Sizing of vector duty motor for constant power load

[imcjoek]

Hello,

I have a problem which keeps coming back to me in my line of work involving how to size an AC vector motor.

We build many winding machines. Usually these are "center wind" in that they drive the shaft and the material wraps around the spindle.

We try to hold a constant tension on the material, and also a constant linear speed of the material as it winds. This leads to the following problem:
As the size of the winding roll gets bigger, the torque the motor must overcome is greater. (Constant tension, with a bigger moment arm).
As the size of the winding roll gets bigger, the speed of the roll also must decrease. (To hold constant linear material speed).
This results in the drive needing to supply a lot of torque at low speeds, however the power required is very low. The speed must also be achieved when the roll is small (and so this limits my pulley reduction based on motor max speed).
I believe this is referred to as a constant-power load?

My question then,
Let us say that I have a job which requires 40ftlb torque at 180RPM and 3ftlb at 1750RPM. These are both approximately 1HP.
Even a 2.0HP motor however will not even stand a chance, since it cannot make 40FT-LB at 180RPM. So clearly I cannot size on power required alone.

If I go by the motor figures I can use a 5HP since its lock rotor is about 50ft-lb. So it can physically "do it".
However I will be greatly exceeding its "full load" torque (15ft-lb) for a long duration of time. Will this be an issue considering a 5HP motor is only being asked to output about 1HP? I have seen some websites that suggest sizing base on highestTorque x highestRPM. This seems ridiculous as I end up needing a 15HP motor! However that DOES put the torque below its "full load".

I realize "full load torque" is at rated speed, but what I dont understand is how you size a motor that does not operate at that rated speed 90% of the time.

The motors we typically use are Marathon "Black Max" vector duty AC, run with Parker/SSD drives.

I would like suggestions. Thank you.

 

[Compositepro]

If you use the the design you describe then you probably do need the 15 hp motor. In addition it should have an separate cooling blower so it has adequate cooling at low rpm and high torque.

However, very few center-wind operations should be run at constant tension. Torque must transfer through the roll of material from the core to the outer wrap. If the roll increases in diameter by a factor of 10 then the torque must increase by a factor of 10 also to maintain constant tension. This, in effect, increases the tension on the inner wraps of the roll by a factor of 10 also, which often results in the roll tightening on itself, telescoping, and damage to the inner wraps. Theoretically, constant torque winding should be used, but that often doesn't work well either because reducing the winding tension by a factor of 10 causes other problems. So a compromise is taper tension where the winding tension is reduced as the roll increases in size, but usually only by a factor of two. This will also cut the calculated size of your motor by a factor of two.

When winding very large diameter rolls pure center winding is usually not the best way to go. Constant tension winding is usually required because because the process upstream of the winder is affected by this tension. For this reason the winding tension should be isolated from process tension by pull rollers and nips.

 

[DickDV]

You do need the 15hp motor for the exact reasons you mention. And, you also need the drive at this size, even tho in both cases you never need all of the kw or hp.

But, since you need both the speed and the torque, you are stuck.

One suggestion tho. The Marathon Black Max motor is certified to be constant hp up to 120hz. You will optimize your system and reduce the component size and cost by choosing your power train so max speed on the winder is 120hz on the motor, not 60hz. That way, your minimum speed is also doubled and it cuts the motor and drive size down considerably.

 

[imcjoek]

We do use a taper tension and surface driven rolls when the customer specification allows it. It does not always do so.

I have raised the suggestion of upping the motor speed to our electrical guy a few weeks ago and he agreed that it may be a good solution.

I just wanted confirmation that what seems like a tremendously over-sized motor is actually required.

Thank you for the replies.

 

[PaulBr]

You should also analyze how fast you are accelerating and how fast your diameter builds up. You max speed will be when winder shaft is empty and no material on it. But you are not reaching max speed instantaneously. Most likely you are ramping up gradually, and by the time you have reached max line speed you already built up some diameter and your shaft speed is slower than empty shaft.

PaulBr.