How to make an over-current fuse/switch to avoid damage to a brushless motor

Discussion in 'OnBoard Electronics & Controls' started by BertKu, Aug 15, 2015.

  1. ElectricKayak
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    ElectricKayak Junior Member

    I think that would be OK. I can't really see a big problem with putting it in the minus line.


    Documentation says around 500kHz.
     
  2. BertKu
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    BertKu Senior Member

    Thank you, that gives me the confidence, that should I not be able to find more P type Mosfets, I will finish it with the 110 Ampere , 8 milli Ohm N type.
    Thanks for your consideration.

    At that frequency, I assume the the Henry's will be higher. In my calculation, it is a formula for a single wire wound around the 4 metal frames. I have no idea how to calculate it when it is 100 wires, but soldered on both ends. I have a formula found for multiple wires, but not for stranded single wire.

    Bert
     
  3. ElectricKayak
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    ElectricKayak Junior Member

    Personally, I wouldn't make that assumption. Papers have been written about whether physical L will increase or decrease with frequency and under what conditions. In a motor it probably changes more due to current than frequency. Also apparent L must be considered also. I doubt all these differences will turn a 60uH motor into a 60nH motor (or vice versa) so I intend to just measure it and call it good. :)
     
  4. BertKu
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    BertKu Senior Member

    Hi EK,
    For sure you are right, but that is not what I meant. My apology, I should have better explained it. I normally measure at 1000 Hz and whether the core is an iron core or a ferrite core, I do get a reasonable accurate reading.

    Without core the L in air will be substantial lower. Should I measure the same coil with an iron core at 500 KHz, I have my doubt that it will be accurate and my gut feeling is that it shows a higher reading for L. Those units, I think are made for measuring high frequency coils with or without ferrites. Not for iron cores. But I maybe wrong.

    I think, it could be calculated what the inductance is for our motors by putting the ESC at 50/50, read the frequency from the oscilloscope screen. Take the Voltage /divided by the (average)current, which gives you XL . You know the Ohmic resistance. From there one can work out: XL = Omega L . Thus let say the current is 10 Ampere, your Voltage is 12 Volt. Therefore XL = 1.2 Ohm , the Ohm’s resistance is 20 milliOhm and let say the frequency is 10 KHz, thus L = 1.2 divided by 2 x 3.14 x 10.000 = (Lets ignore the Ohmic resistance of 20 milliOhm, which is so low versus the XL) = 0.000019108 Henry or 19 uHenry. If you really want to accurately calculate L , we could do that by taking Ohmic resistance and XL and look up in the table what the opposite length is, which is probably very close to the above calculation and give you 19 uH. What the real outcome is, can only be calculated by you for your motor.

    That is what I actual meant. Sorry for that.

    But something else I thought to enclose the final product in a 40 mm plastic pipe with on both side a plastic cable gland. I will, on my lathe turn, 2 end pieces, which you can glue after you have put your cables in. I trust that Porta also is in agreement to do it that way. If you have other suggestions, please let me know.

    By the way. I blew the 2 IRFB4710 MOSfets, when I reversed the polarity accidental, I mentioned this, because my 8705 blew. The moral of the story is, don’t do it also !!! I do not have more MOSfets then the 2 needed for your unit . I will only go to Cape Town on the 29th of September. I still have enough IRF4905 for Porta (to blow ????).:)
    Bert
     
  5. BertKu
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    BertKu Senior Member

    EK, Porta, I have now blown another IRB4710, but know now exactly what to do and what not to do. Those MOSfets are much more sensitive then the robust IRFP064. I have now only 2x 4710 leftover for you EK. I better put them in "cotton wool" and be very careful. Bert
     
  6. ElectricKayak
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    ElectricKayak Junior Member

    OK, understood. These units can also measure at a low frequency (500Hz?) if one is concerned about the iron core. I'm not sure if it will be a better or worse representation of the motor's inductance though. Perhaps best to measure the L/R time constant for the effective inductance.

    By the way, I found via Google someone who has already measured the motor's inductance (calculated directly from the phase currents). He gets around 50uH. ( https://endless-sphere.com/forums/viewtopic.php?f=30&t=36479&start=15#p540296 ). My motor is the 148kv version and will be a little higher. So we have our answer and no need to wait for the arrival of my LC meter. I will use it only to verify his number.
     
  7. ElectricKayak
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    ElectricKayak Junior Member

    Dear Bert...please put your ground strap on! :)

    But in all seriousness, do you expect them to be robust in the circuit? Otherwise perhaps one should go back to the N type?

    Also your plastic pipe enclosure works well for me.
     
  8. BertKu
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    BertKu Senior Member

    Yes, but I do not need it actual, I tested the gate at what level it will blow.
    Yes, except if you reverse the voltage accidental. They may or may not survive. The problem with N type cutting the minus potential, is when the motor windings ever make accidental contact with ground/earth/negative, the motor will blow and the unit will not switch off, as the current will go via the casing and probably via earth to your battery. However that maybe the case with yachts or aluminium boats/canoes, which has the engine block centrally earthed. In your case maybe the battery only sees your unit and the ESC/motor. But I still think cutting the plus is a better option. THUS I HAVE TO BE CAREFUL with the last 2 x 14 milliOhm P types. And if it really comes to the push, you have to wait until the 29th of September, when I can get a few more 12 Volt DC to DC invertors.

    OK when we are so far, we will put a photo on the thread. I also have been able to get some more Hydrogen Peroxide (it was already 5 years old)On Friday I will give it another bash to get some good pc boards. Bert
     
  9. portacruise
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    portacruise Senior Member

    PC: As regards the enclosure, I would have twin lead going in and out about 5mm or so and it is not round, but flattish. Just about any caps might work, if it is less work. My small high speed rotary tool can cut the holes if it is a solid cap and then silicone can be used to seal, air tight if that is advisable. Your prototype board was about 85mm X 40mm so maybe a little shave to get into 40mm pipe....


    PC:Will there be a heat sink for such a low power, and is ventilation required? What are good guesses to the vulnerabilities of your switch besides reverse polarity? Wonder about shock (drop height), moisture (rain, splashes), heat (45 degree C outer case if in blasting sun), hundreds of on (100%)-off cycles (pulse currents) within a single outing, etc.

    PC: I have to admire you and your skill, as it can't be easy taking this on and working only with information that has to be simulated- even if it is fun for you. :)


    PC
     
  10. BertKu
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    BertKu Senior Member

    that is good news. 500 hz will give you a good reading.

    That is approx what your motor is according to the calculation 10Khz, 1.2 Ohm XL, at 10 Ampere and 12 Volt. Maybe you can just put your ESC at half and measure the current and judge the frequency. Thereafter it is a piece of cake to calculate the L value. (ignore your milliOhm DC resistance)
     
  11. BertKu
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    BertKu Senior Member

    Hi PC , that is fine, it is a rubber tule which clamps around anything, also flat twin wire. The hole in the tule just has to be smaller. Or I must try to get 00 gland and not a 0 gland. You turn the large plastic nut by hand , until the cable is tight and splash water proof.

    No, your power is so low 2 Ampere x 20 milliOm (4905) = 80 milliwatt. The extra copper on the pc board is enough, For EK I have to make a small aluminium heatsink. No need for air cooling. Otherwise it will not be splash waterproof.
    Will the temperature go over the 100 degrees Celsius ? It may affect that the regulated Voltage is no longer 5 Volt, but 4.98 Volt and maybe the MOSfet resistance will go up to 25 milliOhm from 20. However it should not affect the effectiveness

    Thank you.
    You both at the end, will have to verify what I am doing, is acceptable to make it a success. Time will learn. I have made many successful items in my life and I hope also to make this a success for you and for myself.
    Bert
     
  12. BertKu
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    BertKu Senior Member

    Hi Porta, EK,
    Making the PC boards was not to my satisfaction.
    The differentiation between black and see-through is not good enough from the HP printer versus the Canon printer. On Monday I will go into town and will make a photocopy on a overhead projector slide.
    On the photo you can see what I had in mind. The small black 00 glands are for you Porta and the larger 0 glands are for you EK. The two sides you have to seal it yourself, after you also have properly tested the units, thereafter you should seal it with silicone. I normally use a German product Reinzosil, 300 degrees Celsius, black excellent stuff.

    It glues on anything and stay flexible. Also you should use silicone stuff to seal the cable in the gland. You should have a waterproof unit. The 10 mm LED’s I will fix in the casing also with that black stuff. I am in two-mind whether I should get you a reset button, brought to the outside, but we have to discuss whether it makes sense. If you have a stalling situation, you have to switch the system off in any way or not? For you Porta it maybe make sense, as you may have some weed on the prop and you may have a different way in removing it, without switching the system off and just resetting the processor thereafter.

    EK has only basically a stalling sitiation and he needs to switch the system off.
    Bert
     

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  13. portacruise
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    portacruise Senior Member

    Thanks, Bert
     
  14. BertKu
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    BertKu Senior Member

    Hi Porta,

    Yes it would work. Sometimes I use insulation tape and wound this a few times around the twin leads, to make it easier to clamp it very tight. But in anyway you should use a little bit of silicone glue.

    You see Porta, for that reason when the unit gets switched on, we have a 1 sec delay and then it test for stalling and the again 1 sec delay. I love to reduce the 1st delay to only a half second and assume that already enough torque is applied to the prop and the current is already reduced to below stalling current level. However it is your call. Whatever happens, if the client pushes the reset button on and on and on again. It gets reset and then
    the red LED comes on again and the it gets reset and then the red LED comes on again. That is the only consequence. The motor may get hot and hotter when you have a dummy as a client and start pushing in pushing without removing the stalling obstacle.

    But will this happen? They say a Donkey never bang his leg against the same stone twice. This is an old saying.

    It is your and EK's call.
    Bert
     

  15. portacruise
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    portacruise Senior Member

    Hope this helps.

    PC
     
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