driving brushless DC motor

the motor.

I know neither. Does the MTD6505 output enough current to drive the motor? (~50mA with not load and more with a load.)

George H.

I haven't played with either that motor or that chip, but I still have some suggestions:

What's the motor mechanically connected to? Sensorless motors get very difficult to control if there's high starting torque, and a fan chip is certainly not going to expect that. Also, the chip may be expecting a high inertia -- if you have no flywheel-ish thing connected to the motor, it may not be happy.

Have you put in a resistor to select the motor constant? Sensorless drive algorithms can certainly depend on this, and they want you to do it

-- so you should.

Define "does not seem to like". What happens when you try to run the thing? Buzz? Twitch? Smoke? Turn slow? Nothing at all?

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My conservative friends think I'm a liberal kook. 
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Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

Twitch? Depends on whether it's a leftist motor or not ;-) ...Jim Thompson

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I love to cook with wine.     Sometimes I even put it in the food.

Something like this should work for driving *micro* BLDCs:

But, yes, it's rather intriguing - what do you mean by "does not seem to like"? Something like twitching back and forth or simply nothing is happening?

motor-677981-.htm

stroboscope-project/

That circuit has no feedback, and is therefor using the BLDC motor as a stepper. If you want the thing to be robust in the face of changing torques then you need feedback.

"Sensorless" is accurate from a bill of materials point of view, but a misnomer from the point of view of what's happening -- "motor as sensor" is more accurate, but far more klunky-sounding.

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

motor.

Sorry, I failed to "Define "does not seem to like". MTD6505 eval kit ($60; has very nice GUI with bunch of interesting options) comes with small fan motor. I can control the kit motor (start/stop, vary speed), speed is stable, it is proportional to duty cycle setting the GUI shows motor slowing down when I load it (finger). When I connect motor I need to use (Namiki), the story is very different. It may start only after bunch of unsuccessful attempts (not very good for servo motor; one cannot see any movement because of very high gear down ratio); the speed does not seem to depend on duty cycle (weird!); the speed reading is not stable. I did set motor constant according to the motor spec. There is no appreciable starting torque (the motor is geared down at 79:1, I painted the shaft to see it turning).

motor.

comes with small fan motor.

proportional to duty cycle setting the GUI shows motor slowing down when I load it (finger).

servo motor; one cannot see any movement because of very high gear down ratio); the speed does not seem to depend on duty cycle (weird!); the speed reading is not stable.

starting torque (the motor is geared down at 79:1, I painted the shaft to see it turning).

So what is you're conclusion ?

I think you aren't appreciating what's "appreciable".

Servo motor? My definition for that is something that has to generate torque from a stop, and may need to generate torque in the opposite direction of rotation. That's not the domain for a sensorless motor -- you need commutation if you're going to make that work.

You are using the motor in a significantly different mode of operation than what the chip expects: you're asking for more starting torque, and unless you're stirring molasses with the output of your 79:1 gear box, the torque is not rising with speed as it would with a fan.

For the sake of the experiment it's probably worthwhile to connect the motor to a fan or at least a flywheel and see how well the chip drives that combination. If you slap a fan on there and it all of a sudden works, then you've found your problem which means you're more than half way toward solving it.

If whatever assembly it is that you're building needs to hold position while it's stopped then you need a commutated motor, or you need to play weird games with the motor you've got.

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

I call it servo motor because of the way it is used, not what it is designed for (I do not know what it is, BTW). Our ME group specified this micromotor with gear box (4mm in diameter, 16mm long whole assembly), it will be coupled with the target (filter wheel) after another

1:5 gearing. I am told (by ME designer) that the target stops instantly after power is disconnested (no inertia to worry about). Similar system (with larger motor, lower torque, lower cost) has been manufactured for years. As motor controller generates 1 pulse/turn, the target position is defined with better than 1 degree accuracy - more than enough for the application. In other words it looks like perfect servo motor, provided it starts right away when voltage is applied (as opposed to after, say, 30 seconds worth of stuttering)

My definition, too.

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"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
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