View Full Version : Need Help! VFD to control CNC Spindle Motor

07-30-2009, 02:52 PM

i am just exploring my options here seeing as my AC spindle drive has gone up the creek wanting a huge repair bill if fixable. anyways

i was looking around to see what i can do to replace this 24year old tech and i see the new VFDs are getting pretty good and cleave on what they can do so my questions is has any one replaced there old spindle drive with a more modern VFD to control the spindle?

and if so how do they rate it? ie does it give the same torque out the motor as the old drive did. is acceleration and deceleration as quick if not quicker. as this is my main concerns.

i have been doing quite abit of searching reading on here (thx to all thoes that share so much grate info on here and take time to help others) as well as reading around on other places (sites)

the motor that is on the mill is as follow (as much info i have as of now unless someone can shed some more light onto it)

Motor - Mitsubishi SJ-7.5A
Three phase Induction Motor
RPM 1500 4500 8000
cont 5.5 5.5 3 (KW)
30min 7.5 7.5 4 (KW)
Volts 200V
frame B112F
Poles - ?? (if any one knows)
(see photo on post for plate on motor)

motor only used to go to 6000RPM so im happy to only do that but if i could do 8000rpm that would be very nice

from doing some reading i seem to get impresion if i want to keep torque im getting now from the old FR-SE inverter i need to have a VFD that can do a Constant Torque control not varaible torque control
is this correct?
how does control differ?

as for acceleration speed and deceleration of some good times 5s or less will i be needing a external resistor pack, of which size i am unsure yet the motor is belt driven onto the spindle unit. im presume the motor is a NON perm magnet motor so it will be safe to DC inject it for stopping.

im not to worried on encoder feedback as the 3/4% speed match using flux vector would be good enough so for spindle orientation i plan on using the encoder on spindle and have the control control the VFD over +-10v analog to get it into position and orientate it (as well as direction and speed in normal operation).

or am i going to need a drive with encoder feedback to have a zero stop holding torque (dont need huge amount of torque just enough to stop spindle turning while changing tool). The control can do a PID loop for +-10v on the spindle just as if controlling the AC servo motors so there is options on this side of things.

the one drive i was looking at was a Mitsubishi FR-A720 7.5KW model as it seems to have a wealth of functions to do what i am looking to do with a VFD and to me it looks to do Constant Torque & Variable Torque control.
i have spoke to Mitsubishi and as they are not CNC minded in UK office it is very hard for them to understand true nature of what im looking to do and get end result of.

i have also looked into some of the baldor & control techniques VFDs any others people can say please let me know.

thanks for your time.

07-30-2009, 03:06 PM
I have fitted both VFD and Mitsubishi Spindle controllers, the spindle controllers are intended to replace either an older DC spindle and drive or just the controller if an AC motor is already fitted, they are basically a souped up VFD.
I would say that the depth you want to go depends on what spindle characteristics the old one had and if you want to reproduce the same performance.
IOW, if you have an AC motor in place, I would say yours is a 4 pole motor BTW, and you want tight control and features like threading or rigid tapping and thread milling, then the ideal is to try and reproduce this with a VFD, the motor in all probability has a PWG, pulse wheel generator, so you would need a VFD that has this input feature, Mitsubishi VFD's can be ordered with this option, the other is dynamic braking by external brake module.
This set up should give you all the features that normally come with a commercial CNC spindle.
Other features sometimes required is spindle orientation for tool changing, many machines use an encoder on the spindle for this.

07-30-2009, 04:55 PM
Hi al

thank you for replying so quicky, i see you have answered many questions before on subject in past in my reading up. so thx for taking the time.

when you replaced VFD on CNC spindles (presuming u have) how did you find (or customer etc) the dynamics compared to old drive.
as my consers are with deceleration (but i think right dynamic brake option i would be abele to stop just as fast as i was doing if not faster)

torque from motor output was it same or very slight dropoff?

i was trying to lookup new Mitsubishi spindle drives but i could not locate much info more interested in what new ones could do etc but i know once we go CNC out come the telephone numbers on pricing.
i see most of them control everything in one now pritty much, same way axis servos are going on new controls.
the drive that is on there now is a Mitsubishi FREQROL FR-SE 7.5KW from when machine was made
with output IOs the FR-A700 has all same ones (at speed, zero speed, etc)

when you say suped up VFD in what way to do you mean, Mitsubishi tech was sort of saying i would not get motor torque range out of the A700 compared to old spindle drive but he was only a VFD guy for motors pumps etc not realy a CNC/machinery side of guy so he couldn't rely get to what i was trying to do.

old drive does not have much to play with , u set accel/deaccell motor type , base, max spindle speed thats pretty much it on dip switchs its just do i spend £1500+ to fix it and it goes down again as its getting pritty old now and more and more hard to find bits. or do i spend that money on some new tech, for same sort of cost.

if i can get the accel & deaccell out of new one and still get pritty good toque i cant see it being a no brainer at end of the day. its just nice to hear from thoes have been there and how it realy does turn out etc.

for rigid tapping cnc control takes encoder from spindle unit encoder mounted on side (not motor) and controls axis to follow it. so if i can drive a M6,M8 tap at 800-1000rpm that will do me. thats where like u say i need to reverse the spindle nice and sharp, old spindle would take quite some time to reverse from m03 to m04 at 1000rpm+ although drive was set at max deaccell time. compared to my fanuc spindles which are quite sharp indeed

as u say motor its self also has an encoder built in for old FR-SE as it used it as a feedback for speed so it knew when it was over speed or not responding to speed command etc as it has no other way of knowing what motor is doing in terms of speed, with out it it does not run the motor it would just alarm out right away.

for spindle orientation this was done buy a special option card on front (same as what the FR-A700 can take on it) and would then stop motor in a set position, and hold it there by switching forward/reverse at a speed to compensate so it never really held it in place in terms of torque just speed control in a closed velocity loop.

so my plan was to control the new drive with +-10v analog input so i could position it into place.
tech did say if i put the option card in for encoder it will keep better than 1% speed as apposed to 3/4% speed error,(so 6000rpm now becomes ~5820rpm) but to orientate i would have to give it a binary number in serial form to give it a position where to go (which seems long way to do it to me if i could do it from just analog in as im not worried on 3% speed dif from commanded to real motor speed as control will do what spindle is doing under load etc in regid tap mode)

thank you for your time.


07-30-2009, 07:00 PM
So far my experience has been to reproduce the exact same spindle performance, it has been with the Mitsubishi SPA-x spindle retro-fit.
These are about the best but many $$$.
But second in line is a PWG or encoder on the motor and brake module and regular VFD.