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Saturday, August 22, 2009

Noise Vs Heat

Well, that rubber noise dampener might not be the best idea. The motor and it's gearbox gets too hot to leave my hand on for more than 10 seconds, after my 2.5 mile commute to work with many full stops and starts, plus a few 15-18MPH stretches. The controller itself barely gets warm (I think the sunlight heats it up more than the operation).

Part of the motor's heat comes from sunlight on it's black casing, but most of it is from the trip, because at night I only get two more seconds of holding my hand on it before I have to let go. I'm a wimp, so many people could probably leave their hand on it much longer, if they had to take it off at all. I don't know it's actual temperature. I'm sure it's not good for the magnets over the long term.

Realistically, I think that the motor will probably work for quite a while like this, but definitely not forever--it was designed for quite a load for long periods, but not what I'm putting it thru. :)

I've been considering some heatsink fins around it, secured to it's main cylindrical casing, but it is more likely that I will add forced-air cooling to it by drilling some holes in that cylindrical casing and adding a vacuum-cleaner hose to a hole cut in the flat end of the plastic ex-brake-solenoid cover. The hose will go to a fan up on the "deck" above it, facing rearward and downward, with a filter ring over the holes in the motor casing to keep dust out. The fan will suck air thru the filter, holes, casing, along the motor shaft, then out the fan exhaust. That will keep it at least somewhat cooler, with minimal work and parts added to the bike. The fan will probably just be one of my 12V 5" high-volume PC case fans if I can find them.

If I also add the heatsink fins, it should help even more. I have plenty of aluminum to do this with, from various bits and pieces around here. I even have a bunch of huge heatsinks I could bolt to the flat bottom of the gearbox casing, which would help spread the heat.

Since that rubber pad between the motor and the mounting plate keeps it from transmitting heat thru the bike frame (even though steel is a poor conductor of heat it is better than nothing), I'm probably going to take that off, and live with the noise the bike makes with the induced vibrations and whines in the frame. :roll:


  1. Hi Mike. One thing to keep in mind is the DC resistance of the motor. My 12V motor gets up to 48V supplied to it, but only when the load on them is low enough. That is why the controller that I designed is current mode control. It would burn out my motors pretty quickly if I let the 48V go full into the 12 Ohm resistance. They are probably only about 50 watt long term power handling motors.

    So, they can handle about 7 amps. I^2R is the power dissipation. I derate that for higher voltages just to compensate for brush arcing. Yet, say 5amps times 40V means that my 50W motors can do 200W @ 40V. That is why I can hit close to 40mph if I peddle furiously in tenth gear at full throttle. --Joe

  2. Eventually I will start worrying about the condition of the motors, and how I'm treating them, but just at the moment I am going to press the system as hard as I can to see what breaks first, and how.

    Sure, I could probably calculate out what would happen, but it somehow doesn't seem as much fun. :)

    The 2QD isn't exactly a current controller, but it does do current limiting in a different way than many. It senses the voltage drop across the motor drive MOSFET in the half-bridge. That doesn't help protect the motor itself, but it does help protect the controller/MOSFET. If it heats up enough to raise the RDSon significantly, then the current limiting kicks in to help prevent it from getting any worse.

    I doubt that current limiting is even *going* to activate with the MOSFETs I have on it now, given that they barely even get warm under the loads they currently have, and that's pretty hefty load as far as the bike itself is concerned.

  3. ah, I can see you possibly considering trying weaker MOSFETs. I devised that type current control before I knew anyone else was using it--like the 2QD uses. I have since gone to simpler method I thought of.

    I have a minor correction that the DC resistance of the motor I use is about 1 ohm, not 12 ohm.

  4. I'm considering using the standard TO220 MOSFET size that the 2QD originally ran a parallel pair of for each leg of the half-bridge, just because they'll fit right on the PCB as designed, and I can bolt the whole PCB into an aluminum ex-modem case, which is smaller and thinner by far than the plastic one I'm using now (which does not fit where I had hoped it would, by only about 1/4").

    The capacitor would have to stick out of a hole bored in the top of the case, but that's ok. It'd get better cooling that way anyway. :)

    The MOSFETs I'm considering downgrading to from the NTY100N10s are some Fairchild 60V 80A 3.8mOhm, FDP038AN06A0, because I have several that all test ok, and they should be high enough voltage to not risk destroying them from BEMF and the like.

    At a load current of 80A (which I will not be drawing on this bike with these poor batteries!) with gate drive of 10V, the RDSon will increase from 3.5mOhm typical at ambient (25C) to 7.1mOhm at 175C (which I hope I never reach!).

    The NTY100N10 is 9mOhm typical at ambient, 10V gate drive, 50A load current, and 19mOhm same conditions at 150C, which they'll definitely never reach unless I take the heatsink off. ;)

    I guess that means that the already-in-place NTY100N10 is actually *better* for the current-sense than the other one. Hmmm.

    But the FDP038AN06A0 is lower RDSon, lower gate charge (so it can be switched faster by less of a gate driver), and easier to mount, so maybe I should use them anyway, especially since the PCB is already designed to use a pair of them in parallel for each leg of the half-bridge. I'd get theoretically 160A capability (for bursts, less for sustained operation depending on my heatsink and airflow) at an RDSon of say, 1.8mohm, about 1/5 that of the NTY100N10, thus only wasting 1/5 of the power currently being converted to heat inside the MOSFETs, in theory.

    The only real disadvantage is that the NTY100N10 has a 100V D-S limit, but the FDP038AN06A0 has only a 60V D-S limit, making it more likely that a spike somewhere in the motor noise could kill them.

    AAARGH. Decisions decisions.

    Maybe I should just carry a portable soldering iron and spare MOSFETs with me in case the worst should happen. :)

    Really, it would be better just to carry a whole spare *controller*. It'd be about as light or more so, and certainly faster and easier to deal with changing out!

  5. Weighing of options can be oppressive!


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