OMO, back EMF brushed dc motor speed control

[Bibio]

so who amongst us knows what i'm talking about and if so i'm needing a circuit. needs to be temperature compensating and no PWM.

i already have a hall effect servo design which i can hack the speed change out of to bolt onto the front end of the back EMF circuit.

its only two speed with no reverse of brake needed.

once i have a schematic i can work out the voltage etc.etc from the motor and adjust the design.

Bibs

[Trev B]

Not a clue but does it look like this ???

[shiftin_gear98]

Alas I have no idea what you are describing. But that photo is brilliant. ☺

[SIII]

I do understand what he's talking about, but way above my pay grade to actually do what is being asked !!!


What's wrong with PWM ?

[Bibio]

PWM can cause instability under varying load.

PWM is ok for RC cars and such but not as good in critical speed situations.

[ophic]

I hope this is an electric SV project. SV240VAC anyone?

[SV650rules]

this is interesting

https://sites.google.com/site/hobbydebraj/home/dc-motor-control-using-back-emf-sen

[Trev B]

Don't think that lot will power a SV very far !!

[TicklinJock]

Never designed one, just bought and used industrial versions. They had AC supplies so used Thyristor bridges, and used tacho-feedback for best speed control.
Used some small DC supplied ones that used MOSFETs, similar arrangement to a SMPS Buck circuit so with PWM.
I have old textbooks on power electronics that give the basic circuits.

[Bibio]

well i designed one and have sent the pcb files off for manufacture. will it work.. ermm will have to wait and see. whats it for.. its for a hifi turntable.

this is the reason i have not been on the forum.

[DarrenSV650S]

How does the load on a turntable vary?

[Bibio]

How does the load on a turntable vary?

well ac motors which most commercial tt's have take their speed control from the mains Hz wave but as we all know this varies so therefore the speed of the ac motor varies.

place the needle on a record and you get what is called needle drag depending on the modulation of the record (loud quiet music) the motor system has to overcome this to keep a constant speed. you also have flywheel effect of the turntable platter and spindle bearing drag depending on the viscosity of the oil in the bearing, add to that coupling of the motor to the plinth/sub chassis. ac motors are noisy so you have to decouple them from the arm/platter/cartridge with dc motors you can bolt them straight onto the plinth/sub chassis which gives better speed control but only if the motor and electronics are good enough to keep up with the flywheel effect of the platter mass.

you did ask :-)

[DarrenSV650S]

What a load of pish. You are such a dork, you do know that don't you :smt003

I bet you £50 your ears can't tell the difference



Anyway, isn't that what the strobe markings are for?

[Bibio]

lol

strobe markings on a platter are there to tell you that its running at a constant speed. strobe markings are usually associated with direct drive turntables which is a whole different ball game but still suffer from the above. the strobe has nothing to do with actually keeping the speed constant.

as for my ears.. well nowadays yes your right but at one time yes. a classic test is piano notes, if the speed is not constant and at the right speed the notes will waiver.

however you can have the most accurate speed control in the world but its all down to how well the record was produced in the first place, which is a whole subject on its own but let me just say that no two batches of records are the same as regards to speed.

[Bibio]

its alive :-)

16432

16433

motor shaft speed adjustment 55rpm to 2900rpm. that equates to any belt drive turntable ever made :-)

[Bibio]

on and lit up.

for 33 1/3
16434

for 45
16435

[SIII]

That's gorgeous.

[Bibio]

Thanks SIII :-)

its taken me all winter to do this but it will be worth it

wait till you see it all together and on the turntable. that wont be for another few weeks/month

[madcockney]

In days gone by and when vinyl was the main source of listening to music, many hifi systems and those used in places such as radio stations used strobe systems to control turntable speed. (It may still happen today with high end systems.) There was a feed back circuit using the actual rotational speed of the turntable that corrected the rotational speed to compensate for things such as mains frequency or voltage fluctuations.

Most things can handle slightly varying voltage but frequency is another matter and has to be taken into consideration where critical.

[Bibio]

In days gone by and when vinyl was the main source of listening to music, many hifi systems and those used in places such as radio stations used strobe systems to control turntable speed. (It may still happen today with high end systems.) There was a feed back circuit using the actual rotational speed of the turntable that corrected the rotational speed to compensate for things such as mains frequency or voltage fluctuations.

Most things can handle slightly varying voltage but frequency is another matter and has to be taken into consideration where critical.

the strobe system was mostly magnetic gear tooth very much like the speed sensor on the SV.

a DC motor creates voltage as a by product of spinning, its this extra voltage that you can monitor to create speed accuracy.

each of the above are dependant on how well the drive circuit has been implemented. a badly designed encoder can be worse than back emf and visa versa.

people like myself are resorting to using back emf due to the silly prices charged for gear tooth/optical encoders.

back emf can be used with pwm but its very complicated to get right.

i'm still only half way there with the deign as i still have to test under load on the turntable. so far though while using a laser speed measuring device on the spindle of the motor the speed accuracy is bang on.

[SV650rules]

In days gone by and when vinyl was the main source of listening to music, many hifi systems and those used in places such as radio stations used strobe systems to control turntable speed. (It may still happen today with high end systems.) There was a feed back circuit using the actual rotational speed of the turntable that corrected the rotational speed to compensate for things such as mains frequency or voltage fluctuations.

Most things can handle slightly varying voltage but frequency is another matter and has to be taken into consideration where critical.

In the UK the electrical power suppliers are legally obliged to keep supply frequency within +/- 1% of 50 Hz.

Synchronous AC motors (single phase shaded pole) do produce a 'mains freq hum' (electrical noise) but are normally quiet mechanically.

Well done to Bibio though for his quest for a better, cheaper way.

[Bibio]

here is the realtime frequency of the UK grid. as the needle moves up and down this would be the effect of an AC motor speed drift. this would sound like a piano note not being constant or slightly out of tune.

http://www.dynamicdemand.co.uk/grid.htm

now for even more complicated issues surrounding the use of AC motors, most of the older TT's that use AC motors in the UK have a 110vac motor and as such have to use capacitors to bring the voltage down. thats all very well but you also have to remember that the UK mains voltage was taken down to 230vac then add to that the tolerance of cheep capacitors being +-20% then add in degradation of component and you might as well turn the TT with your hand. nowadays though there is another way to 'lock' AC voltage by converting it to DC then set the frequency using crystal oscillators then turn that DC back to AC, this solution works great but you still get AC motor magnetic radiation which can cause havoc with some MM cartridges.

my design uses a dual (+ 0 -) linier DC supply so even if the mains does shift the output DC stays the same.

[DarrenSV650S]

thats all very well but you also have to remember that the UK mains voltage was taken down to 230vac

Only on paper. It's still 240