Can an asynchronous induction motor be considered as a precise servo axis for cutting feed and angle setting in a miter band saw machine?

Yes! I just completed a cut-to-length project that used a 75HP asynchronous motor for the positioning. This was for a steel line. Positioned to less than 1/32" over 144" max cut length and peak feed speed of 300FPM. I used an ABB ACSM1. There was a load mounted encoder for position feedback, and a motor encoder for velocity feedback and jogging.

Thank you Brian!

What kind of band saw provides a cutting length of 144"? I would like to know more about this project, please.

Anyway, as far as it concerns positioning, I agree with you but with restrictions, although you will need an additional brake to stop the part in place since asyncronous induction motors don't provide holding torque.
But what about the down feed of the saw frame utilizing an induction motor? Would this be steady enough and would it be possible to auto-regulate the feed speed depending on the load while cutting? What about termal issues when running the motor on low frequency? For cutting feed, a very low speed is needed.
Would it not be better to use a "real" servo axis?

Very best regards from Thailand

Axel Girt

Hi Axel-

It wasn't a band saw, but that is not important. The point is that positioning applications can be done using an induction motor. The one I did used a roll feed, but it could have been a ballscrew, rack and pinion, etc. You do not need a brake. The drive will control the motor very well, including holding position at zero speed.

The frame feed can use an induction motor also. You will want to chose your gearing wisely whether you use synchronous or asynchronous motor. The motor may or may not need to be blower cooled. That depends on the motor design.

A traditional servo will almost always provide better performance. They become very expensive in the large kW or HP applications, though. That is when you should look at an induction motor instead. The ABB ACSM1 isn't the only drive that can do this, but it has been doing this for quite a while. Same drive is used for synchronous or asynchronous motors. You just change the parameter settings based on what motor you are using.

Regards,

Brian

I have done several positioning applications with VFD's in the 1/32" range. I used Hitachi but ABB is a great brand also. Many of the high end VFDs have some way to use encoder feedback and will hold position. Hoists, elevators and lifts are common VFD applications.

The 1/32" range was what the customer was looking for. It could have been tighter if we closed up the in-position window.

Dear Barry and Brian,
thank you for your inputs.
However, I am particulary talking about the cutting feed of a metal band saw machine wit a 4 kW motor.
Would you ever consider a VFD drive for a CNC machine tool like lathes or mills?
Or would you consider a VFD drive if two axes need to be precisely interlinked?

Off hand, I wouldn't use an induction motor on what could be a highly dynamic CNC application. I think it would work fine on your bandsaw application, but 4kW size might still be in the servo range. You could cost it both ways. Interlinking axes is more a function of the controller. Remember that an induction motor is probably going to have a much larger rotor compared to a servo motor, so it will have higher inertia. That may help or hurt you depending on the application. Also, encoder options are limited, and tend to be expensive, for induction motors. That is why I say servo is probably better for 4kW size.

I doubt that(band saw) is an induction vs servo issue. It is probably an implementation issue. Either sizing, tuning, or both. Keep in mind that if you ask for an induction motor with an encoder that you will probably get a 1024PPR encoder. That is on the very low side for servo applications. Yaskawa servos come standard with over 1 million cts/rev. The winder sounds like it might need a servo.

Thank's Brian, I really appreciate discussing with you.
One more objection regarding my band saw. VFD for the saw blade speed is fine, no
doubt. But I am still not convinced regarding the cutting feed.
Imagine you are cutting a solid round piece of carbon steel with a diameter of 500 mm / approx. 20".
The saw band speed in m/min remains the same, but the length of the cut, the contact-length
of the saw blade in the cut, becomes longer as you get deeper into the material hence the cutting feed should be reduced, depending on the length of the cut.
Can a VFD really handle this precisely since the rotor speed is asynchronous to the field speed? How?

Yes, a VFD can handle precise speed and position control. I am not saying all of them can, but the better ones can. Just put an encoder on it and tune it properly. The drive will do whatever it needs to to control rotor speed. It is a closed loop system at that point. Did your band saw originally have encoders on it with the VFD?