Multiple motor & controller approach?

General BVS related area
User avatar
Jeremy
Posts: 472
Joined: Thu Jul 31, 2008 6:35 pm
Location: Salisbury

Postby Jeremy » Thu Nov 13, 2008 8:21 pm

Good point about the final drive torque, Bob, I'd not thought of that.

I'm pretty sure that the magnets are reasonably secure, as they are fitted into machined slots in the bell end casing, as well as being bonded in place. A bit of forced air cooling down the array should keep things reasonably cool, I'd hope, plus the heat should be mainly in the stator core anyway. I'm not sure that it'd be needed, but another way to get heat out of the core might be to machine up some radially finned spacers between each motor, thermally bonded to the mounting flange.

The twelve stator slots are just to obey the "stator poles must be a multiple of three" rule, I think, and get a reasonable compromise between ease of winding, relatively low torque pulsing and a sensible commutation frequency for the desired operating rpm.

Jeremy

soapmachine
Posts: 8
Joined: Thu Nov 13, 2008 7:31 pm
Location: Halstead Essex

small motors

Postby soapmachine » Mon Nov 17, 2008 12:05 am

Hi all

I am in to R C helicopters and use these motors a lot, they have a masive amount of power to weight, but the bearing are the first thing to suffer with failer, but that can be got over by replacing with higher grade ones, the other thing I think you mite find is that the timeing of the speed controlers will be hard to get them to run to gether, and that will make them over heat, if you need gears most of the heli parts will be of use to you, and as for your one way bearing clutches most of the large heli's have these, and they are quite cheap to buy of the shelf from H K.

hope this helps Owen.

User avatar
Jeremy
Posts: 472
Joined: Thu Jul 31, 2008 6:35 pm
Location: Salisbury

Postby Jeremy » Mon Nov 17, 2008 7:30 am

Owen,

Good point about the bearings, I've heard the same from reading stuff on model forums.

What sort of rpm do your RC helicopter motors run at? Looking at the various model forums I've gained the feeling that RC helicopter motors tend to be inrunners run at very high rpm (15,000 to 25,000 rpm seems fairly normal), with gear reduction systems fitted to get the relatively low rotor rpm needed.

The motors that we're playing with run at relatively low rpm and are intended for use on big, aerobatic, fixed wing models. The Kv for the motors I have is just 215, giving a max motor rpm at 24V of just under 5200rpm. I might be wrong, but I think that the RC helicopter inrunner motors usually have a Kv of up around 3000 to 4000, which would mean that one of these would run at around 72,000rpm to 96,000 at 24V (although I strongly suspect that few, if any, of these motors would tolerate working at 24V, due to the very high rpm such a voltage would give).

I'm pretty sure that the bearing problems seen in RC helicopter use might well be as a consequence of the high operating rpm of these particular motors. Ball races, especially cheap ones, have an upper speed limit that, if exceeded, tends to massively reduce their operating life.

Controller synchronisation doesn't seem to be a particular problem if the motors aren't electrically linked, although I'm sure it might be if the motors were driven from a common controller. With a separate controller per motor, each controller responds to it's individual motor back EMF feedback and times itself up accordingly. The go cart video's that Malcolm posted earlier in this thread show that this approach works OK, particular the one showing three HXT motors running on a common shaft using three controllers.

Jeremy

soapmachine
Posts: 8
Joined: Thu Nov 13, 2008 7:31 pm
Location: Halstead Essex

Multiple motor & controler approach

Postby soapmachine » Mon Nov 17, 2008 7:53 pm

Hi Jeremy

I was at work to day and thinking about you Idears and it came to me that a lot of the tork a motor prejuced is jew to the size and weight of the spining com so is this a piont to look at in the testing stage of this idear I used to race 125 yamahas in the seventes and this is one of the problems with high reving motors and as soon as you lose speed the tork gose out of the window and you have yor work cut out to gain it back.

Getting back to the motors I use in my heli's they are 1000kv and higher and they skream all the time.

User avatar
Jeremy
Posts: 472
Joined: Thu Jul 31, 2008 6:35 pm
Location: Salisbury

Postby Jeremy » Mon Nov 17, 2008 8:50 pm

The torque produced by a permanent magnet motor is directly proportional to the current it's drawing. These motors have two key constants, the voltage constant , Kv, which gives the no load rpm per volt, and the lesser known torque constant, that gives the shaft torque per amp.

Current does tend to drop with rpm in a fairly linear manner, because the motor back EMF increases with rpm. This increase in back EMF reduces the voltage difference between the supply voltage and the motor back EMF, which in turn reduces the current that's able to flow through the motor (assuming that the winding resistance is fixed).

Torque is also proportional to the diameter of the rotor, which one reason why big outrunners are better at delivering high power at lower rpm than the smaller diameter inrunners.

1000Kv is pretty high, so I'm pretty sure this may be why RC helicopter bearings have such a tough time. Rather than the modest 5160rpm that my motors will spin at on 24V, a 1000Kv motor would spin at around 24,000rpm.

Jeremy

soapmachine
Posts: 8
Joined: Thu Nov 13, 2008 7:31 pm
Location: Halstead Essex

Multiple mortor & controler approach

Postby soapmachine » Mon Nov 17, 2008 10:41 pm

Hi Jeremy

I have looked at some high power motors and can not see what you are paying for, so your idear makes a lot of sens as there is going to be a lot of weight saving and this is quite an ishew on a motor cycle as the hole package needs to not be any more weight than what you have removed.

I have been working on a brush motor out of a hotpoint washing machein and I have removed the windings and are about to rewind it with some very large transformer winding out of a old welder and have to work out how meny turns to put back on the feilds the armacher is only goint have a single turn on each segment and to see what sort of power I can get out of it I have some contactors from milk floats and by using some stainless welding wire as resisters to control the speed.

joncot
Posts: 1
Joined: Thu Jan 03, 2008 7:27 pm
Location: Nr Andover

Multiple motor & controler approach

Postby joncot » Fri Jan 09, 2009 7:34 pm

I only came upon this thread last night. Fascinating insight into multiple motors and r/c sources.

Then today I picked up the latest copy of Motorcycle News and there is a report on page 6 of the News section about KTM producing a 2wd motorcycle with an electric front end. And apparently they have patented the idea. The drawing reproduced there MAY show one geared motor or three linked in the hub.

Its not available on the MCN website yet.

The article goes on to describe using an enlarged starter/generator with 160V going through the gears to produce 189ftlb of torque. And a capacitor is incorporated into the system to give an extra kick.

Sounds quite a handful.

But if you take out the ICE too and replace that with electric, hmm. Interesting. The electronic control box may be the clever bit. Enjoy!

ScottC
Posts: 7
Joined: Thu Jan 29, 2009 2:26 pm

Postby ScottC » Thu Jan 29, 2009 7:15 pm

Jeremy wrote:1000Kv is pretty high, so I'm pretty sure this may be why RC helicopter bearings have such a tough time. Rather than the modest 5160rpm that my motors will spin at on 24V, a 1000Kv motor would spin at around 24,000rpm.
Jeremy


Jeremy,

I use the HXT motors 63/74 a lot and have experience of running them on a common shaft.

On my mtb I have one motor and have had to replace bearings after a little over 1000 miles - this with the system running on 48V for maybe 100 miles then the rest on 24V. At 200rpm/v this motor doesn't see past 5000rpm so I am not sure at all that failures in this case are down to rpm.

I am however certain that they suffer a lot with moisture/water and dirt ingress due to the manner in which I have mounted it (not enclosed). I have not replaced the bearing with 2RS yet though I am sure that would sort the issues at some performance loss. Next time it fails (always the 'output end' one) I will.

As a result I strongly suggest filtering inlet air wherever possible.

My common shaft prototype used a 2mm wall ali tube 80mm diameter. 6mm thick plate was cut into circles with OD 79.5mm with the shaft hole in the middle. Cable exit was by way of an oval cut into the wall. I pushed air in a hole between the motors which passed out each end of the tube.

Finally, I have fried a LOT of these particular motors - at great cost. If anyone using them intends to run them over 60A for more than a second or two you really must have forced air cooling - as Jeremy has already alluded to - they have no real mass and can be taken over 80C VERY easily. Of course, used as intended they will have a 30+mph wind flowing directly through them....

BTW Jeremy - you are living my dream!!!

Scott


Return to “General”

Who is online

Users browsing this forum: No registered users and 22 guests