[chrispy]

I am running a three phase motor on my Bridgeport Series I CNC machine. Already have the rotary converter, and plan on leaving it that way.

I am really in the dark about spindle motor speed control. I see a number of people using a VFD, but they are also converting (if you will) there single phase to 3 phase with the thing. Plus these seem like a costly option.

I have searched on the subject to no avail. What I am looking to do is control the speed of the spindle through MACH3. Not looking to lose the 3 phase power. What are my options?

I have heard of people using a servo / stepper on the dial, at the top of the head (to be honest I have no idea how this dial works, I know it changes speed, but how) I would rather control it electronically, is it should be cheaper and more reliable.

Anyone know an option that will interface to MACH3 nicely? Cheap would be nice too. Also looking for some type of tach that would interface with MACH3 as well.

I am using the Bob Campbell breakout board.....


Thanks in advance,
Chris

[ger21]

I am using the Bob Campbell breakout board.....

Bob also sells a board that will control the VFD. Look under Mach2 spindle speed control on his website. About $100. The VFD is not the cheapest option, but is probably the best. And most VFD's do use 3 phase. Only smaller ones can run on single phase.

[Al_The_Man]

If you only use the convertor for the mill, probabally the best way would be to get rid of it and run a VFD with 1ph in - 3ph out, once you calibrate it the speed will be fairly constant and a known as it will be tied to the frequency selected, and can be displayed on the VFD. I got rid of the Vari-speed pulley on my Excello's and used direct timing belt drive with VFD control up to twice the rated rpm of the motor (120Hz).
Al.

[chrispy]

Gerry,
Thanks as always again.

So the Campbell board is a closed loop system (i.e. no feedback to MACH3), and Mach3 is only assuming the spindle speed?

So pretty much any VFD that runs a 3phase 2hp motor should work?? This is assuming that there is some type of standard that will interface with the above mentioned board. Sounds good, but.... it does not look like anyone is giving away these things! Pricey indeed.

I am looking at this as an option to figuring out the original manual stator switch on the mill. Currently it needs a coil on the back of the switch to be energized with 120vac. A momentary e-stop cuts power to the coil, thus killing the killing the switch and power to the spindle.

So far, I ran 120vac to the coil and the motor runs fine. Problem is the momentary e-stop switch only jogs the coil a little with each press. It actually takes 3 presses of the e-stop for the coil to make it far enough to kill the power.

Rather than sort this all out, I am looking at the VFD option. The big question that arises is the e-stop. Can I signal the VFD to kill power to the spindle motor? Obviously I can with MACH3, but thanks to Windoze I am definitely building in a manual (if you will), e-stop switch to kill the spindle motor and steppers. Trying to use the original momentary e-stop switch as no one is giving these away as well.

Next question that arises is calibration. How accurate is the VFD? Does it need calibrating? Of course not many of us have a remote tach around, and would be sort of useless buying one, just for this purpose.

Than comes the "I am an idiot" question. What does the knob at the top of the head do to control the motor speed, and how will this effect the addition of a VFD?

Save for that, and the last question. Is there a tach feedback system that will provide feedback to MACH3? Or is it even really needed?


Thanks again,
Chris

[chrispy]

Al,
Good idea, but being that the VFD will be acting as a static converter, I would be losing 1/3 of the HP. Plus I like the option of running it at 3 phase, for the sake of longetivity.

This is my first 3 phase machine and the convetor came with it. Leaves the door open for more cheap 3 phase machines.

[ger21]

There is no feedback. Al, correct me if I'm wrong, but none is needed woith a VFD. I also don't think you'll be losing any power. The VFD does provide the 3 phase power.
I really don't know too much about this stuff.

[chrispy]

Gerry,

From what I know (and I could be wrong), without a converter without some type of rotation, there is no L3. I am pretty vague at how a static converter gets the motor started (I think it just jumps it like a cap start motor but on L3), but after the motor has started, the motor is now running at single phase. Thus your 1/3 power loss.

Chris

[ger21]

I think the VFD takes care of that, but don't ask me how.

[chrispy]

Gerry,
Yes it does take care of it by cap starting on L3 (if you will), than dropping back to single 204v. As a matter of physics there is no way to generate an L3 that is out of phase with L1 and L2 without some type of rotation (at least with any decent amount of output).

Although I might be wrong.

Chris

[Al_The_Man]

The VFD takes the 240 1ph (or 3ph), and rectifies it and creates electronically 3phases, so there is no direct relationship with the incomming ac and the artificial 3phase, Some (not all) VFD manufacturers derate there product on 1phase, because they do not all guarantee the level required for the DC source whith out full 3phase for full power.
I am running my 2hp on single phase and have had no problem, the VFD is operated by +-10v analogue and there is a scaling factor in the parameters to relate the level of signal to out-going speed. The motor then runs at a speed related directly to frequency, so the motor runs around 10% lower than this due to slip which is normal on a typical AC induction motor.
On the question of e-stop, it pays to keep the VFD active, i.e. NOT break the incomming AC power by contactor etc, but to use the stop input on the VFD and this allows dynamic braking otherwise the spindle would coast to a stop.
The vector control VFD will also maintain fairly good speed under load without resorting to optional feedback device.
You can also operate on the 3ph from the converter if you wish.
You would need to keep any mechanical variable speed device in one place when calibrating and lock it at that point.
Al.

[chrispy]

Al,
Thanks for the good info. I was just looking at some VFDs and it is pretty clear how to integrate the e-stop now. You are 100% correct on not cutting the AC to the VFD (besides it would be costly to switch that much power), and use the inherit braking of the VFD. This is a plus for me, as I would not have to deal with the stator switch on the original unit. Seems like it would be easier to figure out a new VFD than to deal with the stator.

The only thing left for me to figure out is how the original variable speed control works on the machine. At this stage I would assume that I would just set it to the max, like you mentioned, and let the VFD do it.

As well I need to figure out what to do with the hi/lo speed selector (the manual gear one).

First thing on both of them is to figure out what the heck they are doing.

Hopefully the VFD will make it a little easier to design the e-stop. I have a partial system in place for the steppers, but I am just cutting AC power to the power supply for them. I am concerned about the capacitance of the DC supply, and if it will be safe enough to stop the steppers. Using the Gecko drives, but the newer ones do not have a disable on them. Should be telling us all something???? Relays after the Geckos would be nice, but complicated. I might just try cutting the AC and see how that works.

In looking at eBay, I see that I might be at an advantage in sourcing a 3 phase only VFD. Seems like the smaller ones go for a lot less $ than the single phase ones. Hopefully something will pop up in the cheap category soon.


Thanks again,
Chris

[chrispy]

Al,
Afterthought..

Do you know if the VFD is "recreating" all three legs, or is it just creating L3? Seems like no reason to recreate L1, L2 if you already have them. Or is it just inherit that it creates all 3?

After that, than the next question would be: Does it just create it when starting the motor, or is the motor running on all 3 phases after startup? After seeing the derating that the manufactures are doing, I can only guess that they are back to single phase after startup. Or am I wrong again?


Thanks,
Chris

[Al_The_Man]

It cannot use the existing AC supply directly as it has to vary the frequency, so the motor 3phase never see the incoming supply, In other words all three phases are created electronically.
The derating is as I mentioned in the previous post is that with only single phase coming in, the capacity of the DC power supply produced is considered somewhat less to be able to produce full motor power. There are many manuf. now that say theirs do not need a de-rating factor.
Al.

[Ken_Shea]

I got rid of the Vari-speed pulley on my Excello's and used direct timing belt drive with VFD control up to twice the rated rpm of the motor (120Hz).

AL,
This is exactly my consideration on a Hardinge CNC conversion.

In using a VFD as in the above quote, is there a VFD duty cycle factor to be concerned about, assuming bearings and motor are rated appropriately.

How about at minimum speeds ?

Is extra cooling required or recommended ?

Thank You
Ken

[Al_The_Man]

If the motor was used for long periods at low speed you may want to monitor the motor temp and put a fan on if necesary, in my case the Mill had a two speed gear box which I retained so I can keep the motor revs up quite high, by using the low gear for the low speeds.
I have found it is very rare to require very low speed and very high load at the same time. Most good VFD's have several monitoring functions that act to protect the VFD if you are stressing it.
Al.

[Ken_Shea]

Thanks for that information AL

Ken

[qtronpowerson]

If you only use the convertor for the mill, probabally the best way would be to get rid of it and run a VFD with 1ph in - 3ph out, once you calibrate it the speed will be fairly constant and a known as it will be tied to the frequency selected, and can be displayed on the VFD. I got rid of the Vari-speed pulley on my Excello's and used direct timing belt drive with VFD control up to twice the rated rpm of the motor (120Hz).
Al.

May I ask why the need for timing belt - your quoted text being most apt right now!!
qtron.