MagicJigPipe - 16-12-2011 at 02:11
So, I was working on this tonight and I just woke up thinking about it. Please excuse any lapses in judgement...
I received an analog, Corning, Hotplate/stirrer that was damaged. Some ICs (particularly the ones that switch the stirring mechanism and heater I
believe) had been destroyed by some acid amongst many other things. I replaced those and everything seems to be working but there is a strange
occurrence:
The stirring mechanism will only "go" at one speed no matter what the knob is turned to. Since I don't know exactly how these things work, I can't
decide exactly what is happening. There is a fan-type device with little fins on the bottom of the motor. I can't imagine any purpose for this.
This may be far off but I suspected that it might be some sort of speed sensor. Why? Because there is a little device placed right underneath the
fins that looks like 2 partially exposed LEDs in a housing (I've never seen this before). The fan-device doesn't look like it was designed to be
extremely efficient for cooling, hence my sensor "theory".
It could be that, due to my repairs, when I place the main power board (the one with the transformer) into the device that something is touching the
casing and causing it to stop working because I can cause it to start working by fiddling with the board while it's in there (while "fiddling" I can
hear a relay quickly clicking on and off). This could also point to this theoretical sensing mechanism as well, I think. Is this mechanism real? If
not, what is that component that is coincidentally placed right underneath those fins? What are the fins for if not for sensing speed or something
similar?
Anyway, if anyone has any experience with this I would certainly appreciate any advice.
Dr.Bob - 16-12-2011 at 08:24
I think that the fan is there to cool the stirring motor and electronics, which cannot handle 300+ degrees which the hotplate can produce. So if you
plan to heat stuff with it, that would be the benefit of it. I think the stirring speed is not feedback controlled, I don't believe that there is a
speed sensor in the older stirplates. So if the controller is not quite right, they seem to default to high speed. I have a few like that, which
stir fast no matter what the setting of the knob.
MagicJigPipe - 16-12-2011 at 11:24
Then why does that fan seem to be so inefficiently designed when it comes to maximum air-movement? I mean, the fins are flat and regular instead of
at an angle to the rotation plane. There are 4 notches sticking up on the inner part of the "fan" but they don't do much. All-in-all, it seems like
that "fan" barely moves any air at all. I can barely feel it.
I guess I believe you, but until someone can explain to me this apparently poor design, I will be skeptical.
Rosco Bodine - 16-12-2011 at 23:46
That cogged plastic wheel is a beam interrupter, and the cogs run through a U slot in an optical emitter detector assembly which sends signal pulses
to a tachometer IC which is a digital to analog converter outputting a linear voltage proportional to the pulse rate. That output voltage from the
tach IC goes to one input of a voltage comparator IC, and the second input of the voltage comparator
gets a reference voltage signal from the speed control pot. The output of the voltage comparator probably controls the gate voltage of a triac
through which the motor is powered. The way the thing works is if the signal voltage from the
tach is lower than the reference voltage from the speed control pot, the output
of the comparator goes high and fires the triac which sends power to the motor,
and as the speed of the motor increases the tach voltage increases till it matches
the reference voltage from the speed control, the comparator output goes low
and the triac interrupts power to the motor which then slows until the comparator
senses and corrects by powering the motor....and so on .....thereby speed is regulated. Omron is popular manufacturer for those U-slot emitter
detectors
which are also used as load sensors on garage door opener motors.
There are variations on the theme that are found in different models. It's basically a cheapass poor substitute for electronically enhancing
performance of an AC motor in a forced speed controllable mode of operation .....where a more expensive high torque DC servo motor is really what is
needed but costs ten times more.
[Edited on 17-12-2011 by Rosco Bodine]
MagicJigPipe - 17-12-2011 at 10:51
Thanks so much. That helps a lot because now I know the solution is just to position that sensor in the right place relative to the fins. And the
fins are metal in this one, by the way. It is somewhat warped so that might be part of the problem.
I knew that wasn't primarily a fan.