Multiple motor & controller approach?

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qdos
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Postby qdos » Wed Nov 12, 2008 11:43 pm

I'm really keen to see how you guys fair on these as I'm thinking on going this route too. Bit busy with a body at the moment hopefully will get it all sorted over the winter period and then will be able to let you all see what I've been upto. Or if you're down at the Exeter show this weekend say hi and I'm happy to chat. I'll be wandering about a bit but shouldn't be too far from the BVS stand.

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Jeremy
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Postby Jeremy » Thu Nov 13, 2008 7:28 am

bobc wrote:2 motors arrived today.
I note that the prop connection is at the turning end of the outrunner & I'm a little concerned about the practicalities of keeping 4 motors in synch on a single 8mm shaft if they're just attached by grubscrews. Basically I'm thinking that a keyed driving collar will have to be made to take the torque from outrunner to shaft via the 4 screws provided for the prop adapter.
Suggestions & links to suitable hardware gratefully recieved....


When I stripped one of my motors I found that the 8mm shafts had what looked like CNC machined flats at the places where grub screws fit. The shaft flats were configured so that the shaft could be re-positioned to take the drive from the bearing end, rather than the bell end.

If a new, longer, shaft was similarly machined with accurately located flats, then I think it would be OK. These motors have a tough time in the big aerobatic models they are often used in, so I feel reasonably confident that they will tolerate EV use OK.

One way around any potential sync problem is to use a distributed controller topology, effectively individual controllers per motor. This is what I'm doing for my test rig, with separate, very cheap, controllers for each motor.


They don't feel like 2500W when you pick 'em up do they ;^)


No, they certainly don't! Holding a motor rated at over 2kW in the palm of you hand brings home just how tiny these things are for their power. Apart from the advantages the light weight and size has in an EV, one of the big advantages is that they are relatively cheap to post, giving a significant additional saving over bigger motors.


Kelvin,
I'm not sure if I will get down to the show this weekend, I'm currently finishing off a home-brew CNC machine for carving moulds for composite parts. I want to make decent bodywork for the bike, as it looks a bit Heath Robinson at the moment. Hopefully, the CNC machine will make it much simpler to make complex 3D part moulds, up to about 500mm x 600mm x 300mm.

Jeremy

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qdos
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Postby qdos » Thu Nov 13, 2008 7:35 am

I'm definately going to have to pop by and see you sometime. Let me know when's good.

bobc
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Postby bobc » Thu Nov 13, 2008 11:16 am

OK - I have a plan to improve shaft coupling: the outrunner end has the 4 threaded holes to attach the prop adapter; my plan is to use these to pull a boss onto a split tapered inner (mucho like a taperlok). This should squeeze the shaft enough to do most of the work & use the flats on the shaft for indexing. The whole shebang could be about 10mm long allowing motor spacing of 70mm on the shaft,the stator brackest would go round this coupling like the supplied bulkhead brackets do with the (now redundant) collar supplied.
Regarding accuracy - these are 28 pole motors so 360 electrical degrees is just 25.7degrees mechanical. Mechanical errors need to be better than a degree really....

johnspeed
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Postby johnspeed » Thu Nov 13, 2008 1:34 pm

Hi Chaps

At my place of work we couple to servo motor shafts by a keyless Trantorque bush.

You can find details at www.fennerdrives.com/
Click on keyless bushings

Hope this is of some use.

Regards

John

bobc
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Postby bobc » Thu Nov 13, 2008 3:48 pm

Pertinent factor there is that trantorq specify they achieve 23Nm torque capability onto an 8mm shaft with such a compression fitting.
23Nm @ 7000rpm is 16.8kW !!!!
And 4 of those little motors is just 10kW - safety factor of 1.7, that's good enough for me.....

MalcolmB
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Postby MalcolmB » Thu Nov 13, 2008 5:08 pm

I was looking at those trantorque and fenlock fittings myself the other day. They do have surprisingly high torque ratings with a relatively short gripped length. Anyone know roughly how much they cost?

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Jeremy
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Postby Jeremy » Thu Nov 13, 2008 6:33 pm

Malcolm,

I've just checked the online shop on the Fenner site and it looks like an 8mm bore, 19mm OD one is around £13 plus VAT.

Bob,

Each bush only needs to transmit the power from one motor bell, doesn't it? As such, the reserve will be much greater than 1.7.

Also, these motors aren't 28 pole, I think. Mine have 12 stator poles and fourteen magnet poles, so are 7 pole-pair motors. I am pretty sure that most of these use the same configuration, although I may be wrong.

Jeremy

MalcolmB
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Postby MalcolmB » Thu Nov 13, 2008 7:05 pm

:oops: Thanks Jeremy. I didn't notice the shop link last time I looked.

bobc
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Postby bobc » Thu Nov 13, 2008 7:24 pm

Yes Jeremy, but at the end of the shaft all the torque has to end up in the pinion... I was thinking of making a thicker extension shaft at the end for the pinion that could be directly coupled to the nearest outrunner so I only have 3/4 the torque to transmit. Anyway I just noticed a classic cockup above - I counted the magnets & said it was that many pole pairs DOH! sorry, 7 pole pairs, 360degrees electrical = 51.4 degrees mechanical - rather easier to manage ;^)
Also just noticed that you pointed it out above Jeremy - Ta!
We should get 10kW from a 2.5" cylinder less than a foot long & costing £120 - that's pretty unbelievable!
There were comments about magnets falling off on the vendors website: lets hope these are after (model) plane crashes..... (they don't say how bad the landings were) (or how hot the motors got)
PS, the 12 stator slots - I'm assuming that it's simply been made different to the rotor to avoid cogging & that the stator is sort of "wave wound" - but these things are pretty new to me & I'm learning as I go....


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