[Al_The_Man]

Whilst I am rambling on. You are going to see people saying things like amps / current = torque. From the math I have done, it's voltage and current together that have to be considered when trying anticipate motor performance. I think everyone assumes fixed voltage because that's the norm.
At a given voltage, yes more amps is going to equate to more torque. At a given motor load, yes more voltage is going to yield more speed. Also note that as speed goes up, induced resistance goes up and current goes down. A stalled motor has the least resistance and thus allows the most current to pass.

Torque does in fact = current. With a permanent magnet motor the speed is regulated by the back EMF, which once it reaches roughly = the applied voltage, speed increase will cease.
If the motor is loaded at all then the speed will drop and so will the back EMF allowing an increase in current.
There are ways of increasing the voltage, therefore current, by either a linear amplifier (0v to max DC), or by the increase in energy by PWM signal.
Obviously if max voltage or PWM was applied at stall then you would burn the motor out if some form of current limit was not applied.
Al.

[jdelaney44]

Al, yes, I wasn't trying to say current did not equal torque. Sorry if that's how it read.

What I was trying to point out was that you can't just treat current like it's totally independent. More current at a given voltage will generate more torque.

Question then-

Fictional example:

If:

70V 10A yields torque of 5 ft lbs and rpms of 2000

Then:

35V 10A yields torque of something less than 5 ft lbs and rpms of something less than 2000

But, in theory there is a current at 35V that could yield the same torque at the lower speed. Assuming the motor will take it.

Or, more accurately, there is a shaft load at 35V that will cause the motor to draw the same current as iut did at 70V.

Conceptually, is that in the right ball park?

Thanks,
-jd

[Al_The_Man]

. More current at a given voltage will generate more torque.

Question then-

Fictional example:

If:

70V 10A yields torque of 5 ft lbs and rpms of 2000

Then:

35V 10A yields torque of something less than 5 ft lbs and rpms of something less than 2000

But, in theory there is a current at 35V that could yield the same torque at the lower speed. Assuming the motor will take it.

Or, more accurately, there is a shaft load at 35V that will cause the motor to draw the same current as it did at 70V.

Conceptually, is that in the right ball park?

Thanks,
-jd

No, Torque will always be equal to current, this is the Torque constant of the motor Kt=oz-in/amp.
You cannot have 35v @ 10amps at rpm of 2000 and 70v @ 10amps at rpm of 2000.
You can have the same torque (current) with different terminal voltage, but the rpm will not be the same.
The Torque will be the same but the output power in watts will be different.
Al.

[FPV_GTp]

I have a millport mill (similar to a BP series 2) that I have installed ground ballscrews & am getting ready to CNC it. I can buy a ready built kit from AJAX for about 4000$ but I think I would rather do it myself for the learning experience (and less $). Initial questions are:Steppers VS servos, I've been told that servos are more accurate (Servos have a built in encoder/resolver & steppers do not?) but steppers are simpler to setup & less $. I want this machine to run off of a PC type computer so can I use either steppers or servos? In addition to the PC what components do I need for each axis? If I want servos is there a company that makes servo drives for home builders? If I buy the servos off Ebay do I want brushed or brushless? AC or DC? High voltage or low voltage? Similar questions if I go with steppers? How do I know what size motors to buy, what holding torque vs running torque?

hi

i found this site to be extremely useful http://www.machsupport.com/artsoft/index/index.htm

and it seems all the guys and alot of dealers selling cnc kits out there are using artsofts Mach 2 and mach 3 cnc software

also go in there link section you will find all the right sources for electronics that you will need to make you project reality here is a link http://www.machsupport.com/artsoft/links/links.htm
you will have to do some heavy reading

see what you think , hope it was helpful

cheers

[jdelaney44]

OK, I think it's sinking in to my thick head.....;-) Thanks.

Torque constant. Key word constant.

-jd

[NC Cams]

Al: not only is there a torque constant (Kt = X in-oz/amp) but also there is a RPM-vs-voltage constant of "Yrpm/volt", yes/no?? Naturally the RPM constant is for UNLOADED rotor.

As you apply load, rpm drops, the back emf drops in proportion to rpm drop and current flow increases until you ultimately reach locked rotor torque. At locked rotor, absolute max torque and max amp draw occurs (of course assuming terminal voltage remains constant).

My understanding - oversimplified perhaps but my understanding. Please correct or verify.

[Al_The_Man]

Yes but as I mentioned earlier if the occurence in the last statement happens, you destroy the motor, if max voltage is applied.
I am still putting together a check sheet for empirically coming up with the motors specs when they are not available from the manuf. It will hopefully have some of the above questions made plainer.
Al.

[machworx]

I have a question: does anyone know if there is a system or components available that would allow one to use their existing DRO scales as a feeback loop if driving your axes via stepper motors VS the traditional encoders in servo motors?

[ger21]

It's supposed to be implemeted in a future release of Mach3 with the G100, but I believe it's dependant on a G100 firmware upgrade, and may be quite a ways off. (maybe up to 1 year? but hard to say)

[NC Cams]

Word of caution re: optical scales for F/B loop =

The original Bridgeport Eztraks used a DRO scale on the Z for feeback in lieu of the encoder. They found that they had stability problems when/if vibration were to occur - system hysterisis/slop was supposedly the root cause of the problem. Ultimately they had to add an encoder and ran simultaneous position sensing.

The servo ecoders to deal with position F/B while machine was controlling Z axis - the linear scale was used in manual mode (servo disconnected) for encoding of position for DRO.

This does not mean do imply that linear scales won't work, rather, it only re-emphasises the fact the need for a robust, rigid system that is essentially as free of slop/backlash as you can make it.

Otherwise, you could be plagued with oscillations as the system "hunts" to find/correct itself.

[Al_The_Man]

I have a question: does anyone know if there is a system or components available that would allow one to use their existing DRO scales as a feeback loop if driving your axes via stepper motors VS the traditional encoders in servo motors?

Unless you have almost zero backlash there can be a problem with just using one feedback loop, If you need a very good explanation of dual loop feedback on systems that have inherent backlash and require accurate positioning by using linear scales, check out the instructional videos on the Galil Motion site (Dual feedback positioning), to see what it entails for tuning the PID loop.
This can only be done successfully however with servo's fitted with an encoder together with the scales.
Al.