SErvo Vs AC motors

haythame78

Member
Join Date
Feb 2005
Posts
8
Hi

i am in the process of choosing motors to actuate the motion in a Slice and Scan machine that was discussed earlier on one of the forums (Horse power calculation for an AC motor)

i mainly have two options either to use a servo motor or a small AC motors which is a 3phase 230V (generated by the Inverter) in addition to an encoder for the position feedback. and here arises the question what is the difference between servo and other AC motors is it only the the feedback unit? i know there must be more than this

4 of these motors will have to move synchronously in steps of 1 mm which means that those motors will need to be geared to produce the motion needed. According to the calculations these motors will be 0.25KW and 7.2 RPM. those motors will have to ramp up and down in terms of speed each time it travel a step of 1 mm. Only position feedback is required in this application. however, do i still need to a speed feedback in order to control the ramping up and down in speed? would i need a multi axis controller to synchronise the motors or is it enough to use a PLC which is going to be connected to each of the motor drives?

do u think that this can be acheived using 3 phase AC motor (230V) or do i need servo motors?

do u think PID control should be applied to control those motors (which can be integrated in the motor drives)?
 
Positioning is almost always a job for servos. The only way AC drives are suitable for positioning is when there is a positioning controller ahead of the drive and the drive slaves off the positioning controller.

By the time you have done that, you would be money and performance ahead by using the servo system.

Servo motors usually are designed for very fast accel/decel times. This usually means as light as possible rotors and deep overload magnetics.

Servo motors also routinely have encoder mounting provisions and almost aways are rated in torque rather than horsepower.
 
DickDV said:
Positioning is almost always a job for servos. The only way AC drives are suitable for positioning is when there is a positioning controller ahead of the drive and the drive slaves off the positioning controller.

By the time you have done that, you would be money and performance ahead by using the servo system.

Servo motors usually are designed for very fast accel/decel times. This usually means as light as possible rotors and deep overload magnetics.

Servo motors also routinely have encoder mounting provisions and almost aways are rated in torque rather than horsepower.

what do u mean by positioning controller? is that different from the variable speed controller?

as far as i know servos need Servo drives. which will be the interface and signal conditioning between the PLC and the Motor/Servos

Both ways i will need to use a PLC to cordinate the the operation of other components as well.

Do u think servos would still be better?

thanks
 
First off, when it comes to motors, DickDV has repeatedly demonstrated that he is the local Master round these parts, so I for one would recommend listening to him....

Now, with that said: Have you looked in to stepper motors for this application? From what you describe, the need to move a specific step distance with controllable accel/decel characteristics with relatively low force requirements, it strikes me that you are describing a normal stepper motor application.

Not only would three steppers with identically programmed drives give you a higher level of control than a similar AC VFD solution, the price should be much less than the servo solution.

IMHO.

Steve
 
Golly, Steve! Thanks for the nice complement.

Haythame, by positioning controller I meant any control device that keeps track of location and outputs simply a speed and forward/reverse signal to a drive/motor system.

Most often, it takes the form of a PLC with a positioning card or cards in it, one for each axis of motion.

Having said that, I think Steve might very well be on to something. I'm not real familiar with stepper motors but it does seem like you have an application that could use that technology and maybe save some money.

But, again, positioning and AC drives (even the best precision drives) are not a good mix.
 
I agree, steppers are probably the way to go. Servos would also work but are probably more expensive.

As far as the controller, what type or brand were you considering?
What PLC, etc.?
 
Gerry M said:
I agree, steppers are probably the way to go. Servos would also work but are probably more expensive.

There are some pretty cheap servos out there now.

With a servo, you may be able to lose the gearboxes, though you may need them for inertia matching.

You mentioned synchronizing, that would probably require a servo drive. With many drives, you can program a step motion. All you will neeed from the PLC is a digital output saying "go 1mm".
 
If you have VFD and abs-encoder in fieldbus and PLC, maybe you don't need the servo. You have distance to destination (ref-act). Speed is SQR(Distance*k) where k could be as 3.14 or what ever. Or you can use linear ramp, if you like.:D
 
how slow you want to run this? most ac motors will smoke at low speeds....
how precisely you need to stop? servo can stop on a dime way faster than what you can get with vfd.
motion controller is device that processes sped/position/accel/decel and other information and controls the servo drive (amplifier). you can use PLC to do the same but it will lightyears behind speed and performance of motion controller...
 

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