Based on the feedback of my recent roasts, I am convinced that the increased rpm (from 43 to 52) contributed positively. The roasts are more even and the heat transfer efficiency increased. The roaster response also increased slightly as the result of the higher rotation speed.
The next step in this experiment is to replace the stock motor with a higher rpm one and install a motor speed controller so the rotation speed can be profiled throughout the roast. After watching ebay for several weeks, I finally found a unit with the right rpm and torque that is affordable as well. I quickly snatched up the motor and eagerly waited for its arrival.
Here's a quick comparison of the new and stock motors (running at 24 VDC):
Stock motor:
Manufacturer: Shayang Ye Industrial Co.
Model#: RB400150-06202
Reduction ratio: 100:1
Rated speed: 54rpm
Rated torque: 6.1 lbf-in (97.2 oz-in)
New motor:
Manufacturer: Pittman
Model#: GM8723G567
Reduction ratio: 96:1
Rated speed: 86.5rpm
Rated continuous torque: 7.1 lbf-in (113.3 oz-in)
The new motor is a much heftier unit and the construction is very solid. It is also not cheap if purchased new. The NOS unit I got is very reasonably priced. As usual, trimming and fitting are necessary to get it go into existing housing. The current coupling sleeve was drilled to accept the circular shaft. I was initially concerned about slippage and mis-alignment, but so far it seemed to be fine.
I had originally planned to purchased a PWM (pulsed width modulation) motor control unit from ebay but after conversation with a very generous co-worker (Dr. Gizmo), he designed/installed a analog motor speed control for me. Below is his schematic of the circuit:
The new motor/controller setup allows me to vary the speed from 20 to 80 rpm. At speed below 20rpm, the tachometer no longer register the speed and I do not think such low speed is necessary. The maximum rpm dropped from 83 to 80 due to the voltage drop at the transistors. Noted that in actual use, the rpm will go up 1 to 2 more when the roaster is fully warmed up.This latest mod puts the little huky a step closer to a "commercial" sample roaster that I desired. I am very excited to start experimenting with speed profiling and anxious to see if it can solve one of the remaining deficiencies of the roaster.
