Servo motor type

[slcman 9]

Hi , I start to work with servo motor. I would like to know what is the difference between synchronous & asynchronous servo motor? Can I use the same drive for both type of motor? thanks

[paulengr 44]

I haven't heard the term "asynchronous" to describe a motor before. I believe you are referring to AC motors in which case there's the AC induction motor, and the synchronous motor, and also some other types. You definitely need a different type of drive to deal with a synchronous motor vs. an induction motor although you can run synchronous motors in induction-only mode. This is just the tip of the iceberg though. There are actually about a half dozen different servo motor designs. I suggest you spend some time on Danaher's web site reading up on this before you go much further.

[slcman 9]

It's hard to start in servo, too much info! I found synchronous and asynchronous servo motor in siemens web site: http://www.automation.siemens.com/mc/media...complete_E2.pdf

[panic mode 246]

term "servo" has nothing to do with type of actuator (and it doesn't have to have motor for that matter). it is a control mechanism that uses feedback so it can do precise positioning. imagine conveyor or fan driven by standard motor. you can start and stop it but you cannot guarantee that it will stop preciesely in position you wanted. this is what servo can - thanks to feedback. servo controller will keep moving it back and fourth until it's in right place. feedback also can be anything. industrial servos use encoders, resolvers and what not. hobby rc servos use potentiometer etc. if someone turn potentiometer while power is off, controller will know the position right away when you power it up next time. with other types of feedback this is not always true (resolver, non-absolute encoders). in that case you will have to home it (reference it) every time it is powered up.

[slcman 9]

thanks

[paulengr 44]

Anything that includes feedback on motor/machine position and allows for variable speed control (effectively anything except bang-bang control) constitutes a "servo" in the general definition. There are two key things that drive servo systems in general to have a different design. If you take a basic position control system with a permanent magnet DC motor driving a linkage for say a robot (and this would be the typical specs for a robot circa a few years ago), the control system is 12th order...too hopelessly complicated to be useful. However, if the position feedback sampling rate is roughly 30 times the mechanical system bandwidth, and we keep the maximum speeds below 1/5th of the motor maximum speed, we can end up with a very stiff and responsive control system that can overcome any sort of torque variation and such...stiff enough for machining applications. That kind of sampling rate and control algorithm ends up being a few milliseconds. This is right at the edge or generally just beyond what a PLC-5 or SLC system can handle, or just about any PLC from the 80's or 90's. I have personally written some hydraulic servo controls in a PLC-5. It's not easy but can be done so long as you recognize that system bandwidth is pathetically low at best. Hence up until recently, servo control systems for machining operations were built with custom controllers integrated into the drives. The PLC just supplied commands "start, stop", and perhaps supplied setting information. It wasn't until recently such as with the ControlLogix series that the PLC's were actually fast enough to take on traditional "servo grade" applications. Looking over the Siemens catalog you refer to, an "asynchronous" motor is just a conventional squirrel cage induction motor in a typical "servo motor" package (square box). The "synchronous" motor is an AC permanent magnet motor. See this web site for a description: http://www.freescale.com/webapp/sps/site/o...=02nQXGrrlPZL8l See also this one for the whole list of available motors: http://en.wikipedia.org/wiki/Electric_motor Note that it does miss a couple others. If you "unroll" the motor into a long, linear line, you get a linear motor. Also, a simple solenoid is technically in some ways a "motor" although the linear motion it generates is very small. You can use a conventional AC induction motor in servo applications, and that's what the "asynchronous" motors Siemens is selling are. But, be aware of two key characteristics of the motor. Since you will be driving it at low speeds, external fan cooling (or some other type of cooling) is required. Second, a lot of conventional AC motors are so cheaply made that they will fail mechanically (especially the rotor). Look for "vector duty" or "VFD grade" or similar motors that have beefed up components if you are considering doing this. As to literature and other information...I'm at a loss here. There doesn't seem to be a lot of good information out there that isn't either highly theoretical or so basic that it's useless.