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! http://www.automationdirect.com/adc/Shopping/Catalog/Motors/AC_Motors_-_General_Purpose_and_Inverter_Duty_%280.25_-_300HP%29/AC_Motors_-_Inverter_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. 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
