Sensor less vector vs closed loop vector

Good evening All
Once again I'm in need of help! :wink:

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.

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I'm looking for a VFD for a cnc lathe conversion

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Have you decided what you are going to control it with ie MAch3 Linuxcnc etc.
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.

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Originally Posted by Ross77

Good evening All
Once again I'm in need of help! :wink:

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

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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

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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.

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Well I am not an expert but if your spindle has enough torque it should not slow down under normal threading use. EMC (LinuxCnc) is very fast and can handle fast encoders I take it you will use a mesa 5i25 with a 7i76 or similar I also have put to jog wheels on to use the lathe in manual mode. Perhaps you should ask your question over on the linuxcnc forum for the correct cards especially if you want a tool changer as well. I will follow your conversion with interest:applause:

Thanks 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 ?

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Have you got a build log for the Myford ?

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Unfortunately I haven't I just made an encoder disc driven off the spindle (not the motor) 1:1 and used steppers on the X and Z with the original screws (these might be changed). I also got rid of the compound and made a solid block on the cross slide instead. Hence the jog hand wheels for manual use etc.

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Originally Posted by Ross77

A_Camera, thanks for the link. That looks like a better option especially with the detachable panel.

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I agree about the panel, but if the ability to detach the panel is important then if you decide to go with the Bosch you should also buy the cable so you could easily detach it. I bought a 3m long cable with it and yes, it is nice to be able to detach it but I am missing a simple feature or two from the panel. One is that there is no cable fix at the back of the panel or in the VFD where you remove the panel from, so this is something to be solved by the user. The other is that if you remove the panel it becomes a tiny plastic box without any screw holes or screw ears, so it can't easily be moved to a control box somewhere away from the VFD without a modification or special fix. None of those two points are a deal breaker, but it is a bit annoying to have to come up with your own solution. I will eventually make a cover panel which allows seeing the LEDs indicating the status even when the panel is detached and which also will hold the cable at the VFD end. I will move the panel to my controller box for better accessibility since my VFD is not going to be where it is easily seen.

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.

Attachment 19050

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.

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Originally Posted by Ross77

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.

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I don't have any other experience than with the 24kRPM motor I have, but guess you are right. Anyway, for the task you have, that motor is definitely better than using a high speed motor at low RPM, even if you would have a real closed loop vector drive.

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Originally Posted by Ross77

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

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The way I understand is that PID means that the VFD have speed feedback, so it can keep up the speed according to the PID setup parameters, but in a real closed loop system you even have information of the shaft angle, so you know EXACTLY by the degree accuracy of the used sensor where the shaft is, even if it would be at zero speed, stopped. That way you can apply exactly the right voltage/current/frequency combination to pick up the speed or to start the motor the way you need it. An elevator may not be a good idea to start with maximum speed from zero the fastest possible time, while a CNC spindle is just fine by that. So it is not just about the speed but also about the position of the shaft angle. It simply provides a more accurate regulation than simple PID.

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.

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.

So how is the performance with the original lead screws Clive? can you zero out the backlash in the cross slide?

Thanks A_Camera, yeah I think I will invest in the cable and brake, they are out of stock now so will have to see how long it takes them to restock before I make my final decision. Having looked at the PID in more detail I think you a right and it is more suited to the slower response of temperature control rather than machine spindle control. How does the jog control work? is it like a servo motor for indexing?

Thanks M_C. the motor I was planning to use is an inverter duty 4 pole so should be good from 150 to 3000rpm. 2.2kw specs are:

For Variable Speed use:-
Part Time Output when Inverter powered is...
7.30Nm - 2.2kW ( 2.95HP) x 2880 RPM at 100 Hz
14.59Nm - 2.2kW ( 2.95HP) x 1440 RPM at 50 Hz
14.59Nm -1.1kW ( 1.47HP) x 720 RPM at 25 Hz
14.59Nm - 0.22kW (0.29HP) x 144 RPM at 5 Hz
Continuous Output when Inverter powered is...
10.4Nm -2.2kW ( 2.95HP) x 2016 RPM at 70 Hz
14.59Nm -2.2kW ( 2.95HP) x 1440 RPM at 50 Hz
12.40Nm - 0.94kW ( 1.25HP) x 720 RPM at 25 Hz
7.30Nm - 0.11kW (0.15HP) x 144 RPM at 5 Hz

The lathe is only 125mm centre so not much capacity for large stock and low rpm. I was more looking for indexing at low speed. I will check the cutter forces though.

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So how is the performance with the original lead screws Clive? can you zero out the backlash in the cross slide?

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I don't zero out any backlash because I find that on the lathe you (or I) always back off from X at the end of the cut run and then then any backlash is taken up when you start the next cut. If that makes sense. The conversion I did was from this guy http://www.jeffree.co.uk/modelengineering.html