Replace Servo with AC motor

Big John T

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Join Date
Nov 2006
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Poplar Bluff
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Has anyone replaced a servo drive with an AC motor with a feedback for position? I have a Cleveland Machine Controls ACM4000 servo motor that is bad and need to replace the servo system. Any thoughts or ideas...

John
 
The first question that pops up to me is duty cycle of a servo system versus an AC drive and motor:

Can the new motor and drive system start and stop as fast as the servo motor all day every day?

The 2nd thing that will trip you up is positioning accuracy.

Are you currently asking the servo drive to position the load with great accuracy, and how can your new system replicate that action?


Once those two hurdles are conquered, then you need to consider the position controller interface (if one is used.)

Most of the time, the servo was chosen for a good reason, and a regular AC drive can't do the same job.

JMHO
Paul
 
I personally wouldn't recomend using an AC drive unless you are willing to put in a great deal of time and effort to re-engineer the system.

Servo drives are fantastic at positioning; while AC drives are great and running at a specific speed.

BTW, what is the application?
 
Originally posted by OkiePC:

Most of the time, the servo was chosen for a good reason, and a regular AC drive can't do the same job.

It kind of depends on the age of the system. 15 years ago there were very few AC induction systems that could even come close to replicating the performance of a servo motor/drive combination. That is no longer true. In fact many manufacturers now make general purpose drives that operate both PM servos and AC induction motors. The widespread use of DSPs and advanced control algorithms have made the current generation of induction motor/drive combinations very capable. I know my previuos employer used servos in many application 10 years ago that would be a no brainer for an FOC or DTC AC drive today.

Having said that, if you are under the gun now may not be the time to jump into an AC system. It will be much easier to cross-reference into a servo system than it will be to do the same into an AC induction system. Just getting the right motor frame size can be a big deal. But to say that an AC system won't be capable of handling the job without having any application details is oversimplifying the selection process.

Keith
 
The application moves a tapered rod via a right angle gear box and a acme threaded rod and is coupled up with a lovejoy spider coupling. [font=&quot]Accuracy [/font]is not an issue. I just need to stop the rods in different locations depending on the product being produced. That is the reason they went with servo's back then to be able to set the stop and start positions. The rods stop in three positions unload, load and extend. I can't find much info on the servo except that it uses 240vac feed to the drive and is fused with 35 amp fuses on the load lines. The one thing I can read on the name plate is 145 Lb/in. and 6.0 lb/in/amp... Recalling that from memory my notes are at the office.

I know the AB flex drive can utilize an encoder feedback on the AC motor for position feed back. The machine is controled with a SLC 5/03 and a PV550 so interface to the flex drive should be easy I think...

Any more thoughts on this? I'm more of a PLC guy and the drives I don't get to mess with much so any help is welcome.
 
kamenges said:
It kind of depends on the age of the system. 15 years ago there were very few AC induction systems that could even come close to replicating the performance of a servo motor/drive combination. That is no longer true. In fact many manufacturers now make general purpose drives that operate both PM servos and AC induction motors. The widespread use of DSPs and advanced control algorithms have made the current generation of induction motor/drive combinations very capable. I know my previuos employer used servos in many application 10 years ago that would be a no brainer for an FOC or DTC AC drive today.

Having said that, if you are under the gun now may not be the time to jump into an AC system. It will be much easier to cross-reference into a servo system than it will be to do the same into an AC induction system. Just getting the right motor frame size can be a big deal. But to say that an AC system won't be capable of handling the job without having any application details is oversimplifying the selection process.

Keith

What is an FOC or DTC AC drive?

I am lucky and they have 4 of theses and can keep up with 3.

Do any AC motors have a NEMA style frame? This looks like a NEMA 42... Is there a chart that shows the NEMA frame dimensions? I could make an adapter for the AC motor if need be as it's just hooked up with a lovejoy spider coupling to a gear box. Not a high [font=&quot]precision [/font]servo type gear box.

John
 
The big thing is speed, not accuracy. If it is running at low RPM then the AC motor is not going to have the power. Also it will have trouble jogging into position

I still don't understand why you want to get rid of the servo. Even if it is obsolete why not just upgrade to a newer one. Price isn't that much different for a servo motor and drive compared to a AC motor and drive that can handle servo appications
 
I have a system that uses AC motors with drives and an encoder to position a lead screw. Accuracy requirement for me is low. I didn't design the system, I just maintain it once in a while.

It sounds like your application is similar and it can work, as long as you don't need high accuracy or performance.
 
Gerry M said:
I have a system that uses AC motors with drives and an encoder to position a lead screw. Accuracy requirement for me is low. I didn't design the system, I just maintain it once in a while.

It sounds like your application is similar and it can work, as long as you don't need high accuracy or performance.

Do you know what motor and controller you have on your system?

John
 
TWControls said:
The big thing is speed, not accuracy. If it is running at low RPM then the AC motor is not going to have the power. Also it will have trouble jogging into position

I still don't understand why you want to get rid of the servo. Even if it is obsolete why not just upgrade to a newer one. Price isn't that much different for a servo motor and drive compared to a AC motor and drive that can handle servo appications

The rpm is 1500 to 2200. The only reason is cost...

John
 
Originally posted by Big John T:

What is an FOC or DTC AC drive?

FOC - Field Oriented Control
This seems to be the favorite control algorithm these days for both servo motors and 'vector' controlled AC induction motors. The reason volts/hertz inverters don't perform very well is they don't specifically control the strenght or relative angle of the magnetic flux in the motor. 'Vector' control does this for you. FOC is just one of the 'vector' control methods. There is a ton of information on FOC on the Internet. Just search on 'Field Oriented Control'.

DTC - Direct Torque Control
This is another vector control methodology that is not used as much. The only company to use this that I am aware of ABB. There is a pretty decent body of information on the Internet on this also. The ABB-provided info is pretty good but be a little careful. It is in ABBs best interest to make DTC look good against the other control algorithms since ABB is one of the few if not the only manufacturer doing this. Having said that I have heard nothing but good things about the actual output performance of ABB drives using DTC.


Originally posted by Big John T:

Do any AC motors have a NEMA style frame? This looks like a NEMA 42... Is there a chart that shows the NEMA frame dimensions?

At that size I would stay servo. I know they are available but I have personally never seen an AC induction motor that small. Also, you won't be able to get a closed loop AC drive rated that low so you will be paying for capacity you can't use. I start to seriously look at AC induction versus PM servo right around 1HP.


Originally posted by TWControls:

The big thing is speed, not accuracy. If it is running at low RPM then the AC motor is not going to have the power. Also it will have trouble jogging into position

This isn't an issue if you properly select the drive and motor. The drive needs to use a control algorithm that can develop full torque at zero speed. The motor needs to be a TENV or ##BC (##-insert your favorite frame type) in order to stay cool down there.

Peak speed depends on the motor winding. While there are alot of servos out there wound to get to 5000 RPM, that's not an automatic. There are also servos wound to get to only 1500 RPM.

The big differences are torque to inertia ratio, peak torque and torque density. Servomotors have higher torque to inertia ratios. This is due mostly to the use of permanent magnets on the rotor. These produce higher magnetic field strengths than an induction motor rotor of comparable mass. Higher torque to inertia ratios mean higher acceleration rates.

Servomotors generally have a higher peak torque capacity relative to their continuous torque than do AC induction motors. Induction motor rotors go into magnetic saturation at about 250% of rated torque (I believe). After that pumping more current into the motor just results in more heat, not more torque. In contrast a PM servo will continue to produce increasing torque with increasing current (more or less) up until you de-magnetize the rotor. This is often 400% or more above continuous torque levels. Again, this means higher acceleration rates.

Torque density is torque per frame size. Again, because of the PM rotor, PM servos have higher torque density than a comparably cooled AC induction motor.


Originally posted by TWControls:

Price isn't that much different for a servo motor and drive compared to a AC motor and drive that can handle servo appications

I need to find out who you are buying from. In the sizes I typically work with (5 - 20HP) AC induction is much less expensive.

Keep in mind I'm just trying to be the regulating opinion here. We simply don't know enough about Big John T's application to know if it really requires a servo or not. However, I do know that 'because we have always done it that way' is not a valid design rationale. I've lost count of the number of hours I've spent getting out from under problems caused by that rationale. A servo may be the right answer. Just make sure you know WHY it is the right answer.

Keith
 
Keith, I was incorrect on the frame size it it a 143/145TC C face mount. I was just guessing on the Nema42... We measured it this am and found the specs on c faced motors. The shaft is not the same size but that will not matter it is a lovejoy coupling.

Thanks for the indepth information.

John
 
kamenges said:
Keep in mind I'm just trying to be the regulating opinion here. We simply don't know enough about Big John T's application to know if it really requires a servo or not. However, I do know that 'because we have always done it that way' is not a valid design rationale. I've lost count of the number of hours I've spent getting out from under problems caused by that rationale. A servo may be the right answer. Just make sure you know WHY it is the right answer.

Keith

I have worked with servo systems in the past and this application is slow and does not require the [font=&quot]accuracy [/font]of a servo. The reason the servo was used when the equipment was built was because they needed to be able to set different stop points. The slides are driven by a right angle gear box with a lead screw through the gear box. The reduction is such that at 2200 rpm the slides move about 18" in 5 seconds. I need to stop them +- 1/8" of the set point.

John
 
So it doesn't look like you have any real tough acceleration requirements. You probably have some size to give also.

The first thing to do is find the continuous stall torque of the servomotor you currently have and find an induction motor to match that. Assuming you use a 4-pole AC induction motor (1800 RPM sync speed) you will get roughly 3 lb-ft (36 lb-in) per HP. Then see if that size motor will physically fit in your machine.

If that checks out, what is the machine currently using to control position? Is there a free-standing controller or did the servodrive have an integrated controller (often called an indexer)? If you have a free-standing controller then you just need to make sure you can accept the interface between the new drive and the controller. If you curently use an idexer then something else needs to take on the position control function.

Keith
 

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