Back EMf DC motor supply

light fantastic
backtotheblack
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Back EMf DC motor supply

Post by backtotheblack » 11 Apr 2017 00:01

to stop cluttering up the other thread i decided to start another.

boards are designed and in the post as i type.

the intended motor is http://uk.rs-online.com/web/p/dc-motors/2622157/

i have designed the kit to fit every PT. and will be a sort of drop in replacement or off board housed in a case with umbilical. the only problem is that the Export and the LPT will need 3x 3mm holes drilled into the plinth, the LPT will also require some routing of the MDF base board, this is if you require the speed select knob to stay in the original position. if you require it to be off board then i can supply a blanking plug with pink triangle on it to cover up the hole so looks like a badge.

my plan is to keep with the PT theme and have the selection knob on the off board housing the same as the acrylic LPT, Export and anniversary.

i have added a facility on the boards for LED lighting of the platter if desired and will change colour with the knob and any colour of LED's can be used preferably pink and amber. what this means is that when you change speed the knob and the platter will both be the same colour, you dont have to use it but its been included.

mounting the motor will be by way of carbon fibre bracket with height adjustable as well at belt tension motor mount and directly coupled to the sub chassis (top plate for LPT). this will give better speed stability, the placement will be via the original motor mount position so no need to drill things. or if one desires the motor can be mounted via the original means. the motor will come with a pulley so no need to go hunting for one.

i'm expecting to have a finished prototype in around a month if not sooner.

tim_bissell
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Re: Back EMf DC motor supply

Post by tim_bissell » 11 Apr 2017 00:07

That is great news!

cheers,

Tim

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Re: Back EMf DC motor supply

Post by backtotheblack » 19 Apr 2017 22:29

boards are here :D

needed to order up some components so with luck get them stuffed at the weekend then test next week.

the pic doesn't really show how small these things are. everything including the transformer, power supply and motor control will fit in a case W.103mm x D.160mm x H.53mm.
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backtotheblack
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Re: Back EMf DC motor supply

Post by backtotheblack » 21 Apr 2017 01:30

wish i had know this before i ordered the circuit boards. asked the PCB manufacturers (online instant chat) if they also work in carbon fibre and make profiled boards but without the circuit, yes sir no problem. why cant the UK have this kind of customer service.

well that's going to save me a BIG headache :-)

Motor Mount
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backtotheblack
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Re: Back EMf DC motor supply

Post by backtotheblack » 25 Apr 2017 02:12

boards populated.

regulated supply
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motor control
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regulated supply tested and works great.

will test motor control over the next few days.

backtotheblack
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Re: Back EMf DC motor supply

Post by backtotheblack » 25 Apr 2017 18:28

its alive :-)

all works as should with a cheep crap motor that the circuit was not designed for, just got a bit of hunting which i expected with the cheep motor :-)

33 trim pot works as should
45 trim pot works as should
hunting trim works as should

turning switch Left lights the two pink LED's and selects 33. turning switch right lights the two amber LED's and selects 45. middle is off.

just waiting on snubber caps to arrive as back order with supplier, hopefully tomorrow.

i'll swap out the cheep motor for the maxon later.

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Re: Back EMf DC motor supply

Post by backtotheblack » 25 Apr 2017 19:41

swapped in the maxon motor and yeeehaaawww. very very quiet running :-)

just as well i have held off on getting the pulley's made. i calculated for around 1200rpm but i think i'm only getting around 500. ill measure with laser speed gizmo later. not a great worry as i just up the zener for more voltage. or bigger pulley.

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Re: Back EMf DC motor supply

Post by backtotheblack » 25 Apr 2017 20:44

ok i was not expecting these results.

55rpm to 2900rpm adjustment range. to put this into perspective with the pulley i intend on using that makes this control unit work with any belt drive turntable ever made inc outer rim drive.

i'll get the pulley's made up and then i can test in situ for speed accuracy/hunting under load.

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Re: Back EMf DC motor supply

Post by backtotheblack » 25 Apr 2017 22:27

let there be light

33 1/3
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45
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Re: Back EMf DC motor supply

Post by backtotheblack » 29 Apr 2017 14:23

update:

its pretty much perfect :-)

using my laser speed measuring device i get +- 0.3rpm on motor shaft speed at any rpm from stall to 2900rpm. this is due to not being able to use the compensate trim as there is no load on the motor. once the drag of the platter/needle is in place then i would say its going to be rock solid once trimmed out :-)

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Re: Back EMf DC motor supply

Post by backtotheblack » 02 May 2017 21:26

update:

i lashed up the PTtoo platter and the LPT pulley onto the motor and control. the LPT pulley is ermm a bit wobbly and with it being huge in comparison the the ones i intend on using things are very good indeed. considering the motor is running at about a 1/3 of its intended speed (aprox 260 instead of 630rpm) things are great. takes a while to get up to speed about 20sec but thats due to the low speed/volts which in turn reduces torque.

now remember that i'm using an old worn belt, the motor is being held down with blu-tak and the platter bearing is just sitting on the same surface i think my results are great. whats the score? a fluctuation of aprox 0.1rpm for every 10 revolutions of the platter according to my laser gizmo, strobe looks solid.

its getting there :-)

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Re: Back EMf DC motor supply

Post by tim_bissell » 03 May 2017 01:55

That looks fantastic! I need to finish my son's hifi rack so I can build up a new mini-anni as a test bed. not to mention a suitable case for the controller and transformer.

It would be intriguing to try two or three motors driving the same belt (á la Voyd) and see if they can be synchronised, or whether they just end up fighting.

-- Tim

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Re: Back EMf DC motor supply

Post by backtotheblack » 03 May 2017 14:16

sounds good Tim

tbh i have not thought about 3 motors but it might be possible if run in parallel and the emf adjusted.

regarding a case, i have a hammond enclosure here that i'm going to stuff everything in but its going to be nippy drilling it. what i might do is get one of the mates to punch and form cases for me as they would be around the same price. they wont look as fancy (maybe) and will just be powder coated but they will be perfect size with no messing.

Lance

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Re: Back EMf DC motor supply

Post by backtotheblack » 05 May 2017 20:45

update:

sorted the torque issue :-)

grabbing the motor shaft now results in burnt fingers as it just wants to keep going where before, the shaft would just stop dead :-)

tbh i had my doubts that the motor was good enough but now i know it is.

i'm waiting on some precision zener's to come so i can try that.

in the mean time i'll do some opamp rolling as i have a few from rolling on headamps and phono stages.

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Re: Back EMf DC motor supply

Post by backtotheblack » 06 May 2017 17:36

update:

eureka... lashed up the LPT pulley a bit better and with the mods i have done i'm now getting 33.3 rock solid and 45 rock solid. torque is incredible for such a small motor.

adjusting the pots is a bit finicky and needs very very fine adjustments. have to remember that its not spinning with its designed pulley and the motor shaft speed at the moment will be around 250rpm instead of around 620rpm for 33.3.

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