Emco Mentor Mill conversion

[Ross77]

It's not quite true you only need one cap? i had to use a few to get the balance right on all the phases. Takes a bit of messing but worked well in the end and saved me using a VFD for the job. Ross it is horses for courses, for me i prefer to use AC and VFD because it is easier. Brushless DC would be second choice but expensive (large servo would be even better). How deep are your pockets?

Cheers, I always thought that ac with vfd was more reliable but restrictive on high rpm also I had heard that torque at low speed can be an issue.

I was more looking at cheap treadmill motors, mainly because I picked one one up on ebay for £1. Not had chance to break it down yet but the specs say 1.75hp continuous and 3hp peak. geared down to 3000 rpm for the mill as it has roller bearings then that would be about 3hp continuous. geared up 1:2 on another spindle it should still achieve 1hp at 12000rpm

[Swarfing]

Sounds like your most of the way there then. Just need an appropriate controller and your away :-)

[Jonathan]

Cheers, I always thought that ac with vfd was more reliable but restrictive on high rpm also I had heard that torque at low speed can be an issue.

You can run a lot of standard induction motors at well over their rated frequency using a VFD and you will still get approximately rated power at the higher speed, however for that to happen clearly the torque drops proportionately. When you reduce the speed with a VFD, ideally the torque remains constant but in reality it drops a little. Even if the torque did remain constant, your power output will reduce with speed as power=torque*angular velocity.

The cheap VFDs from China seem to be good:
http://www.ebay.co.uk/itm/VARIABLE-F...item3cc89d5929

I use one of those to power the 2.2kW motor on my lathe and it works well.

the specs say 1.75hp continuous and 3hp peak. geared down to 3000 rpm for the mill as it has roller bearings then that would be about 3hp continuous. geared up 1:2 on another spindle it should still achieve 1hp at 12000rpm

Ideal gearing doesn't change the power output - if the input is 1.75hp, then whatever gearing you put in between the output will still be 1.75hp continuous, not 3hp. Clearly in reality you loose a few percent due to the inefficiency of the gearing.

[Swarfing]

Looking at the size of the machine i think the DC motor will do you fine. Mine will be running with only 1HP motor and that is more than enough for my needs.

[Ross77]

Looking at the size of the machine i think the DC motor will do you fine. Mine will be running with only 1HP motor and that is more than enough for my needs.

Hi Swarfing, you posted the same time as me so missed your reply. yeah think the DC will be fine for this little mill but I was just trying to work out which would be best. Didnt what to spend time making mounts etc if the ac motor on vfd would be as good, it seems that the DC wins on low rpm to torqure ratio which is mainly what I need for this machine.

The sad thing is i need to get the ac motor working to make the bits to convert to dc, that is providing the spndle is ok.

Sounds like your most of the way there then. Just need an appropriate controller and your away :-)

I was hoping to cannibalise the motor controller from the tread mill and control it with a Adruino.

[Swarfing]

That DC motor is it a 90v or 180v by the way?

[Ross77]

just opened it up and the motor is 180v and 4800rpm so a little slower than I was hoping for. Driver board looks quite compact and control panel is fed from one muiti core wire so shouldn't be to difficult to hack the speed commands.

On the down side it is 10mm to big (diameter) to mount on the existing motor plate so to use that motor I will have to make a new mount and move the motor position over to clear the top of the quill. Looks like I'm going to have to have a go at rewiring the AC motor for 240v and fire up the vfd.

[Swarfing]

Go for it, at the end of the day your other options are to spend some money. If i does not work...then just spend some money :-)

[Ross77]

Thanks Jonathan, is the torque drop off noticable? or is it dependant on the motor?

With referance to the gearing I was talking about power at the tool cutting tip. obviously the motor will always be the same power. Or have I got that wrong as well?

edit yes I have.....increase in velocity when gearing up and increase in torque when gearing down.