Quote Originally Posted by Ross77 View Post
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

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.