Good evening All
Once again I'm in need of help!
I'm looking for a VFD for a cnc lathe conversion and I'm looking for the best one (and cheapest) for the job. I will be threading so either need a closed loop vfd or a sensor less vfd that can hold constant speed under load.
The dedicated closed loop ones are a bit pricey so I'm looking for advice as to whether I need a full blown closed loop system? the motor and drive will be 3hp so should have plenty of power.
I've seen these inverters which are sensor less but in the manual they say it can be configured for PID. is that not then closed loop? this would seem like a good option as I could switch to closed loop if problems arose.
I've read it a few times and does seem to be capable of closed loop but not sure why its not advertised as closed loop.
Thanks in advance
Unless you're going to replace the V-belt with a toothed belt, running the motor closed loop will be of no benefit.
I can't remember the exact details, but essentially sensorless vector drive measures the motor back EMF to determine how it is, or isn't, moving. It's essentially a very complex open loop drive that is able to drive motors very slowly and smoothly.
A full closed loop drive, will most likely use very similar technology, however it allows the connection of an encoder so the drive knows exactly where the motor is.
In terms of a lathe spindle, the most important thing is whatever controller you're using gets a suitable signal from the spindle. Unless you're planning on doing something that involves really accurate spindle control like rigid tapping, the only main requirement is a well controlled and steady spindle speed.Avoiding the rubbish customer service from AluminiumWarehouse since July '13.
18-08-2016 #3I'm looking for a VFD for a cnc lathe conversion
I have done a lathe conversion Myford S 7 using LinuxCnc with a 64 slot encoder on the spindle using (A + B + index) that controls the Z in time with the spindle you can even rotate the chuck by hand and thread...Clive
Here is one I have, (but mine is a 1.5kW) and although so far I have only been test running it, it seems to work very well. Pretty easy to configure with any motor and also has PID if you want to set it up. I have just yesterday managed to create a Mach3 brain and macro to control it via RS485 modbus, so all the VFD inputs and outputs are free for other use than starting/stopping and speed controlling the spindle. It is a very nice VFD, well made and easily configured using a free software from Bosch and an USB port on the VFD. Of course, if you want to fiddle with the panel that is also possible... Documentation is excellent and the price is also very good. I bought mine from Inverter drive supermarket as well and am very happy with the communication and the quick handling. The VFD can be used in both V/f and SVC modes and the auto tuning helps you to set up the motor parameters. I have no experience with any other VFD than the Bosch Rexroth EFC 5610, perhaps others are just as good, but when I selected this one I checked out quite a few others as well, and ended up with this one because it has some features others I looked at don't have, plus it has an excellent manual. Today if I'd need to buy a new one I'd probably still buy this one, but perhaps I'd go for the 2.2kW version.
I think that the term "Sensorless Vector Control" is a bit wrong, since it is not sensorless at all, but the sensors used are built in the VFD. It is actually a closed loop system, but the feedback loop comes from within the VFD itself instead of an external encoder. There is a mathematical "model" of the motor operating parameters inside the VFD. As the motor operates, the VFD monitors the output current (mainly), compares it to the model and determines from experience what the different current effects mean in terms of the motor performance. Then the VFD executes the necessary error corrections just as the closed Loop Vector Drive does.
The only drawback is that at slower speed the ability to detect the changes in magnetics becomes more difficult. At zero speed it is not reliable enough, as opposed to a closed loop system, which works down to zero RPM. Never the less, an SVC controlled motor can also be used down to pretty low RPM with very high torque, assuming your spindle motor can actually be used at that low RPM. Most high speed spindles have a minimum usable speed of around 6000 RPM and you should not go below that, at least not for a longer period. But... at 6000 RPM if you are not using a SVC VFD then the torque is pretty low already, so there are indeed benefits even at higher speeds.
PID in itself is not the same as closed loop vector control. Closed loop vector drive uses a shaft encoder on the motor to give position indication back to the microprocessor in the VFD, so when the processor says move x radians, the encoder will indicate the error if there is any, and the VFD corrects this error. You can't really do that with PID.
Anyway, if I was doing a lot of threading I'd select a low speed high torque motor, not a high speed motor driven at low speed.
Thanks everyone, that's really helpful.
m_c, yes I will be replacing the vee belts with timing belts to eliminate any slip. I want to be able the do fine threading in stainless so I was under the impression that I needed good control over the spindle.
Clive, I think I have settled on EMC so I can use the multi pulse spindle encoder like you suggested. so if the controller reacts to the spindle speed is it quick enough to pick up the change in spindle speed as cut starts? Im looking at as high a speed and max depth of cut as possible.
A_Camera, thanks for the link. That looks like a better option especially with the detachable panel. I will be using an inverter duty 4 pole motor to get the best performance over the speed range needed. they do one that can go from 150-3000rpm so should be perfect. As you say the advantage of the encoder feedback is that it goes down to 0 rpm so good for indexing.
Im still confused about the PID feedback as the inverter I mentioned shows an external connection to a speed pulse.
For the extra money of the closed loop VFDs I think I'll try the senorless first and upgrade later if and when needed.
Many thanks All
18-08-2016 #6is it quick enough to pick up the change in spindle speed as cut starts? Im looking at as high a speed and max depth of cut as possible...Clive
I finally have a bit more time and a purpose to get my CNC projects finished. Yes I will probably end up with the mesa cards, it depends if I can get the Kollmorgen servo drives to work and what I need to interface to them. Build should start in a month once I have a clear idea of what to do. biggest problem is the cross slide!
I will ask on the Linux forum and see what the consensus is but from reading around it seem to like you said, as long as the spindle isn't under powered and it is a good quality VFD then the spindle speed should remain constant. 3HP should be ok but I suppose I could do some test cuts in manual mode and monitor the rpm.
Have you got a build log for the Myford ?
19-08-2016 #8Have you got a build log for the Myford ?
Last edited by Clive S; 19-08-2016 at 07:41 AM...Clive
Anyway, if you decide to go for the Bosch you should also consider buying some other items, so have a look around before placing the order. My order looked like this, and I think this is the minimum you will need.
If you decide to order a Bosch 2.2kW and consider buying a brake resistor and the cable clamp you must check that those fits even the 2.2kW version. I am pretty sure that the panel cable is the same.
They have even other interesting items which I would have bought if I not already had those, so have a look around. Anyway, it is a good company to deal with, my experience is positive even with the after sale support. I had a few questions and received all the help I needed.
That is my understanding of the differences, but there must be more than that, since the price difference indicates that. Anyway, no, I don't think you need it, unless you really need EXTREMELY low RPM, like below 10 RPM or so.
Last edited by A_Camera; 19-08-2016 at 08:10 AM.
You might struggle to get enough torque by only using one gear/ratio for the entire speed range.
Going by the online calculator I just used, 1.5Kw at 3000RPM gives 4.77Nm, which will remain constant over the speed range. Certain motor/VFD combos can boost that for short period of times.
You might want to run some figures through something like FSWizard or G-Wizard, to see what the estimated torque is for the work you're planning on doing.
If you're planning on running the motor at low speeds for extended periods of time, you might want to consider a motor with a separately powered cooling fan, otherwise there is a good chance it'll overheat.Avoiding the rubbish customer service from AluminiumWarehouse since July '13.
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