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Wires of my brushless motor have different resistances, what can I do?


Old 09-16-2017, 09:26 AM
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Question Wires of my brushless motor have different resistances, what can I do?

Hi people
I'm new to this site and to the hobby.

I'm scratch building an FT Tiny Trainer, and I'm having a stuttering problem with my Emax MT1806 motor. It spins but it stutters. The stutter is sometimes better and sometimes worse. I crashed once and after the crash the stutter got so bad it wouldn't spin at all. It would just go back and forth real fast. Then it magically got better and started spinning+stuttering like before.

It was acting funny since I bought it(more info about that below*) and after some research I measured the resistance between all three combinations of the wires. AB is 0.2 Ohms. AC is 0.2 Ohms. But BC is 0.6 Ohms.
According to my research this will cause exactly the stuttering that I'm experiencing by confusing the ESC about motor position.

Another odd thing about the motor's behavior is that it will not stutter if I start it when it's already spinning. Let me explain, for example when the motor is stationary and I throttle up to 1/4(no-load), the motor will stutter and draw high amps(~2Amps). Then I throttle down to zero quickly and before the motor stops spinning I again throttle up to 1/4. This time the motor does not stutter and the amp draw is 300mA. As long as it's spinning I can throttle up and down as much as I like and it won't stutter.
It only stutters when I start from stationary position. I changed the startup mode to Soft and Softest in the ESC but it got worse.

I have removed all the bullet connectors and checked the connections.
I have tested the ESC with another motor and there is no stutter.
I have measured the other motor's wires' resistance and all of them have the same resistance, 0.2 Ohms. (I think 0.2Ohms is as low as my multimeter can measure).
Unfortunately I don't have another ESC to make certain that the problem is with the motor.
I measured the actual wires. No connectors attached. Measured multiple times. Made sure contact is perfect. But the numbers are always consistent.

Now I know from research that my motor is most likely faulty, most importantly because of the resistance difference, but none of the posts I've read have mentioned if there is anything that can be done about a motor with such condition.
I opened it to investigate the internals and there is nothing suspicious. The magnets are firm and there is no physical contact. I cleaned it in hopes that it will magically get fixed but it didn't.

So my question is, is there anything I can do to fix it? I really don't want to buy another one due to budget issues.

*More details:
After I bought the motor it had a little friction when I spun it with my hand. It felt like something was grinding in there. As a noob I thought maybe it was normal and that it would probably wear off eventually due to friction. So I spun it with my hand for a little while and the grinding thing got better. And after I powered up the motor a few times it was gone.
But then I had this strange stuttering problem. Being a noob (and excited to get in the sky asap) I thought it wasn't very important. I decided I would do something about it later. Because the stuttering only happened when the throttle was very low, and when I increased throttle it would go away.
I put the motor on the plane anyway and flew it. It crashed.
Now one week after the crash I tested the motor again today. And it would stutter all the way. It would not spin at all.
So I started researching and found out about ESC timing. I changed the timing to Medium from the default Low and the stutter improved. But it was still there. I again changed the timing from Medium to Low, but nothing changed. I expected it to get worse again but it didn't.
It's strange. It looks like the more I use it the better the stuttering gets no matter what timing I set for the ESC.

I also did the dry testing thing(forum doesn't let me post the link).
It passed test 1: Motor does not get hot when I spin it with a drill.
It also passed test 2: All combinations of wires generate the same amount of voltage(although I'm not sure about the accuracy of this. My digital multimeter is a cheap one, and it would only give me 0.5 volts, I'm not sure at that voltage the effect, whatever it is, will get amplified enough to be measurable by my multimeter).
The odd thing was test number three. That post says there should be considerable drag when you short all the wires together and spin the motor with the drill. But there was absolutely no drag when I did it. I didn't even hold the motor. My drill was spinning at 2500RPM and the only thing holding the motor in place were the alligator clamps shorting the wires together.

Thank you for reading all this.

Last edited by pouria19; 09-16-2017 at 09:33 AM.
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Old 10-04-2017, 07:46 AM
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The fact that the problem stays with the motor when you swapped it out, means that the motor is defective. Since you know there is a difference in resistance between the windings, ie one higher, there is an internal fault in the winding of that pole. Because of the high resistance,the ESC is not driving that pole hard enough to to rotate the motor rotor past the magnet on start-up and thus the "shuttering". Probably a manufacturing defect that is not correctable. If you can contact the supplier maybe they will replace it for you. I have had good experiences with such situations. Otherwise you may be out of luck.
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