You want to use an AC drive for a simple positioning application. As everyone has pointed out you'll need to have encodder feedback to the drive for it to be able to hold the load at a position accurately. I have recently been playing arround with an ABB ACS800 drive with the position software installed. The motor is 11.5kW with forced cooling and a 1024pulse encoder (two chanels and zero pulse). The positioning accuaracy is fantastic, and in that particular application we are loading up the positioning drive/motor with an identical motor on a regen drive in torque control. It was very entertaining to have the position drive hold a position and then dial up a torque on the regen drive. There is no observable movement of the motor shaft, yet the drive displays were showing lots of Amps and matching torque values, as you'd expect.
If your application does not require the accuracy of a servo drive system then I would urge you yo consider the postioning drive.
Hey, thanks for responding. So many times we go skiping along down our tangential paths around here, we lose sight of an OP's original question. Glad to see you were paying attention and appreciate it.
Eng-Tips: Help for your job, not for your homework Read faq731-376
450x, as jraef say, thanks for responding.
now quickly down my tangential path...
Curt, I obviously bow to your far greater knowledge on this subject and was by no means trying to counter your description. It was my description in this thread that was wrong(quite possible) or I'm fundamentally wrong (also quite possible). I was simply trying to say (maybe if I put it the other way round) that 'with' movement in a closed-loop vector drive then the function of generating the active current, and therefore 'holding' the shaft stationary (if your setpoint was zero) will occur. Using your hoist with the 747's as an impressive example, the 747's are trying to create the error in the rotor angle and it is there you find out how well your drive performs.My point earier about zero speed and zero torque was simply (and maybe wrong) that if there was zero setpoint and no error, then certain drives will overflux due to the controller oscillating, effectively looking for an error. So the torque generated is not actually necessary but an indication of the performace of the control algorithm. This is the point where I think, "do I hit submit post and make a complete d*&k of myself", but as you can see I did and therefore the need to learn is a stronger urge than the other.
Not to worry. The whole point of this forum is for everyone to learn (except me, of course...)
Let me say at the outset that I consider a flux-vector controlled AC induction motor using a shaft position sensor to be a real positioning servo drive. I know lots of people who use them as such. The only thing they really give up to what most people consider to be servo drive/motor systems is that they have substantially lower torque-to-inertia ratio, because the rotor moment of inertia is typically 4 to 5 times higher than for a permanent-magnet brushless servo motor of the same power rating.
In our own positioning controllers, only two setup variables need to be set differently for controlling induction motors as opposed to permanent-magnet brushless servo motors. First, induction motors require a non-zero "slip gain" (slip-to-torque ratio), whereas for PM servo motors, this parameter must be zero. Second, induction motors require a non-zero "magnetization current" command (direct current) in order to induce current, and hence, a magnetic field in the rotor. This is not required when the rotor has permanent magnets creating its field.
Now, a positioning servo drive, with either a PM or an AC induction motor, can sit all day at zero position error, whether or not there is an external load. It does not need to hunt. The key is integral gain in the position loop. The integrator can "charge up" so that a torque command is output even in the absence of an error at the moment.
It is a common misconception that servo drives need to "hunt" dynamically to hold position. A good servo system can sit fat, dumb, and happy all day long at zero error if there is a constant load (zero or non-zero). Hunting is usually a sign either of response to changing disturbances or bad setup.
Very interesting Curt. Thanks. Usually I hear "charge up" as "wind up". I don't understand how DC current in the stator can induce something in the rotor or is this like a circular magnet with a few gaps in it and only one section has a winding(the stator)?
It isn't DC current in the stator. It is a slowly rotating flux. Three-phase system at low-low frequency. Just enough to produce the torque needed to hold the rotor.
I wasn't trying to speak terribly precisely, and I used "charge up" as a colloquialism. In an analog system, the voltage on a capacitor typically does charge up to perform the integral function. In a digital system, the number in a register gets bigger.
But "wind up" usually refers specifically to the "charge up" of an integrator past where it can do any good -- that is, when the servo output is already saturated at its maximum magnitude. This can be very problematic, because the very large value in the integrator can cause the servo to overreact as the servo comes out of saturation. Most decent servo algorithms now have some kind of "anti-windup" feature that prevents the integrator from charging up further if the output is saturated.
Skogs clarified the nature of what is happening to create holding torque nicely. At 0 rpm (0 Hz) rotor speed, the stator frequency may be 2 Hz. As far as the rotor electromagnetic dynamics are concerned, this is no different than a 58 Hz rotor mechanical frequency (1740 rpm for a 4-pole motor) with a 60 Hz stator frequency. The rotor "sees" a 2 Hz slip in either case.
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