is a vfd the right choice? - Page 1
Forum section: electronics / beginners

author: tomy983
author_rank: Contributor
timestamp: March 29, 2016, 03:48:32 pm
content: Hi everyone!I am helping a friend of mine which has a very old (40+) wood mill.The logs get loaded on a cart, which runs on rails, and get pushed into the band saw.The position control for the cut width was electromechanical, using mechanical dials, reduction gears, electromagnets and microswitches.It was very imprecise.I now managed to install a linear position transducer, and with a microcontroller I managed to increase the accuracy of the control, but it is still an hit and miss.This is probably because the driven axe is quite heavy, carrying hydraulics, the log, vices.. I guess is in the order of a couple of metric tons, and mostly I believe is because this axe is driven by an AC induction motor, the classic squirrel cage type with a cooling fan, switched by relays. It does have a brake, but the stopping position is very inconsistent, better than the old mechanical drive, but still not good enough. (Sometimes is off by 2-3 mm over 80 cm of travel, henche the operator need to reposition).My question is, installing a vfd, increasingly reducing the speed of the motor getting closer to the target sounds like a good idea, something that could work? Does anyone have a better suggestion for me?

author: Delta
author_rank: Super Contributor
timestamp: March 29, 2016, 03:50:58 pm
content: Sounds like a classic servo loop with velocity feedback. It should work well.

author: dom0
author_rank: Super Contributor
timestamp: March 29, 2016, 04:17:46 pm
content: VFDs can cause troubles with very old AC motors (their insulation is not only old, but was also not designed with the higher peak-to-peak voltages of VFDs in mind).

author: tomy983
author_rank: Contributor
timestamp: March 29, 2016, 04:26:47 pm
content: Quote from: dom0 on March 29, 2016, 04:17:46 pmI forgot to mention.. The original motor dedicated to the task passed away about 10 yrs ago.. Hopefully this one may handle the extra voltage...But thanks for your answer, is good to know.. Please, since I never used a vfd, is there anything else I should worry about?For example, should I remove the mechanical break on the motor and rely on the vfd breaking to avoid breaking action while slowing down, or this should not be a concern?

author: SeanB
author_rank: Super Contributor
timestamp: March 29, 2016, 06:38:07 pm
content: When shopping for a VFD also ensure you get both the line reactors ( 3 phase probably if it is the size I think that you will be buying) and a brake chopper for the VFD, as it is likely you will be doing some heavy braking with the drive. If you have smaller motors running at the same time ( 3 phase like hydraulic pumps and probably the saw) you probably will also want to add VFD drives to them, and interconnect the DC bus lines and sharing the brake resistor. This means that your braking the log is used to power the hydraulic pump and the saw, and the inverters also give an advantage of being able to measure power draw in them, and save power in the saw by slowing it down between cuts ( no need to stop it, just slow to a lower speed provided you get it back to speed before the cut) and lowering the hydraulic system pressures when idle so you use less energy.This will also save energy and will also enable things like tracking power required to cut so giving an early warning the saw need dressing, or that it has bound before it snaps. Now you have some computer control and a PLC bus there are many small incremental improvements you can do.As to braking, leave the original mechanical brake ( probably a friction brake operated by a separate cylinder) and use the motor braking. You can do incredible braking after speed reduction as you approach the desired position, basically almost stopping the unloaded motor in a single revolution at max, though the brake chopper will be doing a lot of dissipation, so it will have to be sized to handle this or else you will have a drive shut down ( freewheeling) and complain. Do the mechanical brake after motor DC braking, to hold position.When programming the VFD's also make a choice as to alarm outputs, and choose how it must respond to mains dips, probably you want it to freewheel and catch up, as opposed to faulting. Controlling the motor use the VFD inputs to do things like motor start stop, and use one of the inputs, if not doing via modbus or such, to select speeds and braking action, as most do come with both analogue outputs ( for an analogue meter) and a few digital inputs to select modes and speed. Also remember that the drive can drive the motor forward and reverse, so you can use this to advantage to have speeds that vary in each direction.Do note the warnings about contactors and motor overloads,and the need for output filters on some long lines, and the derating that requires. As well place in a well filtered enclosure, and put in circulation fans and a good filter pack so they do not get full of sawdust. You might even have to pressurise the cabinet to keep it out, and use well filtered plant air.

author: tomy983
author_rank: Contributor
timestamp: March 30, 2016, 10:16:42 pm
content: whoa!Thank you so much Sean for your suggestion, it is actually a very good advise you gave me!Originally I wanted do install the vdf only for the "width" motor, but listening to you I will add one also for the longitudinal one.As for the saw blade motor, today I went to check it out and it has a 20hp motor... I don't even want to know how much it may cost to add the vfd to that one.. (See, I really am helping a friend, there will be no pay for me and it will be done with a kind of low budget..), still, I think is a great idea and my friend too. Hopefully in the future.As per the other suggestion I will just follow them, as I think you gave me a great advice.I really am not shure wich way I will be talking to the vdf, I liked modbus, but I read somewhere that is very very slow, like seconds. So I will probably just use a 4-20mA control for speed and se what the offerings are for the other controls.I will engage the mechanical breaking after the dc break and then release the dc break, so not to heat up things for nothing.The lines will be short as I will install the units directly on the "train", again, thanks to your advice!

author: SeanB
author_rank: Super Contributor
timestamp: March 31, 2016, 07:06:46 pm
content: Most VFD drives also support a 0-10v input to control speed, or simply few switched inputs for coarse speed stepping. Up to 16 speeds though, so you could easily have a full speed, half speed, slow speed and finally stop with max DC braking using digital signals only. 3 relay outputs on the controller and simple enough to implement. With the saw you probably will then only want the single motor then, but include the line reactors, as it is a sawmill I would guess power is from a big on site transformer, so the reactors will help with the life of the inverter.