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#### johniee

• Newbie
• Posts: 14
##### Homemade dc brushless motor (Inrunner)
« on: June 13, 2010, 20:46:47 PM »
hellow guys, i have bult a motor, basicly with materials from home. But the motor is dysfunctional.i will describe my motor in short.
I use a funs controller (fun from cpu with 3 wires) , a shaft form another fun, coil from bike generator, a cylindrical cardboard, wires etc.
first i put the contoler perpendicular to cylider, specify  i make a slot in cylinder  and there i put the hall sensor of contoller. I put 2 coils around the cylinder so the hall sensor detect no field. I connect the 2 coils to contoller in way to have opposite magnetic field. In the shaft i put two small magnets, in pi rads angle, so in the rotor i have one pair of poles ( N-S-pi angle-N-S). Then i put the the shaft inside at cylinder, now the axis of shaft and the axis of sensor (of the side which detecting ) is approximently 90 degrees, so the sensor detects the cos(ωt+fo) of each pole (N,S) while shaft is rotates with angle velocity ω. When one pole have smaller angle the contoller turn on one coil , when detects the other turn off and turn on the other coil so the shaft rotates.
But my motor is dysfuctional:
1) to start, it must be one of the poles in angle  smaller to 40 degrees with sensor .
2)After while the motor stops to rotates, when stops the axis of poles is parallel to axis of cylider and coils.

I believe that the causes  of that dysfuctional is:
that the sensor is not too closed to magnet poles so for big angles , detect no poles. We have  critical position (poles is parallel to axis of cylider and coils) in this position cos(ωt+f)= cos(90)=0 the sensor dedect no field and this position is stable because the magnet of shaft is opposite of magnet of coil. If the torque of friction is bigger thanthe torque of  inertia at this position the shaft is stop at this stable position.
Also the shaft is not good fasten so it removing, so it go away for dedector.
intersting that some times motor don't start when have small angle beteween magnet-sensor i bealive that the reason for this the not good fasten of shaft. Byt if i put the cylinder stator opposιte( at gravity) the motor don't start

SO have anyone any else  ideas??  are other  reason for  malfunction?? if i want to measure the tacho,  i must use the yellow wire and two palses is one full turn right?

#### kam

• Hero Member
• Posts: 1849
##### Re: Homemade dc brushless motor (Inrunner)
« Reply #1 on: June 14, 2010, 07:41:02 AM »
This sounds an interesting experiment. Send some photos to see what you have make.

As for the tacho, you are correct.

#### johniee

• Newbie
• Posts: 14
##### Re: Homemade dc brushless motor (Inrunner)
« Reply #2 on: June 15, 2010, 01:29:40 AM »
ok i  attach some fotos of  my bldc motor. i hope to be helpfull.
(i cant paste it there  )

#### johniee

• Newbie
• Posts: 14
##### Re: Homemade dc brushless motor (Inrunner)
« Reply #3 on: June 20, 2010, 00:49:23 AM »
Guys  for the tacho, i finally believe that in my motor one pulse is a full turn of rotor, because we  count two palses is one full turn for the common bldc that used in pc funs. The common bldc have 2 pairs of poles( N-S-N-S) so one pulse is one of the poles( N or S) and 2 pulses means the full turn of rotor. However in my motor  i use one pair of poles, so i think  that one  pulse is a full turn for rotor, correct?

#### kam

• Hero Member
• Posts: 1849
##### Re: Homemade dc brushless motor (Inrunner)
« Reply #4 on: June 20, 2010, 08:19:00 AM »
Indeed, if you do have only 1 pair of poles, you will get 1 pulse per rev. Why don't you make a test to find it out? Do you have an oscilloscope? If not, you can make a probe with an LED yourself and check if in each turn you get 1 or 2 pulses.

That would explain the reason why your fan fails to start. I suppose also that, when the fan is stopped, if you give power, it will rotate and will stop to a very specific position all the time correct?

#### johniee

• Newbie
• Posts: 14
##### Re: Homemade dc brushless motor (Inrunner)
« Reply #5 on: June 21, 2010, 00:09:54 AM »
I don't have an oscillocope, but i will test it to an oscillocope in my polytechnic school. But i can't verify it with another method, with the eye is not posible.. and with the tachymeter ( a light is flashing at variable frequency) i think it is difficult because the rotor is small and inner.

Kam you are correct. When my motor stops , it stop at very specific position, this position i mark it one photo above as the critical position[the axis of poles (one pair) is parallel to axis of cylider and coils, paraller to magnetic field of stator].Also when rotor is close to this position is don't start.

i can make a probe with an LED ? interesting..

#### kam

• Hero Member
• Posts: 1849
##### Re: Homemade dc brushless motor (Inrunner)
« Reply #6 on: June 21, 2010, 00:51:21 AM »
i'm pretty much sure that you either have the hall sensor in the wrong angle, or the poles are incorrect (as described above).

#### johniee

• Newbie
• Posts: 14
##### Re: Homemade dc brushless motor (Inrunner)
« Reply #7 on: June 22, 2010, 00:33:03 AM »
for that poles are incorrect, yes kam, i believe that too. Because i have one pair of poles, it exist a position at rotation of shaft that the sensor dedect no field, as described above.I didn't found more suitable permanent magnet at the local electronic shop.

I think that is in correct the angle of sensor.The side that 'looked' at the magnet, now it 'look' at the verctical axis at shaft , at poles and i think the sensor dedects the cos(ωt+fo) of each pole( n,s) as described above.

#### kam

• Hero Member
• Posts: 1849
##### Re: Homemade dc brushless motor (Inrunner)
« Reply #8 on: June 22, 2010, 22:03:57 PM »
Unfortunately the images you posted are blur and not clear. When you make the motor work, if you make a video, i will gladly feature it in the hacks n mods section.

#### johniee

• Newbie
• Posts: 14
##### Re: Homemade dc brushless motor (Inrunner)
« Reply #9 on: June 29, 2010, 00:55:39 AM »
ok , now it works but in sometime it stops as (described above). Anyways, When i find some free time i will try to remake it. Also i will try to put more poles ( if  i find more suitable materials ) and i hope then to work it  completely, i will shoot it video