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Just checked my motors speed...

Discussion in 'Pro/Adv Discussion' started by NRJ, Sep 10, 2016.

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  1. NRJ

    NRJ

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    I just got a digital tachometer to check the speed of my motors on my P3A.
    The motors are the 2312A type. I have had a few accidents. I have suspected slower motors for some time now. One motor appears to be slower than all the rest. I hear a noise when hovering which seems to be coming from a motor and sound like it could be a bearing. So, I tested my motors speed while idling to confirm if it is good or bad. Now I need your help. I'm using a SainSonic TA-3 Digital Tachometer.

    Results looking from the front to the back of the drone battery in the back:

    Front Left: 2061 RPM
    Front Right: 2248 RPM
    Back Left: 2948 RPM
    Back Right: 1865 RPM

    It appears the Back right at 1865 is much slower.
    The back left may be normal as it is the only one with the higher speed?
    The front left, front right, and back right may need replacement.
    Maybe I should replace all motors.

    Sure could use your advice and help. Thanks.
     
  2. Sagebrush

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    If it hovers and stays in place, I'd guess it's the craft adjusting the four motor speeds to maintain position.

    If you had a motor problem, I'd also guess it couldn't hover.

    ???

    SB
     
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  3. Lico

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    Your drone could have a fauty motor due to the crashes but don't go by the rpm because any inclination or any wind will make each one of the motors to slow down or to accelerate in the effort to stabilize it.
    If you hold it from the landing gear and move it around you can fill all the motors changing RPMs but be carful, depending on how much you move it some of the motors can even stop momentarily.
    Go by your feelings, and if there is a noise, it is because something is not right and it will eventually become a problem. If you can identify the motor that is making noise, replace it.
    Is it flying well? Do you feel any tendencies when hovering?
     
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  4. flyNfrank

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    Motor speeds will always vary as they are part of a propulsion system.
     
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  5. NRJ

    NRJ

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    Additional information... When I tested the motor speed, it was indoors with no wind or other disturbances. It was also on a balanced surface which I checked. That is why I thought that the motors should be idling at the same speed. Not exact speed, but closer than what I have. Also, I have tried to determine which motor is making noise, but that is very hard to do. It only makes noise when I have flown and brought it down to a hover in front of me to examine. But, I don't get to close either.

    After my last crash some time ago, it has been obvious to me that one of the motors was slow. That is why I tested them. I just don't want one motor to go out when it's up flying because it is working harder than all the rest. I know the controller increases or decreases speed as needed.
     
  6. flyNfrank

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    The only thing that can be done is by feel when rotating each motor by hand while comparing them to the other 3.

    As for having them run, you would need to connect them to a source that supplies the exact power amount the other 3 are supplied with.
     
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  7. NRJ

    NRJ

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    That makes sense to test them on the same power source. Not sure I am able to do that without much expense. But, I did test turning the motors by hand. The one motor which is suspect is the one at 1865 rpm. When I turn the other 3 motors slowly, you can feel a kind of notch (for lack of a better word) or maybe a tug that the motor labors after on all three motors as it is being turned as if it is rubbing against something. But the one with the rpm of 1865 has much, much less of a notch//tug when turning it. This is the one I had suspicions about but could never say for sure. Now with the lower rpm and the less of a notch/tug while turning the motor, would that shed any light on the motor speed? Or would that give an indication that this one motor could be an issue? Would it be wise to replace this one motor or does one thing "not" have anything to do with the other? Thanks.
     
  8. maxwell smart

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    The tug is the magnets pull on the motor. Low resistance in one motor is not necessarily bad.
    You could change it out if that would make you fell safer.
     
  9. NRJ

    NRJ

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    Thanks for clarifying that. That helps. The reason I would maybe do that is because the one motor with less magnetic pull or resistance is the only motor different from the rest. Possibly the magnets have been damaged due to a break in the coiled wire in the motor, hence less resistance when turning. I may do that if for no other reason except like you say to make me feel better, but also to test it again with the digital tachometer to see if the rpm's change and to see if the resistance while turning is more like the other 3 motors once changed out. Thanks.
     
  10. maxwell smart

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    I think they use live magnets. I changed moters out on my P2 for the same reason you are having.

     
  11. NRJ

    NRJ

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    Did it help your situation once you changed the motor?
     
  12. maxwell smart

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    I changed them because I had some crashes. I thought one motor felt week, so I changed it out. I just wanted to be safe.It is still flying.
     
  13. With The Birds

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    I wouldnt suspect a motor problem to explain the speed variations you have measured. RPM is determined by line frequency commanded by the ESC. A bearing or other motor issue would simply be seen as additional load at the ESC with that particular motor drawing additional current. I suspect the ESCs also read the back emf from the motors to determine actual rotational speed and the flight controller would report a motor error if they were outside a given tollerance range. If the motor was unable to lock to the commanded line frequency I doubt the phantom would maintain a hover.

    Your are going to give yourself an unnecessary headache with that tachometer.
     
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  14. Lico

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    Try to turn each motor by hand without the propellers to check if there is any abnormal resistance or plays and if they are crooked or wobbling.
    Also give full power and little power without the props alternating rpm and try to hear any noise.
    Alternating rpm is important because some vibrations or lack of lubrification will only show at an especific rpm.
    Do not try to lubricate by spraying oil.
    If you want To lubrificate the motors remove them and use small drops of silicon oil. Don't use WD-40 as it can cause a reaction with some plastics and rubbers.
     
  15. NRJ

    NRJ

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    You got me laughing about the tachometer giving me an unnecessary headache there. I do agree with you that a "bearing or other motor issue" would be an additional load. I am concerned that because if it is a bearing or other motor load that the ESC is causing an additional load on that motor to compensate for the lack of power or thrust and that is why I would want to "maybe" change it out because any motor with a constant additional load will probably give out or die earlier than the rest of the motors that do not have a bearing or other motor issue. That is what I am trying to avoid if in fact that is or may be a problem. I sure don't want to lose my drone because one motor finally gave out because of an additional load being put on this one motor and it dies in the air dropping it some 250 feet or so. What do you think?
     
    #15 NRJ, Sep 10, 2016
    Last edited: Sep 10, 2016
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  16. NRJ

    NRJ

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    In my other notes above, I have turned the motors by hand. The one motor in question is much more loose (if you will) and is much easier to turn than the other three. There is no wobble in any of the motors, none appear or feel loose in any way. Alternating rpm is interesting. I haven't tried that. I will try it and see if there is any noise, or anything detectable. Thanks.
     
  17. Monte55

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    Could you explain how you used the tach to determine RPM? Most results from the cheap tachs are very iffy. Did you put reflective material on each prop? Ambient lighting conditions has a pronounced affect. I have an old optical tach that is made for props. It sees the breaks in the light. It doesn't emit any light. To be sure it is accurate I aim it at a florescent light fixture and it reads 3600 rpm.
     
  18. NRJ

    NRJ

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    The tachometer kit gave me reflective tape which I put on one end of the propeller. I put black adhesive tape on the other end of the propeller so it does not give a double reading of the same object. Therefore, it is one revolution per minute rather than two if I did not put the black tape on the other end. I saw this on a YouTube video that I watched before attempting my endeavor. I held the tachometer still over the end of the propeller where the reflective tape was and held it for about 30 seconds (one half minute) on each propeller. The readings offered were High, Low, and last reading. I used the High as that is what I was looking for which is at the drone idling speed.
     
  19. flyNfrank

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    Personally here is how I would handle your situation if I had you aircraft given to me and no others. First I would attempt to duplicate any issue you mentioned. If I heard a noise of any kind I would shut the a/c down and look at the clearance between motor's and body. If ok there, I would try to lift up and down to check for slop(movement) per any up and down lifting. If that test passed, I would then again power up the a/c and allow it to idle some 5-7mins. I would then use my infrared thermometer and check for unusual or excessive heat for just idling. Now, if it was to fail this test, that doesn't mean the motor is the problem. It could also be the ESC for that motor. If it passed this test, I'm done doing all test and will then continue to fly the a/c as I normally would.

    When you change any of the parts that require soldering, I feel like you also open yourself to additional problems. I lost a P2V+ after changing out all the motors and 6 flights later a wire broke due to a poor soldering job on that wire.

    Btw, I want to point out to you that using a infrared thermometer can not be relied on unless the same amount of current was going to each motor during warm up. When the P3 sits at idle you will most likely hear the rpm in the motors raise and lower. In some cases this is from the barometer sensor programming. When landing, if coming down too fast the barometer will increase speed to help keep the a/c from landing too hard or crashing into the ground. At some point it may begin to pogo and make it harder to land as well.
     
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  20. NRJ

    NRJ

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    flyNfrank, so, it appears that the same issue exists between the tachometer rpm test and the current that is going to each motor as well as with the infrared thermometer. So, it is only a test and can be accurate or maybe not. Makes sense to me based on the current going to each particular motor. I understand. Also, a big "And" is that there are many variables involved in the current, positional telemetry, and atmospheric conditions that can affect the motors and this is all handled "pretty darn well" within the ESC, whether indoors, or outdoors, much of what I don't understand. This is evidently not a day and night issue. I will take your advice for now and do the tests you have outlined above and note the effects of those tests and see if there is anything that "stands out" as "out of the norm". I'll try to do that next week sometime. Thanks for your expertise and advice and to everyone's help and advice as well. One last question, where would I find an infrared thermometer? Thanks in advance.