Thread: Wow - huge price difference

G'Day Rob,
When you are calculating the gearing for your motor setup, a couple of things to consider are, whilst the motor is rated @ 4300 RPM, limiting it to something more conservative like maybe 2500 - 3000 RPM (easily done with KB controllers) you may extend your brush and commutator life considerably. While my motor is rated @ 1750 RPM it is set it to 1440 max, the same as the original AC motor. As I don't what your lathe is, I personally would be a little nervous about increasing the max spindle speed, will the bearings take it ? and how well balanced are your chucks ?, you may introduce vibration and harmonics which will negate any advantage of increased spindle speed.
Regards,
Martin

Hi Martin,

That's a good point and perhaps one that is often forgotten when we look at changing an existing design.

One thing I would say is just because your design brief includes an increase of max RPM to say 2500, doesn't mean it has to be used if you find it to be unstable or some other undesirable factors come into play. The spindle bearings should be taken into account and treated with utmost respect in relation to this as they can be expensive to replace and having to replace them because of excessive RPM would negate any advantages gained from the mod. Finding out the bearing manufacturers recommended maximum RPM for oil or grease lubrication should be easy to find and keeping under it (by maybe 10 - 15%) should be the go.

The premature wear of the motor brushes may just be an acceptable price to pay for the occasional luxury of increased max RPM available.

On motors, I have read that 3 phase induction motors are so much more efficient and smoother than their single phase counterparts but never really though much about it. The other day I had my 3 phase treadmill motor at full throttle of nearly 5000 RPM and it didn't even look like it was turning. Smooth as silk!

I now love 3 phase motors!

Simon

Hi all,

Thanks for the input.

The actual spindle speed is not going to be much higher as I am using the lower ratios to limit the speed - ie. running in the lower ratio pulleys. That way I get much better torque than winding back the motor RPM.

The only bearings that might get a workout are the intermediate pulley, but ball races should be good for less than 4300 rpm so I don't expect any issues.

My lathe is a much abused CQ9325 Chinese job - but she's served me well.

The point about throttling back the motor a bit is a good one. I will just have to see how much grunt the motor can transmit to the chuck. If it can pull a higher gear I will knock back the motor RPM.

Been rubbing back and painting the motor cradle/mount this morning.

Nice sunny day in Adelaide and it's looking good.

Moved on to trying to find somewhere to mount the big piece of aluminium plate I'm using as an auxilliary heat sink for the controller - 2 HP application has to have one.

Sounds simple, but these things never are, it's angle plate so I have to find somewhere that won't jag me as I go past.

Having a beer right now - a Chang beer in fact, which is not too bad

Cheers

Rob

Originally Posted by toolman49

couple of things to consider are, whilst the motor is rated @ 4300 RPM, limiting it to something more conservative like maybe 2500 - 3000 RPM (easily done with KB controllers) you may extend your brush and commutator life considerably.

But if you limit the rpm for a given power, the current will have to go up right?* So are you better off or worse off?

Stuart

*or am I over simplifying things?

Depends on the controller stu, I don't know enough about the ones that that Martin has. I am probably going to add a pic or arduino to the mix to give real time feedback from the he sensor to the controller.

G'Day Stuart,
The KB controllers have inbuilt current limiting, from experience in industry I found that limiting to about 75% of max speed (when the customer would allow it) increased brush life dramatically.
Ueee,
I don't know which controller you were thinking of using, but the KB controllers have tacho feedback built in as standard.
Regards,
Martin

Hi Martin,
I was getting one of the KBIC's that Rob is getting from the states. I have found 2 donor treadmills so far but both have been 3 phase, not DC Old treadmills don't seem to be quite as common here as in Melbourne (the place not the photo)
Do the KB's regulate on back EMF? I presume they are PWM controllers? The older smaller DC controllers i have used with back EMF never seemed as good as a controller with an actual sensor.

Of course the other crazy thing that has crept into my mind is putting a slide pot on the cross slide and hooking that up to the main speed control pot. Then the speed can be set for a given diameter and the slide pot would compensate for the dia getting smaller as you turn or even speed the lathe up slowly as you face or part........

Originally Posted by Ueee

Of course the other crazy thing that has crept into my mind is putting a slide pot on the cross slide and hooking that up to the main speed control pot. Then the speed can be set for a given diameter and the slide pot would compensate for the dia getting smaller as you turn or even speed the lathe up slowly as you face or part........

Now that's what I call feedback baby!

Then, lets go one step further. If you had a DRO you could take the signal from the Crosslide axis, feed it into a PIC circuit and then tell the PWM circuit how fast to go! As the DRO changes, it changes the speed accordingly, keeping constant surface speed as in your idea.

Nice to theorise, isn't it?!

Simon

"Of course the other crazy thing that has crept into my mind is putting a slide pot on the cross slide and hooking that up to the main speed control pot. Then the speed can be set for a given diameter and the slide pot would compensate for the dia getting smaller as you turn or even speed the lathe up slowly as you face or part........"

You blokes are something else

I just want my tacho to work.

BTW I priced getting the HP resistors in from the States for the KBE controllers, but no go, the postage is a killer. Better off getting them from CBC at about $10 each.

Rob

Originally Posted by Ueee

I have found 2 donor treadmills so far but both have been 3 phase, not DC

While I like the idea of using my 3 phase treadmill motor and the dedicated motor controller that drives it, I'm finding this to be very challenging. Breaking into the circuit to provide my own speed control is not as easy as I first thought. I'm not too keen to gaffer tape the treadmill to the lathe either or having to select 5 Km/h in order to get 1000 RPM!

Worst case senario, I keep the motor and buy a $150 VFD. That's a last resort though as it kills me to throw perfectly good working stuff out and the idea of using the treadmill circuitry to run the motor with an external POT is still my intent! It's killing me and doing my head in though. I have just enough electronics knowledge to create a nagging frustration and not knowing what the whole picture is. I've spent much time looking at the input signal waveform on the CRO and saying to myself.... There can't possibly be information in that!

Simon

Fellas,
This is getting out of hand.
Regards,
Martin

Originally Posted by simonl

I've spent much time looking at the input signal waveform on the CRO and saying to myself.... There can't possibly be information in that!

Simon

He he, i know what you mean Simon, give me an analogue circuit anyday! Without knowing whats been flashed onto the processor's RAM it's near impossible to know what is going on. I just let the 2 i have found go as i have a small collection of 3 phase motors sitting around already, and i really don't need any more. Did i really just say that?

Mild steel, 40mm dia, so thats....6.5km/h.

I recon you should just run a belt to the lathe off the treadmill, and you could get a workout while your turning.



Rob

Originally Posted by toolman49

Fellas,
This is getting out of hand.
Regards,
Martin

Na, its just getting interesting....more interesting than the paperwork i'm supposed to be doing anyway......