I used to work for Siemens. True, you can servo any motor, but whether it can be an effective positioning drive is really the question. It all comes down to how precise you need to be able to position the drive.
We would have loved to have servo'ed standard 3 phase AC induction motors for position control applications. You can sort of do it, at a lot less than full torque, but it is really not at all precise. If Siemens can't get standard AC squirrel cage motors to position precisely, I doubt that anyone can. That's why 3 phase servo motors have magnetic rotors. That's the only way they can operate precisely synchronous to the rotating field. I installed 8 of these on a unwind/rewind paper coating line last year and the motors were Marathon motors with encoder feedback. We used these drives, which are really smart. Each axis is independently programmable. https://www.youtube.com/watch?v=GvIdTvIdrzc They can do precise position control with that drive, but they need a 3 phase servo motor to do it. Fortunately we didn't need precise position control just good torque control for the winder and precise speed control so the paper didn't go slack through the system. Keep in mind that positioning a spindle for a tool change is a lot different than being able to precisely control position at full torque and zero speed. Dave On 1/25/2016 4:29 PM, Stephen Dubovsky wrote: > No mixup. I used to work for GE (and Hughes) designing AC induction drives > for mostly traction applications. You can servo **ANY** induction motor. > The only limit to frequency response is the inductance and that can be > solved with high enough bus voltage (DC motors and steppers have the exact > same problem.) There is ZERO residual magnetism needed to get an induction > motor to hold position (even at zero) during closed loop operation. > > Everyone has played w/ a DC motor w/ the rotor terminals shorted out. They > are darn near impossible to turn w/o a large lever. The DC rotor which has > zero power applied, when moved (even a little) through the stators magnetic > field, induces currents in the rotors shorted windings. Sound familiar?? > Apply DC current to an induction motor and it can produce the same magnetic > flux in the gap as the DC motors magnets do. The shorted rotor windings > thing? Yep, thats already taken care of as its how an induction machine > ALWAYS works. The result is the rotor is just as hard to turn as that of a > DC machine. Now neither of these machines hold the exact position open > loop so what do you do... > > Closed loop! Ok, now we put a shaft encoder on both machines. On a DC > servo, when you see the position error you apply voltage. Voltage = > current. Current = proportional to torque. You apply enough until the > resulting torque is enough to both stop the load and move it back to the > commanded position. Servoing it. W/ an AC induction motor, when you see > the position error you apply frequency (the voltage may already be present > just like in the paragraph above.) You can then command any torque at any > shaft speed within the motors torque/speed envelope - it just takes more > calculations;) So you "apply enough until the resulting torque is enough > to both stop the load and move it back to the commanded position." Exact > same result as a DC servo. > > Another way to think of it is look back at the shorted winding example of > the 2nd paragraph. To get the DC motor to hold position w/ shorted > windings, you could slowly move the stator magnets 'backwards' to conteract > the shaft torque and movement. Not too convenient since they are likely > glued in the frame. Bot that **EXACTLY** how an induction motor does it. > Its 'moving' the (virtual) stator magnets to apply torque to the shorted > rotor and hold it in place. > > Most folks who have never done advanced induction control are surprised to > find the torque vs slip curve is both 2 quadrant and symmetric. You can > indeed provide full torque, in either direction, at zero shaft speed (or > any non zero speed). Once you have a predictable way to respond to a force > command (from the PID loop) you can position servo any system. AC > induction, PMSM, DC, hydraulic, etc. doesn't matter. > > Bottom line is that you can servo an induction motor VERY well if you have > a VFD smart enough to do it. > > SMD > > > On Mon, Jan 25, 2016 at 11:09 AM, Dave Cole <[email protected]> wrote: > >> You are getting your motors mixed up. >> >> Typical AC induction motors suck for positioning. >> >> You are thinking of AC servo motors that use a magnetic rotor. >> They are a totally different animal. >> >> DC servos typically have wound armatures (and brushes) and use magnets >> for the field. >> >> > ------------------------------------------------------------------------------ > Site24x7 APM Insight: Get Deep Visibility into Application Performance > APM + Mobile APM + RUM: Monitor 3 App instances at just $35/Month > Monitor end-to-end web transactions and take corrective actions now > Troubleshoot faster and improve end-user experience. Signup Now! > http://pubads.g.doubleclick.net/gampad/clk?id=267308311&iu=/4140 > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users ------------------------------------------------------------------------------ Site24x7 APM Insight: Get Deep Visibility into Application Performance APM + Mobile APM + RUM: Monitor 3 App instances at just $35/Month Monitor end-to-end web transactions and take corrective actions now Troubleshoot faster and improve end-user experience. Signup Now! http://pubads.g.doubleclick.net/gampad/clk?id=267308311&iu=/4140 _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
