Rob,
The advantage of using a variable frequency supply at full voltage is that the motor retains it's torque characteristics all the way down in speed. A very simple way to get variable frequency AC power to control the motor can be purchased for less that US $6.00 on e-bay. To that needs to be added some simple circuitry, but no programming is required.
This is billed as an inverter driver board. It is sold as a fixed frequency device, but the chip is capable of doing wonderful things. By setting a jumper and lifting one pin, and inserting a pot, the board becomes a variable frequency driver. All that is needed to be added is an H-Bridge circuit, and a source of 5V and 12 Volts, and a rectifier/filter for the incoming line voltage. An isolated 12V wall wart and a three terminal 5V regulator will do just fine.
The datasheet/user manual for the board is here: http://www.egmicro.com/download/EGS002_manual_en.pdf
The schematic on the board datasheet already shows the board connected to the H-Bridge. (Everything to the right of the number blocks) For the low currents involved here, the fan, thermistor and driver transistor are not needed, and if you use the specified MOSFETS, you shouldn't need heatsinks. The value of L1 was left off the schematic, and is 3.3 mH. It has to be capable of carrying the motor current.
The datasheet for the EG8010 frequency control chip has the information for making the device a variable frequency generator. The datasheet, here: https://www.google.com/search?q=EG8010&ie=utf-8&oe=utf-8 is formidable, but the only information necessary is in a paragraph and diagram that starts on page 15 and continues onto page 16.
The parts for the H-Bridge are very inexpensive. The MOSFETS are available on e-bay for as little as 5 for US $0.99.
I started to layout a PWB for the bridge, but put it on hold when I did, what apparently others have done, by melting a gear in my Harbor Freight laminator while testing out my temperature controller. The circuit is so simple, that it should be easy enough to build by hand wiring. The schematic shows the circuit working on 400 VDC with an output voltage of 220VAC, but that voltage is the supply for the motor only, and it looks like it will work on any voltage, including fairly low voltages. The pot in the bridge circuit adjusts the actual output voltage.
This brings me to the usual reminder and warning about not working with these voltages unless you really know what you are doing, and are comfortable doing the work. Line voltages are killers, so beware. The output voltage of this circuit is not referenced to the mains ground, so connecting it there should lead to a nasty surprise involving the release of the magic smoke.
Harvey
ANY hacking of ANY electrical appliance can lead to plastic meltdowns or fire.
That is the nature of hacking.
You don't even think of walking away from one of these before it cools off.
If you do then thats on you.
AND
since I did succesfully raise the temperature without melting plastic structures
I concluded it would be feasible if the nylon plastic gears were replaced with metal gears.
I ruined ONE nylon gear running heaters at 390 degrees.
I would not have let the heater go above 350 if it had not been for the question of
COULD we use the 370 degree BROTHER toner ...not just 340 degree HP toner.
And as far as speed control goes.......
I didn't sprinkle any spice on a 555 to make PWM.
I just told the MOC3020/BTA24 triac circuit hanging off a PIC16F628 pin to stop...wait a few seconds...then
go. I figured I already had triac heater control so it was simple enough to control the motor.
All parts ...soup to nuts cost about $12 for heater and motor control by one PIC16F628 with an LCD
to keep track of settings.
http://www.learnmorsecode.com/laminator/pic/index.html
Noting the pictures show the hot press and not the laminator in use.
On 12/16/2016 09:34 PM, mosaicmerc@... [Homebrew_PCBs] wrote:
The only doubt I have about that approach is that tampering with the heating mechanism to push temps above design specs. can lead to plastic meltdowns, and possibly a fire.
Slowing the rollers however is a different matter as it doesn't risk exceeding the device specs without added protections.
Since you raised the point I gave it some thought and even did a small LTspice sim.
It would appear that rectifying the 120VAC into DC (like a PC SMPS or cellular phone charger) and then producing a stepped replica of a sine wave using scaled PWM fed into a 220 uF or larger cap (as also found in PC SMPS units): we can have a variable frequency sinewave suitable for altering the speed of a <200mA synchronous AC motor without risk of a breakdown or fire.
The caveat here is we're dealing with the 120VAC side of things and must design appropriately.
Now to drive the sine wave PWM...a 555 chip can do it via an optocoupler into t he high voltgae FET (also found in a PC SMPS)
This is it in principle, but not exactly,as we'd need a variable freq sine wave source perhaps from 10Hz to 60hz.
Generating PWM from rectified sine wave using 555