model aircraft BLDC motor ratings

[NiHaoMike]

I'm trying to make a very powerful fan and I have noted that model aircraft BLDC motors are getting very cheap nowadays.
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=3887
How much do they need to be derated for continuous operation? And how would I then match it up to a propeller for use as a fan? (I'm going to design and build the inverter, mostly as a learning experience.)

[sacherjj]

The link you posted has 45A for 30S as max, so your current will be under 45A by a good bit.  You will need to use a controller to drive the brushless motor.  You can't just hook to a battery. 

I'm not sure if there is a good formula to pick a prop.  However, you monitor the current of a running prop to see how much it draws at the duty cycle.  Ideally you will find one that runs up to your max current at full power.

A good resource for motor and prop matches will be aeroquad.com.  They have some good combos of cheap motors and props that give your best match.  Then you just pulse a brushless motor controller with a simple 555 circuit (look for a servo tester circuit.)

[RCMR]

Most motors carry a wattage rating and a maximum voltage or current rating -- you can deduce the missing factor by using Ohms law.

Note that in the case of the cheap motors there is often a significant exageration of the true capabilities of these motors.

You can buy very cheap 3-phase controllers for these motors which are controlled by a PWM input (1ms to 2mS at 50Hz rate) and that's a whole lot simpler than designing your own brushless, sensorless controller.

Note that because these are sensorless motors, there may be some issues with starting torque and delivering large power levels at low RPMs.

Still, I'm planning to retrofit the wife's cordless vacuum cleaner with a brushless motor setup -- it'll be a whole lot suckier than the brushed motor it has now :-)

[thmjpr]

The link you posted has 45A for 30S as max, so your current will be under 45A by a good bit.  You will need to use a controller to drive the brushless motor.  You can't just hook to a battery. 

He wanted to build it.

I agree with RCMR, just buy one, at least to start with. You can analyze their design (usually atmega8, fet drivers, and lots of expensive fets). Then mod it with a larger heatsink/fan if necessary. The code is going to be the hardest part IMO.


Mike:
You can find some discussion here, although its mostly mechanical:
http://www.cnczone.com/forums/diy-cnc_router_table_machines/112743-diy_spindle_idea.html#post829461
http://www.cnczone.com/forums/benchtop_machines/12184-high_speed_spindle_sieg_x2.html#post94477

Also found this site which lists continuous as half of the surge rating: http://www.neumotors.com/Site/2200_series.html

So, 45A peak, maybe 20A continuous. Add some margin for safety/exaggerated specs, probably 10-15A is reasonable (with air flowing over it). If you can measure the winding temp, you could be certain and push it harder.

One more awesome link (ducted fan): http://www.etotheipiplusone.net/?p=828

[Mechatrommer]

He wanted to build it.

i think its she.

[kaptain_zero]

Having just started playing with these types of motors..... a few thoughts.

1. Look for a motor with the required watts and with the lowest KV rating which stands for RPMs/Volt. 1000KV means at 10V, it will be screaming away doing 10 000 rpm! These motors are not quiet, not by a long shot. Granted, with a quiet propeller, it should not be too bad, if you keep the rpm low, but the whine from these high RPM motors will make it sound like you are at the Dentist!

2. There is plenty of info on how these motors are controlled to be found on the internet... you just need to search a bit. A friend is using one of this type of motor as a router motor for a CNC router which he intends to use for making small PCB's. There are devices called "Servo Testers" that are quite inexpensive, as in about $5, that will let you manually control an ESC (electronic speed controller) without the need for the remote control stuff.

3. Low KV rated motors have more poles (say 12 to 16 or thereabouts), which I am going to assume will give more starting torque than a high KV motor which can have as little as 2 poles. The ball bearings used in these cheap Chinese made motors are made cheap in.... you guessed it.... China! I'm not saying that is a bad thing but I've seen them wear out fairly quick and replacement bearings are $0.99 a set, including shipping..... from China. Locally sourced bearings from a bearing supply company were quoted at $18.00.   

4. For quiet, long term use in a fan...... it's hard to beat a good old induction motor, with plain bronze bearings, running off the mains..... cheap and quiet.

Regards

Christian

[NiHaoMike]

The only times when I'll be running it near "full speed" is when noise is not an issue, so no real problem there. The PWM frequency will be just above the audible range (22kHz or so) to avoid switching noise at low speeds.

Using an inverter that makes a BLDC motor emulate a brush DC motor would be very easy, but that's no fun! I'm planning on using a dsPIC to perform FOC. I'm really after the learning experience.

The fan would need to be able to run from a DC supply (12V or so) so a BLDC motor makes the most sense. (Before anyone suggests using a PC fan, the only ones I can find do not rev high enough. I currently use a 120mm, 6500RPM PC fan.)

As for the application, it's just a general purpose fan. From providing temporary cooling for equipment with poor thermal design to providing simulated wind for a video game, I'm not looking at designing it for a particular application.

In my experience, cheap sleeve bearings are a lot less reliable than cheap ball bearings. Not sure about fluid dynamic bearings as I haven't seen enough of them to tell anything about their reliability, but they theoretically would last longer than ball bearings.