Variable speed Motor

[J-G]

This one is 130 mm overall apparently
https://www.ebay.co.uk/itm/12627833...XwgzqajSc-&var=&widget_ver=artemis&media=COPY

Interesting thought but no indication of the speed - for the sellers target audiance of course the rpm is irrelevant, so no real surprise there.

 

[J-G]

Looking at your images, it makes me think of precision lapping machines. In fact that is what it is.

So the parts most critical to quality will be a flat, rigid disk spinning with low vibration.
Rotational forces are probably not too high.

I am reminded of a hifi turntable platter. Heavy and with a good main bearing.

Having the motor decoupled from the platter via a belt drive would help to reduce vibration.
I wonder if 4000 rpm parvalux driving through a 1:4 ratio pulley & belt would work ?

Brush motors with commutators are called "universal" because they can run off both ac and dc. Regardless of their names, they don't care what you use. Only the voltage matters and below their design maximum voltage they will still run but at lower speed and making less power.

There are many cheap modules available on ebay that convert one dc voltage to another. Up (boost converters) or down (buck converters). They are specified by their max voltages and by their power rating.

If you preferred to feed the motor with AC, then a big enough transformer would do.

This expensive lapping machine happened to pop up used on ebay today. Just for your curiousity.

https://www.ebay.co.uk/itm/266695404087

Hmmm. . . Any curiosity I may have had (over the lapping machine) was sated with the first sight of the price ! - though I was minded of my time as an apprentice at Matrix where I would have used something similar when making slip gauges


You are absolutely correct about reducing vibration to a minimum - so platter mass and bearing quality are paramount.

This is my first sectional sketch of the 200mm dia. Platten and bearings - - with a potential pully driven by a 3mm Neoprene belt. I suspect that I'll eventually use a toothed timing belt though and the bearings will be further apart.

The orange components are plastic splash-guards to provide a water reservoir and I'm considering an aquarium pump to recycle the water but that part of the design is some way off. Though it may affect the decision about AC/DC since it could be useful to have a single power source input.

I'm still trying to get my head around 'Universal' motors. If you have a motor which expects 220/240V AC you surely can't expect it to run off (say) 24V DC at (say) 1/10th of the speed.

 

[J-G]

I'm still searching for answers to so many questions - - - -

I've just found this - what I think may be useful - - - AC-DC Converter - - - https://www.ebay.co.uk/itm/225772754265

Would this drive a 'Universal' Motor ?

 

[Sideways]

You are right. A motor designed to run continually on 240v probably won't start moving with less than 20 or 30 volts, maybe more (and ac or dc), but it will start. In practice, choose a motor that gives you the 2000 rpm you need at it's rated voltage and you should be able to reduce the voltage and still get something useful from the motor at half speed.

I think the ac dc converter you linked is a bit scary, it's probably altering the voltage by chopping up the ac waveform and then just rectifying it into lumpy DC.

This is just one of zillions of dc dc converters:
https://www.ebay.co.uk/itm/374539754802

You need another box first to turn mains into dc at a maximum of 50V, but then this little converter will let you dial the dc output to anything between zero and a little less than whatever you are supplying it with. Max input voltage is 50V. Max current is 40 amps (!) You won't overload it easily

But maybe the easiest way to get variable DC straight from a mains supply is to buy a cheap 30V lab or bench power supply ? 5A output may be enough but they make them bigger.

Then for a motor, how about these little brush motors from kiddies e scooters ?
The only problem is that they don't tell you what rpm they make at the designed 24v dc input.

https://www.ebay.co.uk/itm/164841631820

 

[MikeJhn]

Would it not be better to vary the frequency rather than the voltage to lower the speed, used to use one on my swimming pool pump to lower the noise level, did mean running longer for water change over, but was very effective and still powerful to move all that water.

 

[Phill05]

I have a Unimat and that was under-powered so I fixed up a 24v motor and little speed control I still kept the pulley format so I can get the lower speeds with high power more torque and a belt change to the higher spindle speeds it works really well.
For the motor pulley the shaft was 8mm so made a new one, I kept the old pulley with the old motor so could go back to original if needed.





Phill

 

[Fergie 307]

I have a similar set up, however the motor is considerably bigger than the OP wants.

 

[okeydokey]

Have a look again at a domestic sewing machine motor (ebay) or a sewing machine shop/repairers - the controller (foot switch) makes them variable - replace the footswitch with a rotary dial - would be an improvement

 

[StephenArfer]

What I need (I think) is a motor running at 0 - 2000(ish) rpm - probably 120w and physically about 100-120mm long.

How about a brushless 3-phase 200W DC motor with an Electronic Speed Controller (ESC) and a PWM variable control? Input DC voltage can be from say 8 - 18V or higher. I've attached a photo of an example setup. The small 230W motor can take 8 - 14V. I'm not sure about the larger motor wattage, but I run it at about 45A at 14V. There are many motors available in between these two together with more compact lower current ESC's. For the application suggested I would suggest a belt drive about 6:1 or the motors would be operating at a very inefficient RPM range.

Clearly the photo is of a model aircraft setup. I've been using the brushless motors for probably over 20 years now. I've never had a failure operating at much higher power than your requirement, albeit not continuous for long periods. You would need documentation for the ESC in order to set it up. All user settings can be set by operating the PWM input and counting beeps.

 

[Spectric]

When you look at the commercial machines then I believe they are expensive simply because it appears the objective is repeatable precision and grinding small irregular objects. A lot of input on the motor but what about holding the stone and a system of setting the depth and angle which if cannot be achieved with precision would make the motor irrelevant. How do these machines hold a small irregular object ?

The motor that might suit your needs could be from an electric scooter or bike, these use a Dc power supply in the form of a battery and a brushless Dc motor that is often controlled using field orientated control (FOC) which gives both variable speed and constant torque. There must be many discarded electric scooters around which would give you all the parts.

 

[ajs]

Are you suggesting the purchase of a new drill and stripping it down for the motor and controller (switch) ?

That may well be an option but I'd be a little concerned about housing the components - and I might need a great deal more assistance/guidance from forum contributors

That thought had crossed my mind but beginning with one of the mini drill presses such as https://www.amazon.co.uk/dp/B00KBYXPCU/ which are the right sort of power level and can be geared to the right sort of speed range by moving the drive belt.

I have one of those for circuit boards and for the money it's a great bit of kit. The mechanical components are a single unit - i.e. chuck, spindle, pulleys, motor all move as one when the drill is lowered - so should be easy to pull out and repurpose as is.

Alternatively if you want to roll your own I'd suggest looking in the scroll saw section where there was a thread recently extensively discussing a simple circuit for mains universal motors, enough to build one from scratch if desired. It's the speed control switch thread started by mock.

 

[Inspector]

I know you don't need the power and torque but there are scads of free treadmills around with DC motors begging to be repurposed. With considering.

Because of the water I would be looking for a three phase motor, sealed for wet conditions and a VFD to control it. Fractional horse power motors are not large and the VFD's to match are not too big either.

Pete

 

[Sideways]

An amateur reviews a "sterling facetter" machine, claimed to be in widespread use worldwide and hailing from sri lanka.
This is the way I would build one of these. Sub £100 variable frequency drive, 3phase fractional horsepower induction motor and a simple belt drive. That would be both powerful and very durable. Even if you made your own with a wooden frame and used washing up bowls to protect the mechanical parts.

I've tried hard to find a good discussion of the actual cutting speed eg "surface speed per minute" that works best but people discussing these machines haven't a clue about machining and just talk about motor rpm on their brand of machine. This is meaningless unless you know the reduction ratio of any belt drive and the diameter of wherever you typically press the stone against your wheel.

Polishing as I'd expect seems to be done at much lower speeds than the facetting.

 

[J-G]

Thanks to all who have contributed - it's not quite the 'minefield' that I initially envisaged

I think I've had a very productive day today - - the motor suggested by @Sideways is in Coventry (where I lived for 60+ years) some 30 minutes drive away. The lack of information was easily overcome with a simple Google search on the Motor description which found the identiacal unit on Amazon with much greater detail - rpm = 2650 - which is actually very convenient since I can use a 3:4 ratio pully/belt drive to achieve a top speed of 2000rpm.

Further investigation also found a DC-DC controller similar to that also suggested by @Sideways but with a reversing switch. This is a feature that I was going to ask about which is a bonus but certainly worth incorporating. Here's a [Link]

This left the question about providing a 24V DC supply which again proved easy (now I know what to search for!!). I found a company called Digikey and their PSK-10D-24 unit - again a [Link]

My drawings have progressed to this stage :

This is a 'Top' View showing the various components in the position relative to the platten along with the electrical conections.

Below there is a view from the front to show how the Motor and pully/belt system will be mounted.

I haven't shown the Speed Control Knob, Reversing Switch or Mains Input plug since they would clutter the drawing.

To achieve some isolation between motor and platten I've used a clearance hole in the top plate and anticipate using Delrin washers/buffers on the three motor mount pillars, though, as yet, I haven't shown them in the sketch.

Opinions on that arrangement would be welcome

 

[J-G]

When you look at the commercial machines then I believe they are expensive simply because it appears the objective is repeatable precision and grinding small irregular objects. A lot of input on the motor but what about holding the stone and a system of setting the depth and angle which if cannot be achieved with precision would make the motor irrelevant. How do these machines hold a small irregular object ?

That part of the apparatus is a totally separate piece of kit @Spectric. It's already part designed and incorporates an M8 LH threaded bar with bearings top & bottom, actuated by a 40mm dia. dial/handle at the top. It carries a tilting/pivoting unit which has a Collet chuck to hold 'Dops' which in turn hold the gem-stone. The angle that the stone is presented to the platten will be micro-adjustable.

I'm comfortable with making precision kit - even using fabrication rather than castings - but what I knew about electric motors wouldn't pay for a hair-cut! - it's still no where near suficient, but I have learned quite a bit over the past few days

One thing that does concern me is that, to date, I have been unable to persuade my grandson of the benefit of knowing something of trigonometry (he never did any at school !! ) so to make this project viable he is going to have to gain a great deal of knowledge!

 

[Sideways]

Opinions on that arrangement would be welcome

It all looks eminently sensible. I look forward to seeing what you build.

I have a suspicion that 2000 rpm will be too fast and that you may end up fitting a smaller pulley to the motor shaft to slow it all down and increase the torque.
That means including a simple mechanism for moving the motor and tightening the belt could be helpful. Actually some means of belt tensioning is needed anyway. Say attach the motor to a plate. Pivot that plate at one side and have a simple pinch bolt in a slot at the other usually works better than two parallel slots and trying to adjust both sides at once.
A small version like this will be a good prototype if you decide to scale up to an induction motor and VFD later.

 

[J-G]

It all looks eminently sensible. I look forward to seeing what you build.

I have a suspicion that 2000 rpm will be too fast and that you may end up fitting a smaller pulley to the motor shaft to slow it all down and increase the torque.
That means including a simple mechanism for moving the motor and tightening the belt could be helpful. Actually some means of belt tensioning is needed anyway. Say attach the motor to a plate. Pivot that plate at one side and have a simple pinch bolt in a slot at the other usually works better than two parallel slots and trying to adjust both sides at once.
A small version like this will be a good prototype if you decide to scale up to an induction motor and VFD later.

Belt tensioning had occured to me but if I stay with a 3mm Neporine belt (as I have done in the past) I'll specify the length at about 7% less than the measured distance. In this case it would be 600mm rather than the 645.7mm calculated.

If I change to a toothed timing belt then tensioning would be imperative as there is little to no 'give'. In which case I suspect that I'd go for an idler wheel under spring tension. There would then be a concern about noise - - - life can be a compromise

A pivoted plate would certainly merit consideration though.

 

[Spectric]

I have been unable to persuade my grandson of the benefit of knowing something of trigonometry

How did he not do trig at school, it is essential before moving onto calculus and Laplace transforms. Trig is something that cannot be avoided in any field of engineering and many are surprised when they realise it is just as important in electrical engineering as they initially cannot visualise where the angles come in. By the way what is a DOP and how does that hold an irregular stone ?

Pivot that plate at one side and have a simple pinch bolt in a slot at the other usually works better than two parallel slots and trying to adjust both sides at once.

Just like the car dynamo's and alternators of the past before being tangentially mounted, I assume J-G you are like many of us in being of that age !

 

[J-G]

How did he not do trig at school, it is essential before moving onto calculus and Laplace transforms. Trig is something that cannot be avoided in any field of engineering and many are surprised when they realise it is just as important in electrical engineering as they initially cannot visualise where the angles come in.

It's beyond my comprehension to be honest - but he didn't have the easiest start in life - - single parent young mother, outside of my influence 'til late teenage, and has always lacked self esteem. He does have a natural talent for design though

By the way what is a DOP and how does that hold an irregular stone ?

The name is a contraction of 'Dopping Cement' which is used to secure a gem-stone to what is now called a 'Dop'. They are short lengths, usually made of Brass, either flat end, or with a cone or 'V'. Obviously they come in different sizes as well. You could hardly hold a 1mm dia. gem in the same Dop as a 6mm gem

The initial mounting of an irregular rough stone would be preceded by a manual grinding of a flat at a suitable location so that it can be glued and waxed on to a flat Dop. Once the Pavilion ('bottom') has been faceted the gem has to be transfered to another Dop so that the Crown ('top') can be done as well.

Just like the car dynamo's and alternators of the past before being tangentially mounted, I assume J-G you are like many of us in being of that age !

Since my first car was an Austin 7 (non runner) which I bought for £2 - yes I'm familiar with the way that the position of alternators was achieved - I completely stripped that engine (side valve, IIRC) and rebuilt it - - but it was over 60 years ago.
What's 'tangentially mounted' ?

 

[Spectric]

What's 'tangentially mounted

This is where the alternator is bolted directly to the engine block and a belt tensioner is used to maintain the belt tension. This allows the alternator to be kept close to the block, when the alternator was used to tension the belt they needed space to move and with modern cars space is at a premium. Cars like your Austin and many others in the 20th century had plenty of room around the engine rather than being shoe horned in like now and it was common practice to have to tighten a belt to stop it squealing.