... What they did was go back to their web page and change the rpm specs.
Evidently the particular parameters of this inverter or pulley size, or vz controller are what their engineers accepted as their specs, and I just wanted to know if that adjustment to eliminate "hunt" at very low rpm was one that was do-able.
A V/Hz controller (or volts per Hertz) is the most basic type of controller. That type of controller is best suited for an application where the load torque is very predictable and directly proportional to speed. Examples of this type of load would be a blower or a pump that does not have to work against a large head pressure. A lathe certainly would not fall within that requirement, but since the V/Hz controllers are very cheap, that is probably what is most common on the lower cost lathes (not the high end lathes).
A V/Hz controller does not attempt to sense what the motor is actually doing and just goes on the assumption that when it outputs a certain voltage at a certain frequency that the motor will turn a certain speed -- end of story. While this is fine for a well-behaved system such as a transfer pump, it is not the case for a woodturning lathe (not saying that turners are not well-behaved
😀, but the lathe torque load cannot be predicted based on spindle speed). Having the right V/Hz ratio from the controller is important. If the speed is low and the voltage high, the motor's iron core will magnetically saturate, which will lead to loss of speed regulation, a.k.a. "hunting". If the opposite happens, the magnetic field strength in the iron core will be weak and inadequate torque will be developed. In both cases, the motor's efficiency drops which is a fancy way of saying that it will run hot.
BTW, there is no way in heck to make a V/Hz controlled motor run satisfactorily at 100 RPM. I am surprised that it is satisfactory anywhere below 300 RPM and certainly not great below 500 RPM.
One notch up the food chain is the "sensorless vector" controller. It attempts to sense motor slip frequency and load torque, thus enabling it to provide "on the fly" compensation from the controller to maintain the desired speed. They really do a great job, but performance begins to suffer below 300 RPM and while they will actually run at or below 100 RP, I would recommend that you do not do it. This is a good way to overheat a motor very quickly.
At the top of the food chain is the very expensive full vector feedback controller. It not only knows the precise motor speed, but also the exact shaft angle at any instant. They usually use an optical shaft position encoder mounted on the back of the motor to provide speed and position information to the controller. They truly can operate from zero RPM up to the upper speed limit of the motor. This type of controller requires a special motor designed for this type of service. In addition to the encoder, there is a second motor that runs a blower to keep the main motor cool even at full torque and zero speed. These motor frequently can run as fast as 6000 RPM.
.... The manufacturers don't like the really slow speeds for fear that the motors will over heat and burn up.
I do not blame them. If Brent or anybody else built a lathe with a full vector control system, nobody could afford one. In practical terms, ther is very little occasion where a spindle speed below 300 RPM is necessary.
... After you believe that you have destroyed your lathe, step "one" will allow you to get back to factory settings. Also realize that a reset to defaults resets to the inverters default, not the lathe manufacturer's default.
Good point -- the controller manufacturer does not know what motor or application will be used with his controller. Defaults are just a starting place and not the destination. It is handy when one needs to get back to some sort of known configuration.
I have 2 concerns: one, to find out the process so as to know what is involved so that I can understand Grizzly tech support's reluctance so far to address this issued of "hunt" "searching" or "pulsing," whatever is the correct terminology for describing the problem, and two, to find out how to go about having a competent electronics technician do it, maybe later after the machine goes out of warranty.
I believe Grizzly could solve this if they would just do whatever it takes, and would have a very happy customer on their hands, but I am still waiting, and as of now I feel they have skirted the issue for what is easy for them, when it is in the realm of the possible to fix this as other vendors versions of the same lathe do not have reports of this pulsing issue at low rpm.
I think they "solved" it by redefining the performance specs.
😀
A better and more expensive controller might be the answer, but somewhat reluctantly in their defense, I would have heartburn about operating at 100 RPM because of motor thermal concerns. Even a better sensorless vector controller is likely to have "hunting" issues at 100 RPM depending on the drive ratio and moment of inertia that the motor sees. A general design guideline is that the load moment of inertia shall not exceed 10 X the motor's rotor moment of inertia -- otherwise it is difficult to have stable speed control regardless of the controller.
Most woodturners understand it better when I explain it in more simple terms: Bubba, if yew hang that thar 300 lb. chunk of soppin' wet wood on that faceplate, that sucker is gonna' go all wonky on yew when yew flip da switch.
..... I believe Grizzly has the capability, as they rewind motors, and do a number of other technical things.
I'm with Ralph. I seriously doubt that they have the technical expertise. Rewinding motors is more like a skilled trade -- like rebuilding an engine, etc. The scope of a design change should not be minimized. Fixing this issue would involve some specialized engineering expertise to redesign the whole system. On the outside it may seem simple, but it is really much more complex than just swapping boxes as far as optimal design is concerned. In practice, swapping boxes is what many companies lacking the expertise actually wind up doing.
