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 Aug 28, 2008 7:12 pm

Interesting. I think for that layout you've drawn, the sensors have to go in the centre of the stator poles to get 120 degree triggering. For a twelve segment stator it may not be as critical, plus 30 degrees on the stator is really 120 electrical degrees, I think. Shifting the sensor to the edge of the stator segment introduces a 30 (electrical) degree shift from the segment centre, which may not be a real problem.

I'm not sure that the centre sensor needs to be flipped, thinking it through again, but to be honest I've changed my mind a dozen times already as I've tried to work this through!

I know that my old ebike motor had the sensors fitted between the stator segments, but I can't recall how many segments it had to the number of rotor magnets.

Jeremy

Edited to add these pictures:

Image

Image

Image

Image

bobc
Posts: 107
Joined: Thu Dec 20, 2007 1:11 pm
Location: Knutsford

Postby bobc » Fri Aug 29, 2008 8:22 am

Hi Jeremy,
unfortunately I'm an inductoin motor man... I've never really done anything with BLDC. So what follows is surmise & prejudice ;^)
Hall sensors - there's nothing magic about hall sensors. For BLDC control you need position & the hall sensor is a convenient way of getting that considering that the motor has ruddy great permanent magnets stuck on the rotor (incidentally - very interesting to see how the motor is constructed). Putting the sensors IN the airgap,apart from being impossible, feels wrong to me because the actual field will be perturbed by the stator currents, e.g. in high speed field weakening operation.
If I were you Iwould glue some tiny permanent magnets to the outside of the housing & sense them with your hall sensors - that would not interfere with motor operation and would be readily accessible for experimentation.
Or use slotty opto sensors instead. I think it will take a while to get right but should be pretty impressive when you do!
Bob

Delinquent
Posts: 43
Joined: Fri Feb 01, 2008 4:45 pm
Location: Southampton

Postby Delinquent » Fri Aug 29, 2008 8:25 am

interestingly, that motor looks very very similar to one I constructed a couple of years back - which I made using the stator from an aged 110v builders drill! I never really used that motor for anything as the project it was going in developed to the point it was overkill, I might have to dig it out and run some proper tests on performance, could easily knock up 3 or 4 of them then.

User avatar
qdos
Posts: 2089
Joined: Fri Apr 27, 2007 10:26 pm
Location: Dorset
Contact:

Postby qdos » Fri Aug 29, 2008 9:32 am

Jimminy Christmas that is small! it's amazing how reality kicks in when you can see things in scale. I'm very keen to see how this goes but I must say the size does really start to make me wonder if it's up to the job but then when you look at an ICE and break it down to the single piston that 's firing on the power stroke they too are really pretty small.

Looking forward to hearing how things develop

Delinquent
Posts: 43
Joined: Fri Feb 01, 2008 4:45 pm
Location: Southampton

Postby Delinquent » Fri Aug 29, 2008 10:17 am

qdos wrote:Jimminy Christmas that is small! it's amazing how reality kicks in when you can see things in scale. I'm very keen to see how this goes but I must say the size does really start to make me wonder if it's up to the job but then when you look at an ICE and break it down to the single piston that 's firing on the power stroke they too are really pretty small.

Looking forward to hearing how things develop


A very nice analogy - never thought of it like that before! I've always been amazed at the power in these small brushless (You should see the damage one of these in an RC heli can do with just just 1/4 turn of momentum, suffice to say I'm glad my arm was 1/4" further back than usual!) but I'm still a tad concerned about overheating - hopefully Jeremy will prove me wrong soon!

User avatar
qdos
Posts: 2089
Joined: Fri Apr 27, 2007 10:26 pm
Location: Dorset
Contact:

Postby qdos » Fri Aug 29, 2008 10:53 am

Heat has crossed my mind particularly given my 14 years experiences in fire protection including real and lab work running fire tests etc but I think it can easily be sorted and Jeremy has that in hand.

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

Postby Jeremy » Fri Sep 05, 2008 4:50 pm

I'll try and post an update on the mechanics of the double motor module I'm working on in a few days time, but in the meantime I've been trying to resolve the controller availability problem.

After a lot of digging around, it looks as if I may have found a possible solution. I've managed to track down a supplier who will let me buy brushless controllers as populated circuit boards only (no case), suitable for operation at up to 72V and about 80 amps, with conventional analogue throttle inputs (0 - 5V) and the ability to be run without Hall sensors.

The best bit is that it looks as if I can get these controllers for less than £20 each, delivered.

I've ordered a couple to play with, the plan being to build both controllers into the same box, in keeping with the "double motor module" philosophy. I've also found a source for small blower fans that look as if they may be OK for motor forced cooling, using the idea I originally had of ducting air through the motor mount.

At 48 volts, such a double motor module should be OK for about 5kW or so peak at an output shaft speed of about 5000rpm, or, more realistically, about 3kW continuously. It should be straightforward to add more modules to a common output shaft to increase the power output. I expect each roughly rectangular power module (excluding it's double controller) to be about 250mm wide, 120mm high and 120mm long, with the output shaft emerging from the centre of the 250mm dimension. Power module weight is likely to be less than 2kg.

If it all works out OK, then the total cost for a double motor module, with all reduction parts and including the cost of the double controller boards, should come to somewhere around £140 to £150 maximum, which seems pretty reasonable to me.

Jeremy

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

Postby Jeremy » Sat Sep 06, 2008 8:53 pm

I've attached a sketch of the prototype "double motor module" that I'm building to test the feasibility of using these cheap brushless RC motors.

Image

I've tried to make it easy to manufacture, so the main body is just a length of 2" x 3" x 10g alloy box section. This forms the mount for both motors, plus the plenum chamber for the cooling system. The motors have a cooling air "intake" around the mounting face and can exhaust hot air out through the vents in the end face of the rotor bell.

The central output shaft is supported by a simple bearing holder machined from a short length of 1 1/2" alloy bar, with two bearings pressed in at either end. Flange bearings could be used for those who want to splash out another £15 or so!

The module can be bolted down with a couple of through bolts either side of the output shaft, which are probably best fitted with internal anti-crush sleeves.

I may look at fitting the fans inside the box section on small bulkheads, with intake holes drilled in the top face of the box.

By fitting a longer shaft, two or three of these modules could be stacked together. If intended for driving a chain, then it might be an idea to fit the chain sprocket between two modules,

Jeremy

User avatar
qdos
Posts: 2089
Joined: Fri Apr 27, 2007 10:26 pm
Location: Dorset
Contact:

Postby qdos » Sun Sep 07, 2008 10:20 am

You could do a radial array too.

I'm wondering about a better cooling fan set up as I don't think they are in positions which will draw much air over the motors the way you have them set up there. How about locating them on the open ends of the mounting plates perpendicular to the plates and parallel to the motors?

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

Postby Jeremy » Sun Sep 07, 2008 11:42 am

You might be right about the cooling. The radial array would be harder to fabricate, and wouldn't have the advantages the box section has of being both nice and cheap and providing a sealed plenum for the cooling system. It'd also be taller - for the application I have in mind, being short is a distinct advantage.

At the moment I'm hoping that by pressurising the centre of the box section (which is pretty much closed off at the ends by the motor rotors which come close to filling the box section) I can force air through the motor cores. These motors have three openings on the mounting face that are cut away to three sides. The way I have it arranged the fans blow down into the central plenum, forcing air through the motors and out through the vent holes in their rotors. The air really has nowhere else to go, other than the small amount of leakage around the rotor sides.

I could improve things further by blocking off the ends of the box section, perhaps. This would force all the air to go through the motor cores, except for the small amount that would leak past the hole that the rotor bell protrudes through.

Only testing will find out if this works or not!

I'm working on slightly changing the design to use a vertically offset 60t driven pulley (the offset is needed to retain the flat mounting base of the box section). This would give a 3.75:1 reduction, with a max output shaft speed of around 2700rpm, which might be better for driving a chain final reduction.

Finding a source of alloy HTD pulleys at a reasonable price is a challenge. It's probably cheaper to buy them from someone in the US, like SDP, than it is to get them here in the UK.

Jeremy


Return to “General”

Who is online

Users browsing this forum: No registered users and 41 guests