator, producing a voltage opposing the drive voltage, called back EMF. Forward voltage is the difference between drive voltage and back EMF so when the motor speed is reduced the forward voltage is increased. If the winding impedance was constant, this would increase current draw BUT winding impedance on a single phase motor is a complex function of load and speed, as below:

The next problem is that the cooling fan is attached to the motor shaft so as the motor slows the cooling is reduced. Some motors have a special cooling fan drive to offset this but yours is not one.

All this is moot if you use a VFD, every one I've seen has a facility to control the drive voltage and current as required.

BTW I don't think the speed is you actual problem, I think it's the combination of knurling and undriven second roller. Real malt mills run quite fast, up to 4 m.s-1at the surface (about 500 RPM for a 150mm roller, though rollers are usually larger than that).

The problem with the undriven roller is obvious when you ask yourself: what's making it move? The answer is the drive force is being transmitted through the grain. Since you've got drive force on one side and resistance on the other, this places the grain in shear. Shearing grain tears the husk.

 

[Matplat]

Been meaning to update this thread for a while....

I took lyrebird's advice and made some effort to drive the slave roller, I got a few decent wide rubber bands and wrapped them around the drive roller.

It made adjusting the gap a little harder, but it worked! I wasn't sure that it would, because the feeble grip exerted by the rubber band contact patch on the dusty slave roller, didn't seem like it would make much difference...

The immediate difference I noticed was when mashing in, almost zero dough balling compared to before, and, subsequently, the flow through the bed was much better during the mash. I also gained a few efficiency points in the process.

I think I will transfer some of the rubber bands to the slave roller, so it will then be rubber driving rubber, rather than rubber driving metal.

But yeah, anyone looking for a cheap/simple way to drive a slave roller, this is a totally viable option.