Originally posted by dougpdf:
CAN a servo drive be used with an induction motor.
What is and is not considered a "servodrive" these days is a very blurred line. For example, the Bosch/Rexroth Indradrive as well as the the Control Techniques Unidrive SP and the Parker/SSD 890 (among others) can all control induction and PM motors both with and without feedback. So are those servodrives or just run-of-the-mill VFDs?
What most people will consider a servodrive these days is a drive that can control a PM motor, accept high resolution feedback and has sub-millisecond velocity loop closure rates. Basically a high performance drive.
The thing that most users get wrong when trying to use induction motors in high bandwidth applications is they come up short on the feedback. Your "industry standard" 1024 PPR encoder isn't going to cut it for high bandwidth applications. Indramat used to market a "servo induction" motor. It was a 3-phase induction motor but was long and thin for it's horsepower, like a PM servo would be. But is also came with a high resolution feedback device that would produce in excess of 2 million counts per revolution. Worked pretty slick. They were often used for rotary knife applications.
As with most applications like this it comes down to torque to inertia ratio. Can you throw the motor and load around quickly enough to keep up with the process with the torque available from the motor/amplifier pair. If your analysis shows you can do that with an induction motor then an induction motor will work.
The same idea extends to the "plc vs motion controller" argument. A motion controller will provide higher performance in motion applications because it is specifically designed in both hardware and software to perform motion functions. This include the command set but also extends to I/O update rate and I/O update to loop closure synchronization. A plc will not provide those things typically, which has a direct effect on its available control bandwidth. Not to say a plc can't perform a specific motion function. But the user needs to carefully analyze and understand the required control bandwidth and then determine if the selected plc is up to the task.
So the moral of the story is that a motion controller driving a properly sized, matched PM motor/drive combination will provide much greater margin for error in any application that could use either that or a plc/induction motor combination. If you don't know for sure which way to go I would recommend buying the insurance that a motion controller/PM motor combination will buy you. I'm just saying that simply repeating the mantra that "it moves and positions so it has to use a motion controller and "servodrive"" (whatever THAT is) doesn't have a whole lot of value either.
In this particular case, I would guess the non-linearity at lower speeds caused by the vane pump will make this a real challenge to control regardless of platform. I'm thinking some loss modelling and feed forward would help along with some gain profiling to help model the apparent change in flow gain with speed as the vanes seal to the pump body. Or just switch to a gear pump and call it even.
Keith
