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  1. #1
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    Cogging and screaming vxlm question

    Hi.

    I noticed my merv cogging today on low trotttle startup.
    Just turned it off then on. It helped.
    But sometimes it cogging/suttering a few times on start on lower trottle, around 1mm in or around 1/10 it coggle, but if i give 2mm in or 2/10 trottle it stop coggling.

    When i was finish to drive when i had low power on the NiMh it only make a scream sound when i give low trottle, i continue to give more and even full trottle and it only scream, but if i hitted trottle fast it went fine.

    What happen when it scream? To low battery?
    And how mutch cogging is normal?

    I searched and some people say venomline cogging and some other say they replace the esc...
    If i give around 5-8% trottle it coggle but if i just give more from start 8-10% it goes fine.

    Thanks for answer.
    Last edited by Ray87; 07-09-2017 at 09:44 AM.

  2. #2
    RC Competitor
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    Most brushless systems cog when the batt cant supply the power to esc i run a lipo 2 cell batt with a 40c dicharge an 2500 mah i get a lil cog on start up an it gets bad when batt is bout drained some will say gear down a bit to help but i say grab some better batts hope this helps as for the scream if you mean the beep noise yea mine dose that to but most of my esc combos do that brushed n brushless some louder then others its just the inatial power going to motor

  3. #3
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    Hi, i charged the battery and i do not get this biiip and nothing move around anymore.
    But if i slightly give very very little trottle it cog but if i give very little after that it goes smooth.

    Tried the same thing with a stampede vxl an it cog but for a shorter period vs my merv.

  4. #4
    RC Turnbuckle Jr. Squeegie's Avatar
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    Screaming could be a loose slipper...
    Creativity is intelligence having fun. -Einstein

  5. #5
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    The truck still only screm if i give very little trottle one day and one other day not.

    The biip/scream sound is there if i give very very slowly give maby 5-8% trottle.
    It scream and nothing goes around, if i give slowly more trotte until its on full trottle the car still only scream and nothing spinn around. What could this be? If i just give normal trottle it works fine.
    Have never had this scream sound before ever in my previously trucks that i have owned before.
    And the stampede does not screm like this on the same test.

    And if i turn it off and try again it not happen anymore even with very little trottle, only get a small coggling.
    But next day if i try the same thing it could only scream again. And other day not.

    Is this normal for a mini vxl?

    If i get this screm thing i just stop giving trottle and give more normal trottle like everyone else doing when they start to run, then it not happen. So just wondering if it could be normal or not on this mini vxl system.

    My guess is that when it give so less trottle the esc fail from beginning to take the right volt and it fail to start up? Im just guessing
    Last edited by Ray87; 07-10-2017 at 07:36 AM.

  6. #6
    RC Champion
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    What we refer to as cogging is 100% normal on the mini revo, and also any sensorless brushless motor.
    The reason it happens is that these are sensorless brushless motors.
    On a brushless motor, you have three "coils" that are energized is specific order to make it spin. For the ESC to get speed of the pulses right, it has to know how fast the motor is spinning.
    On a sensord brushless motor, it has sensors that tell the esc how fast the motor is spinning, which connect to the esc with extra wires.
    On a sensorless motor, the ESC reads the "back EMF" coming back through the motor wires to know how fast the motor is spinning...

    (Back ElectroMotive Force, basically, its voltage created by the spinning motor, which is going the opposite direction of the incoming voltage. the ESC reads the voltage coming back through motor wires to determine the motors RPM)

    ... but in order for there to be any back emf, the motor has to be spinning, which obviously it is not when the truck isn't moving... To get around that issue, a sensorless ESC will send out a preset sequence to get the motor spinning so it can read the back EMF. This normally works fine, but, sometimes, it doesn't and the ESC cant figure out what speed/order to send the power... letting off the throttle resets it and lets it try again starting with the preset sequence to get it moving....

    The lower your gearing, the more likely the motor is to start working on the first try. Giving very small throttle inputs is also more likely to cause it to fail on the first try, as the motor doesn't get spinning fast enough to generate the needed back emf for the esc to know what to do...
    Dacaur

  7. #7
    RC Qualifier
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    Quote Originally Posted by dacaur View Post
    What we refer to as cogging is 100% normal on the mini revo, and also any sensorless brushless motor.
    The reason it happens is that these are sensorless brushless motors.
    On a brushless motor, you have three "coils" that are energized is specific order to make it spin. For the ESC to get speed of the pulses right, it has to know how fast the motor is spinning.
    On a sensord brushless motor, it has sensors that tell the esc how fast the motor is spinning, which connect to the esc with extra wires.
    On a sensorless motor, the ESC reads the "back EMF" coming back through the motor wires to know how fast the motor is spinning...

    (Back ElectroMotive Force, basically, its voltage created by the spinning motor, which is going the opposite direction of the incoming voltage. the ESC reads the voltage coming back through motor wires to determine the motors RPM)

    ... but in order for there to be any back emf, the motor has to be spinning, which obviously it is not when the truck isn't moving... To get around that issue, a sensorless ESC will send out a preset sequence to get the motor spinning so it can read the back EMF. This normally works fine, but, sometimes, it doesn't and the ESC cant figure out what speed/order to send the power... letting off the throttle resets it and lets it try again starting with the preset sequence to get it moving....

    The lower your gearing, the more likely the motor is to start working on the first try. Giving very small throttle inputs is also more likely to cause it to fail on the first try, as the motor doesn't get spinning fast enough to generate the needed back emf for the esc to know what to do...
    That was very good explaned! Good to know how it works, i guess that explane why it sometimes just making scream noice also when i try this, instead of coggling it scream and dont start the motor since it got to little trottle from the beginning and it fail even if i give full, but release trottle and give little more give the esc enough information to start

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