I will post a new thread perhaps, to align the topic with the issue. Anyone reading can ignore this thread

 

I did not look up the particular motor that you have. But according to retched's post, it is about 58W. That is still on the low end of what is available in model BLDC motors and well within the range of power you project needing in your post #20. It is not unusual to find KW motors in models today, so there is a wide range to choose from in the affordable hobby market.

Planetary geared versions of conventional BLDC motors (i.e., not outrunner) are available at reasonable prices. Their shaft speed is still higher than what you seem to need, but the 4:1 to 10:1 reduction provided by the unit is a good start.

John

 

Oh okey. Re-reading what has been written I guess I could use that little motor @ 7.2 V with about 3 Amps then. So this requires me to just set the electronic speed controller @ 5000 rpm then I guess? So using P = U*I, I get 21 Watt, but what type of efficiency am I looking at here? How much of those 21 Watts become axial power? More than 50 %?

And regarding the power here, a regular PSU for a computer and a parallell resistance would be good? I mean using the 12 V rail. Any harm in connecting the 12 V directly to the ESC?

Is there some simple circuit I can build to make the PWM to the ESC? At first glance I would just figure some sort of microcontroller, but certainly there must be something simple I can simply control using a pot. I guess it will include some capacitor and resistance or whatever? Anything is fine, but I would like to have some variation in it. I recall vaguely that the protocol for the ESC is to vary the duty cycle of a 20 ms wave (perhaps I am using the terminology incorrectly).

Then there is cooling too. How about a regular 80 mm fan from a computer?

 

O
Any harm in connecting the 12 V directly to the ESC?

Depends on the ESC. How much voltage or how many cells (usually meaning NiCd or LiPo) is it rated for? In all likelihood, you will be OK using that 30A controller, but you should check its specifications first. Remember, the ESC passes the voltage to the motor directly. The most important consideration is how your motor will respond with 12V or whatever voltage your power source provides.

Is there some simple circuit I can build to make the PWM to the ESC?

Yes. There are many 555-based circuits. Check Bill Marsden's blogs here. Or, just search on RC servo controller. An MCU will also work, but the hardware pwm functions may not get to the 50 Hz needed for model servos. I have used somewhat higher frequencies (e.g, 120 Hz), which was attainable. You can also just code the pwm in software.

John

 

Depends on the ESC. How much voltage or how many cells (usually meaning NiCd or LiPo) is it rated for? In all likelihood, you will be OK using that 30A controller, but you should check its specifications first. Remember, the ESC passes the voltage to the motor directly. The most important consideration is how your motor will respond with 12V or whatever voltage your power source provides.

I guess I am a little bit rusty here, but what exactly does the ESC do. You say it supplies the voltage to the motor? I was thinking more in the line of the ESC being a current source and supplying "x" amount of amps, depending on the servo-signal it gets. Would you mind explaining the operations of the ESC?

 

How about this model of the ESC:

Three outputs, which form a 3-phase source, and the servo signal varies the voltage of this 3-phase voltage. Is that a useful model of it?

So the ESC takes DC-input and makes 3-phase output with different amplitude on the voltage?


And then I guess there is the feedback circuit which gets the RPM correct? Although the motor is rated at 1550 rpm/volt, i think. But does that matter much really? Is the ESC feedbacking on the RPM (through the back-EMF) or is there still an element of "tuning" if the voltage to the ESC is not spot on? Not that extreme correctness matters for my application.

 

For simple explanations of how brushless esc's work, see here:

http://www.globalrcmodels.com/schulze_electronic_brushless_speed_controllers_esc.htm (The description is the right hand sidebar.)

http://en.wikipedia.org/wiki/Brushless_DC_electric_motor (Note the comment about when starting with a sensorless controller. You will notice with many controllers a hesitancy at the start. That is generally attributed to the controller finding the right direction.)

http://en.wikipedia.org/wiki/Electronic_speed_control

For more detail, see here:

http://innovexpo.itee.uq.edu.au/2003/exhibits/s363281/thesis.pdf

In brief, your controller works just like a PWM controller works with a brushed DC motor, except since it is for a brushless motor, there are three PWM signals. That is, one PWM supply for each phase. It is not the voltage, but the ratio of "on time" to "off time" that is varied. That switching often produces an audible whine from the motor. It allows high efficiencies, because there is minimal heating from the switching compared to using a method such as a resistor to reduce voltage. Your controller is also probably sensorless, which probably means it uses back emf to regulate motor speed.