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Author Topic: Brushless motor driver: Design and build  (Read 2563 times)

andrewh

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Brushless motor driver: Design and build
« on: August 13, 2009, 12:45:10 PM »

Aka the Bodger’s mechanical speed controller

I have been pondering awhile making a Mechanical Speed Controller to drive brushless motors.  I have mentioned it in the Brushless Basics thread and even gathered the parts together for the prototype.

During a sleepless night I was able to sort out the wiring in my head and have now got that sorted and even badly sketched.  Unfortunately the physical layout is easy to build but hard to draw well, so it will probably be built before it is well presented to you all as a drawing or circuit.

I have said a design has been sorted, but this is the Mark 1 of what may be a developing family because initially it will use microswitches to produce the rotating pulses.  The Mk 1 also only produces the rotating pulses in one electrical direction – so half the motor power has been left in the battery!

I aim to lay out here not only the construction, but also the thinking and limitations of the idea so that we can get your (constructive) views and serious improvements.

Whats it for?
Its NOT to power Stavros’ vast speedboats.  It may point to Dodgy Geezer’s economy Keelbilt power train.   It may not ever reach the water in this form.

It IS:
to help understanding of Brushlesses, their needs and feeding habits
To promote discussion and improvement
To possibly develop through two-directional pulses, reed relay switching (no-contact), hall-effect switching to an electronically pulsed version and after that who knows?

The design
Three pairs of microswitches (cheap or free)
Microswitches fixed side by side in pairs and screwed to the face of a disc – with 120 degrees between pairs

Hang it – sketch follows
andrew
usual request - your feedbaCK welcome; I'm happy to explore tangents, improvements and fascinating dead-ends
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andrewh

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Re: Brushless motor driver: Design and build
« Reply #1 on: August 13, 2009, 05:17:43 PM »

First sketch of the BLMSC in LeftiCad



For anyone who gets ahead of me - PLEASE don't connect a real battery across your motor windings - it will be all to easy to do.  Its not a disaster of electronic proportions, but would overheat the coils involved in a few seconds (or tens of seconds)  To avoid this i am going to do my testing with four alkaline cells which do not have the oooomph to harm the coils but will tell me what is happening. 
Please do something like this, or fit a current-limiting resistor to each of the motor leads to minimise the potential for smoke escape.

Might make it tonight
andrew
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andyn

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Re: Brushless motor driver: Design and build
« Reply #2 on: August 13, 2009, 05:47:30 PM »

Best of luck with it mate :-))

You joining us this weekend?
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malcolmfrary

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Re: Brushless motor driver: Design and build
« Reply #3 on: August 13, 2009, 06:02:39 PM »

Whats the MTBF of the microswitches?  If its round about 10^6, at 1000 rpm that gives a life of about 1000 minutes.
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dodgy geezer

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Re: Brushless motor driver: Design and build
« Reply #4 on: August 13, 2009, 06:50:32 PM »

MMmmmmm! Machinery!  Make 'em out of brass!

But you will have problems when the thing gets up to speed. Is opto-electronics acceptable? Why not use photo-electric switches, and a spinning disk with slots cut in it?
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dodgy geezer

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Re: Brushless motor driver: Design and build
« Reply #5 on: August 13, 2009, 06:53:01 PM »

...then you could alter the timing by having the disk cut in a saw-tooth shape, and moving the optical detectors in and out radially. With a big brass quadrant....
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andrewh

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Re: Brushless motor driver: Design and build
« Reply #6 on: August 18, 2009, 12:57:24 PM »

You are all precisely correct, but please bear with me (or better still run ahead and scout the path)

this is, as I hope I sed, the mk.1, and is only designeed to see if and how much it works - I can see a development path which goes in the direction you are thinking of:

For me, a simple rude mechanical, the path would probably include (if the Mk.1 is either promising OR fun)

reed switches - (still mechanical /rotary)
Hall effect sensors/switches (still mechanical /rotary)
servo IR chopper wheel and sensors from a moose (still mechanical /rotary)

If it is still fun and promising the big step will be ditching the mechanical "thing" to cause the switching and adopting an electronic thingy which produces the "rotation" by switching the phases in sequence - then the output stages can be 3-legged beetles of some sort (FETs or transistors)
Thank you for the feedback - no-one has pointed out that the MSC has no feedback - it just puts out the pulses (which are square-edged and unidirectional) and the motor will either follow them, or it won't)
andrew
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malcolmfrary

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Re: Brushless motor driver: Design and build
« Reply #7 on: August 18, 2009, 04:16:13 PM »

Or, to stick with the mechanical thing, you could mount a brass sleeve on the motor shaft divided into three segments wired to the coils, and then feed the electric to them via brushes.  Ermmm, no, hang on......
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dodgy geezer

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Re: Brushless motor driver: Design and build
« Reply #8 on: September 02, 2009, 04:37:56 PM »

This idea is interesting, and might even be a runner. I suspect optics will have to come in at some point - because I can't see how to get the response speed otherwise. With microswitches, I suspect the limiting factor will be the inertia of the microswitch button - the standard spring can't possibly follow the cam, even at car type revolutions of 10k rpm....

Will there be starting issues.....?
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