Rewire Router spindle for constant torque

[janbbeck]

All,

I have not seen this discussed and felt it was useful information for others and so I registered for the forum. I am sharing this to give something back to the community.

My CNC setup used one of those Bosch 1617 routers with a precisebits collet set. What I have done with the help of a friend is to reconnect the windings to make it a shunt motor. The advantage of that is that now the torque curve is essentially flat. I can use an autotransformer to determine the RPMs. This is of advantage if you want to adjust the RPM down for a large cutter, but do not want to give up the torque, as you would if you just put an autotransformer on the router.

This turns the motor into a DC motor. The price you would have to pay is 2 rectifiers, 1 transformer, 1 autotransformer and a second cable coming out of the router.

I would have uploaded pictures, but the forum has a ridiculous file limit size. If anyone expresses interest, I will resize the files and upload them.

Enjoy,

[Al_The_Man]

Are you running with fixed field voltage? or vary both armature and field?
How do you achieve the same rpm as the series connection?
Al.

[janbbeck]

Originally Posted by Al_The_Man Are you running with fixed field voltage? or vary both armature and field? How do you achieve the same rpm as the series connection? Al.

Yes, fixed field voltage. I measured the resistance of the winding and then put the voltage on it that causes the same amount of current the motor would usually run at.

I change RPMs by varying the input voltage into the rectifier that feeds the armature. I do this with an autotransformer.

[ger21]

What range of rpm's can you use now? I'd like to see some pics. I think the size limits on the upload page may not be correct. You might be able to upload a 1280x1024 jpg. try and see. You can definitely upload 1000 pixels wide.

[Al_The_Man]

The series motor achieves high rpm because of field weaking as rpm increases, essentially in a run away condition, I would be surprised if you achieve the normal 20krpm of this type of motor in a shunt fixed field voltage condition?
Did you measure the rpm?
Al.

[janbbeck]

You are quite right. This also causes the motor to slow down as soon as it sees any significant load. It also causes unstable RPM when it sees uneven load conditions.

I have ordered a tachometer to measure the new spindle speed.

If needed, I can also increase the RPMs by using the autotransformer to give the motor a higher voltage than rectified 115V. Since the motor has been designed to accept the higher RPM, the failure modes are only arc-through on the windings and overheating of the motor. I think 50% over the original voltage would be quite safe. As long as you can keep the motor cool enough, this will be fine.

Originally Posted by Al_The_Man The series motor achieves high rpm because of field weaking as rpm increases, essentially in a run away condition, I would be surprised if you achieve the normal 20krpm of this type of motor in a shunt fixed field voltage condition? Did you measure the rpm? Al.

[janbbeck]

Originally Posted by ger21 What range of rpm's can you use now? I'd like to see some pics. I think the size limits on the upload page may not be correct. You might be able to upload a 1280x1024 jpg. try and see. You can definitely upload 1000 pixels wide.

I will see about resizing my images later today. My camera is not high end, and routinely produces 2 MB JPEG images, because I like to leave the quality setting on high. I don't understand why the filesize limit for this would be 500 KB, but other file types can be larger...

But honestly, why can't the web server automatically resize the images I upload to whatever maximum file size and resolution it deems largest? That would reduce the burden from the members who would like to share information with others but are too busy to run all their pictures through an image editing program...

[Algirdas]

amazing! Producers of frequency inverters will bankrupt instantly.
You can upload high quality pictures to www.imageshack.com and here You will post links. CNCzone readers will be aware of heavy illustrations and will be able to choose whether take it to their computer or not. We know, that here, on CNCZone no heavy files.

[janbbeck]

I have put up the pictures I took here:

http://img51.imageshack.us/g/picturesaug2010013.jpg/
They are not in any particular order.

Cnczone's choice of limiting picture sizes is ok. I do mind the fact that the uploading interface does not allow the option of resizing the images to the maximum size, if they exceed it. Some time down the line, the above link will die, and then anyone who looks at this thread won't be able to see it. Why not save forum members the work and make it easy to post pictures no matter what camera you took them with?

Whatever. I like this forum and hope this info helps someone else. I will post RPMs when I get my tachometer.


I'll be glad to explain any details, if there is demand. Just ask.

Originally Posted by Algirdas amazing! Producers of frequency inverters will bankrupt instantly. You can upload high quality pictures to www.imageshack.com and here You will post links. CNCzone readers will be aware of heavy illustrations and will be able to choose whether take it to their computer or not. We know, that here, on CNCZone no heavy files.

[Algirdas]

it's a old good idea about image resizing during upload. We'll see after one year if it will be popular. I guess, not. I want my posts and pictures to be displayed as I designed. Automation is wrong sometimes.
Your pictures will not disappear, just tell me, what is worth to be saved there.
It's just ballast in series with motor windings, as I understood. Is it?

[janbbeck]

Originally Posted by Algirdas it's a old good idea about image resizing during upload. We'll see after one year if it will be popular. I guess, not. I want my posts and pictures to be displayed as I designed. Automation is wrong sometimes. Your pictures will not disappear, just tell me, what is worth to be saved there. It's just ballast in series with motor windings, as I understood. Is it?

It's hard for me to tell what would be useful for others. The biggest difficulty was in finding a way to route the new connections, since there is almost no room at all, and we wanted to be able to undo the modification, if necessary. For that, the best pictures are the ones that show the routing of the new wires.

In http://img695.imageshack.us/img695/2...aug2010027.jpg you can see how we connected the new cable to the existing terminals.

And in http://img51.imageshack.us/img51/613...aug2010013.jpg you can see the original connection.

Next, http://img205.imageshack.us/img205/6...aug2010019.jpg shows how we disconnected the field windings from the original terminals. I cannot find a picture that shows us soldering them together. But I made a quick schematic to show how all this works and am attaching the pictures here.

[Al_The_Man]

With >120 vdc fed directly to the field you are most likely going to have a quite high magnetic field, probably quite a bit higher than in the previous mode, this means that the max rpm will be limited to how fast the armature back emf is generated to reach the supplied armature DC. which could be quite rapid.
Have you run it yet? If so does the max rpm appear to be as high as previous?
Al.