4 wire stepper motor driving issues.

[steven_first]

Hello all. I have a quick question that Google is having trouble helping me with so I figured I would ask here. I have a few DLP color wheels that I have been playing around with recently but I am having trouble driving them. They are all 4 wire stepper motors with one common and 3 coils all connected to the common (guess that is why they call it common!). Any way, I have been using a simple BS2 code to run the motor while using ULN2803 Darlington pair IC to supply the current to drive the motor. This works ok I guess. I have to push the motor to get it started and I am very limited in the RPM I can get to. I was suspecting that it was because of the slowness of the ULN2803 but I am not sure. I have tried to find info on drivers for this type of stepper but all I find is 6 wire and 4 wire dual coil stepper drivers. I guess my questions are these: Why do I have to push my motor to get it started? If I don’t it just jitters and even after I push it, it will sometimes end up spinning backwards. Why can’t I get any higher RPM’s out of this thing, I am maybe getting 750 to 1200 at best; any faster pulses and it won’t spin. And lastly, where can I find a proper driver IC or board for this type of motor? Here is a copy of my amazingly complex program to; it is set as fast as I can drive the stepper. The ‘pls’ is something I was just playing around with to experiment with delay between coils.

pls VAR Word
npls VAR Word

pls = 1
npls = 10

'main
DO
HIGH 0
PAUSE npls
LOW 0
PAUSE pls

HIGH 1
PAUSE npls
LOW 1
PAUSE pls

HIGH 2
PAUSE npls
LOW 2
PAUSE pls
LOOP
END

 

[Mickster]

......They are all 4 wire stepper motors with one common and 3 coils all connected to the common (guess that is why they call it common!)....

That's not a stepper motor. Sounds more like a BLDC motor...

BLDC motors need the speed to be 'ramped' up. Listen to the hard drive in your PC when first booted - it starts off slowly before reaching full speed.

Your program is running at a fixed frequency that employs no ramping, hence the push-start requirement and limited top speed. Changing the pause period(s) will alter the frequency and thus the top speed, but will still require a push-start.

Change your Google searches to BLDC motor and look at example schematics.

This should get you started:
Brushless DC electric motor - Wikipedia, the free encyclopedia

HTH.

 

[steven_first]

Thanks for the help. I am reading up now but I am finding that all the brushless dc motors I am seeing are 3 wire motors and do not have a common lead. I have not found anything on a 4 wire motor just yet… The coil configuration of my motor is a wye configuration with a tap on the central common point and so far no info on driving those (attached a pretty picture of what I have just in case it doesn’t make sense). I have some RC car brushless controllers that I tired messing around with but they won’t drive the motors even with just leaving the common un-connected. Still a brushless dc motor? What do I do about driving it then?

 

[russ_hensel]

The radio control guys sometimes build drivers for brushless motors, sometimes using pics or other mc. Try poking around them using google.

 

[steven_first]

Thanks, I tried driving my color wheel with a brushless esc but the esc would shut down after only a few revolutions of the wheel; probably to high of a resistance seeing most rc motors are about .2 ohms(ish) and this motor is about 6 if I remember correctly. I got some code made up with a ramp up that is doing ok but seeing I am not using any feedback at all it is a hit or miss thing and the speed is very limited but it is a start. I wish I could just get an adapter to put the glass on a real brushless motor, that would help!