On Sunday 14 June 2015 21:03:40 Dave Cole wrote: > Squirrel cage motors driven by VFDs slip. The motors have to slip to > magnetize the rotor. If you want tight control of a Squirrel cage > motor (as tight as possible) they put a encoder on the motor then run > the motor in vector control mode, where the slip is compensated for by > the drive. But there is still slip. The motor doesn't operate in a > synchronous way. > > There are of course motors designed to be controlled by VFDs and more > particularly Vector VFDs with Encoders mounted on the tail end of the > motor. The motors have better insulation and are designed so they > can be run with high torque at low speeds. > > Marathon Black Max motors are a good example of a motor designed for > VFD vector control that can have an attached encoder. > > Warning: Not cheap!
When you are buying cutting edge, expect to pay for the time spent sharpening the tech. :-) > http://www.automationdirect.com/adc/Shopping/Catalog/Motors/AC_Motors_ >-_General_Purpose_and_Inverter_Duty_%280.25_-_300HP%29/AC_Motors_-_Inve >rter_Duty,_Marathon_%280.25_-_100HP%29/Marathon_Black_Max_w-z-_Encoder_ >%280.25HP_to_30HP%29 > > But they still don't operate in a synchronous mode. The only way to > get synchronous operation (to my knowledge) is to have a wound rotor > or a permanent magnet rotor but then those motors aren't typically > designed for inexpensive VFDs. > > >>That in either event does not seem to be a viable way to spin a > >> 1/2-13 > > USS tap at say 300 rpms. << > > I believe that most rigid tapping setups with LinuxCNC are done with > VFD driven spindle motors with some type of encoder attached. Gee, I broke the mold, doing it with a treadmill motor. My problem in doing it reliably is in this case, the basic fault of a poor, made out of plastic, drive train. The motor can do it, but not w/o demolishing the plastic drive train parts. So once I get the screws working ok in the GO704, next is making a sturdier drive for the lathe. But once I get up to about 5 inches and beyond for the spindle drive pulley, they turn into auto parts with huge offsets between the pulley face and the shaft they turn, 2" or more in may cases. So, since my lathe isn't big enough to pull a cutting tool at 3" radius w/o breaking or burning up plastic parts, I expect I'll have to cut those circles out of 1/2" alu plate, on the new mill. There is not room on the spindle for a taperlock, so it will have to be bored to 28mm on the mill. And a hand fitted 5x5mmx10mm keyway cut in the hub. S/B fun. Then I can drive that spindle direct with its own serpentine belt. No breakable stepdowns along the way. But because the 28mm bearing surface is only 10mm wide, its got to be precisely done. Or trim things a bit to make room for the pulley between the adjusting lock nuts. There is not a lot of room for that in the present configuration. Sigh. Compromise and every one made weakens it. > Dave > > On 6/14/2015 8:17 PM, Gene Heskett wrote: > > On Sunday 14 June 2015 17:32:07 andy pugh wrote: > >> On 14 June 2015 at 17:14, Gene Heskett <[email protected]> wrote: > >>> So how do we discern the induction motor,totally unsuitable for > >>> vfd drive except near is design rpms, from the synchronous motor > >>> at home on a vfd drive? > >> > >> I think there is a false assumption here. VFDs are _intended_ for > >> driving induction motors. All my machines have squirrel-cage > >> induction motors driven by VFDs on the spindles. The Harrison still > >> has the original motor from 1970. > > > > Humm, that does not grok with what I understand is the actual motive > > force that turns the induction motor armature. > > > > 1. The slip angle that causes the current in the armatiure is > > basically a velocity effect. Reduce the driving frequency from 60 > > hz to 6hz, and one or both of 2 things are going to happen. > > > > 1a: The currents, because the inductance is still the same, now have > > time to rise to destructive, fusing of the wire in the windings > > levels. This will likely saturate the iron at the same time. > > > > 1b: If the currents are controlled so that they remain the same, > > then the rate of change of the magnetic field is reduced to 10%, > > inducing only 10% of the currents in the squirrel caged armature, > > with resulting severe loss of torque. Not to 10%, but because the > > effects are additive, but closer to 1% of the torque it can deliver > > when drven at 60 HZ. > > > > That in either event does not seem to be a viable way to spin a > > 1/2-13 USS tap at say 300 rpms. > > > > 2. So, do I not understand what I have learned from 65 or 70 years > > ago, or has a new way to drive such an induction motor that cancels > > or defeats the low speed power/torque losses? > > > > Educate me please. > > > > Thanks. > > > > Cheers, Gene Heskett > > --- > This email has been checked for viruses by Avast antivirus software. > https://www.avast.com/antivirus > > > ---------------------------------------------------------------------- >-------- _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
