[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]

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.

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]

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.

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]

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.

[janbbeck]

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.

The winding does not get 120vdc. That is what the first transformer is for. It generates just enough voltage to produce the current the motor was originally rated for. In this case that is 12A.

Yes, I have been running this for about a month now. It does seem to run slower than before, but I won't know how fast, until I get the tachometer.

I can cut through aluminum and steel with no problem, though I can give no good quality details. This is my first cnc build, and I am still trying to understand the combination of cutting depth and speed and spindle speed. I did however run a 1/4 inch cutter through aluminum with this, cutting a slot. The depth was 50 thousands and speed about 8 IPM. The router is not impressed at all with this, but the cutter gave up after a few inches and the last 4 or 5 inches I was simply melting the aluminum. Another time I made a mistake and told it to helically interpolate a circle downwards rather than cutting a shalow circle. By the time I had 1/5 of the circle's circumference done, the cutter was 0.3 inches into the aluminum plate. I can tell by listening that the router stays much more stable on the RPM under load than it did before.

This is not very scientific evidence. If you would like for me to run a specific test, then I will be glad to try. The RPM measurement is on my list already.

[janbbeck]

Well, I got the tachometer in and took the readings. The shunt wound router now runs up to 20000 RPM with 110V Rectified fed into the armature. The original rating on the router is 25000 RPM.

[Al_The_Man]

What field voltage/current did you require to adjust to to get the 20k?
Al.

[janbbeck]

What field voltage/current did you require to adjust to to get the 20k?
Al.

I think the original rating on the router is 12A. That means that the field windings are designed to carry that much current at full speed. So we adjusted the field voltage to get that much current (by measuring the resistance in the winding). This is exactly what causes the torque curve to stay essentially flat - that the field is the same strength regardless of RPM.

In the factory setup the field coils are in series with the armature, which means as we lower the current across the armature to get lower RPM, we also reduce the current through the field coils. This in turn weakens the field and causes the torque reduction.


I hope I am explaining this well. If not, just ask and I will try again.

[Al_The_Man]

But what I don't understand is how you are getting 20k with full field current?
In the series mode, 25krpm is possible due to field weakening that increasingly reduces the back emf with rpm so that a higher rpm is achieved because of the decrease in back emf against the applied voltage.
I would have expected that the high field current would have caused the back emf to rise very quickly and equal the applied armature voltage much sooner than 20krpm?
Just playing the devils advocate!
Al.
Low speed torque was solved for me this way, see post#4
http://www.cnczone.com/forums/showthread.php?t=109789

[Algirdas]

Your diagrams helps to understand. Now I see You modified the motor so, that You can control rotor current (y). Very good solution. Perhaps, You need to inspect/ repolish the motor brushes and collector. It is designed for much lower current and can burn easily soon. It will work just perfect if You will take care of it.

[janbbeck]

But what I don't understand is how you are getting 20k with full field current?
In the series mode, 25krpm is possible due to field weakening that increasingly reduces the back emf with rpm so that a higher rpm is achieved because of the decrease in back emf against the applied voltage.
I would have expected that the high field current would have caused the back emf to rise very quickly and equal the applied armature voltage much sooner than 20krpm?
Just playing the devils advocate!
Al.
Low speed torque was solved for me this way, see post#4
http://www.cnczone.com/forums/showthread.php?t=109789

Remember that the RPM is related not only to the field, but also to the voltage on the armature. I am putting a higher voltage on the armature than there was before (because the field windings are not longer in series with the armature).

[janbbeck]

Your diagrams helps to understand. Now I see You modified the motor so, that You can control rotor current (y). Very good solution. Perhaps, You need to inspect/ repolish the motor brushes and collector. It is designed for much lower current and can burn easily soon. It will work just perfect if You will take care of it.

No, I control the rotor voltage. The motor does not actually pull much energy when it is not cutting. It only starts pulling power to maintain the torque when putting a load on the cutter. It is a constant voltage, constant torque, constant RPM, variable current device. This ignores the I2R and frictional losses, but since these are small its quite true.

What that means is that the power the motor must handle is largely determined by the load put on it. The advantage of the shunt configuration is that mostly the RPMs and torque are constant and it starts pulling more amps to maintain the torque when it encounters resistance. The original configuration of the router (universal motor) cannot maintain a constant RPM with changing load, and therefore you cannot maintain the proper SFM of the cutter.

As another poster pointed out, this could also be done by electronics and a feedback loop, but I don't have a way to put a tachometer on the router.