Free spinning a Brushless motor

[obrien]

I am looking to experiment with something here at work, that would involve using a brushless motor. What I want to know is if it would hurt a brushless motor to be turning say 50000 rpms, if it is not being used. Obviously I will have to get a motor with bearings that will handle that kind of rpm, but for the temporary test, I will just use a cheap motor to see if our idea will work.

thanks

[Steven Vaccaro]

I "Guess" it depends on the motor. Some of the magnets may break if spun to fast.

[ED66677]

take a feigao 540 with appropriate KV according to the voltage you want to apply to it, get a pair of ceramic Boca bearing here at OSE http://www.offshoreelectrics.com/pro...5c-yzz%235-nb2 and you're ready for 70000rpm, but I'm not sure what you mean by free spinning, if it's unloaded, then you'd better use a brushed motor because a brushless system is not desing to work unloaded and will simply burn after a while! Although the brushed motor will not require any ESC if running at fixed RPM, cheaper and almost disposable...
just my though!

[obrien]

basically our idea is to direct couple a brushless motor to the turbo on one of our marine diesel engines. We are trying to get a better holeshot from a ski boat we are testing. The idea is to spin the turbo with the brushless motor when the engine rpms are low, so that we can make boost and hopefully improve the acceleration of our skiboat. I am basically curious to see if the motor will be fine spinning at the high rpms if there is not power being supplied to the motor. We are going to use a feigao motor since it is cheap enought that we won't really care if it gets hurt during the test. This is not going to be a long term solution, just for some short term testing to see if we can make the engine work better at low rpms.

[Steven Vaccaro]

My concern in a setup like that is the heat generated from the turbo. The coil in the motor is not going to like the high heat of the turbo.

[ED66677]

interresting!
if the motor is on the "cold" side of the turbo it might be ok, you may want to use a long shaft between motor and turbo to disipate the heat.
another point you have to think about, if the motor is spinning free at hight motor/turbo rpm the brushless motor wil induct some voltage back to the ESC that might not like it and burn!... at high rpm, when the turbo will start spinning due to exaust gaz, you may want to "disconnect" the motor from the ESC via a 3 phase contactor!

[ED66677]

btw the easyest would be to use a cluch from rc car between brushless motor and turbo!

[obrien]

it will be on the intake side of the turbo for sure. there really isn't a way to put the electric motor on the exhaust side due to the shape and size of the exhaust elbow. I like the idea of the car clutch, I may have to give that some though. THat would certainly be an easy solution to the potential problem. We have enough room on the intake side of the tubo, that we can move the motor far enough away from the turbo that heat should not be an issue.

where would be a good place to find a clutch? I just called the LHS and they don't carry anything. Basically since I don't get into rc cars, what would be a good one to look at?

[Doug Smock]

A sprag clutch would be the ticket.

http://www.renold.com/Products/SpragClutchFreewheels/Sprag_Clutch_Index.asp

Doug

[obrien]

thanks for the input guys. We will look into these things some more in the coming days, and I will let you know what we come up with. If anyone has any more suggestions on any part of this please let me know.

thanks

[ED66677]

don't know what is a sprag cluch, looks like a one way bearing? that's what I was about to say, a one way bearing that would engage as soon as the motor spins faster than the turbo!

[kssi]

we have had ideas like this on a couple of race cars, but between heat, restricting air flow, and the fast that high boost levels depeding on turbo u will not even make the boost you want till the turbo is spining 70k plus it makes it hard. we just use a small shot of nitrous controlled by a cpu, and it seems to work way better. but we also get to slip a clutch or change stall on a tranny. i would love to see a setup work. so if you do make sure you keep us posted. also it the turbo was at the point of making boost there would be load on the motor. let us know how this goes

[Fluid]

You will probably destroy the ESC. When a BL motor is spun without voltage it becomes a powerful generator, feeding the voltage back to the ESC. Depending on the number of turns in the motor it can - and likely will - over-volt the ESC and burn it down in seconds.

What makes you think that the motor will be able to spin the compressor fast enough to make meaningful boost? This is a heavy assembly (for a 540 motor) and when compressing will place a heavy load on the motor. You will only be able to use ~2-3 horsepower from this low-powered motor - that isn't much compared to the HP generated by 1200F exhaust spinning the compressor. The time it takes to spin-up a compressor assembly that weighs several pounds will likely be far too long to help build any meaningful boost.

This idea might work in your application at low boost levels, and if you want to try it more power to you. But you have not yet thought through all the factors that will determine its success. Spend more time on the engineering first, and you'll have a better chance of attaining your goal. Good luck! :)



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[obrien]

thanks for your input guys. Fluid I understand what you are saying. We are going to put a clutch of some sort in there to prevent the motor from waisting the esc. The engine we are doing this test on is only a 3 liter motor, so the turbo is actually very small. For all we know, this may not work at all, but it may work great. As I said before, we are simply trying to make 3-5 lbs of boost at idle so that the engine will spool quicker and hopefully significantly reduce planning time. The other thing you have to realize is that the diesel engines we use only make 200-300 degrees of exhaust temp at idle.


KSSI we have actually tried nitrous on a couple of our bigger engines. Our 440 hp engine loves a little bit of nitrous. however on our smaller 315 hp engine, it cost us a couple of pistons. Since this smaller 3.0 260 engine is made by BMW, is is not exactly cheap. So we have decided to forgo the nitrous on this motor. It might work really well, but since we are hoping to eventually find a solution that can be used in production, nitrous is really not an option. The brushless motor is not likely to become much either. We are simply trying to see if we can build small amout of boost at idle, to see if the engine will respond favorable to it. We are actually looking into a supercharger that will be out in the future, that is actually drivin by an electric motor. however it is still in developent, and it would cost us quite a bit more in engineering costs. the idea here is just to do a simple test at a relatively low cost and see what happens.


again, thanks for the input everyone. I by no means know everything and will listen to any input you may have.

[Rex R]

some time back various companies were 'playing' with a belt driven supercharger, it had an electric operrated clutch ala a car a/c compressor...as I recall it didn't develope much in the line of 'boost'(about 5psi), but it did work.