Inverter/motor physics, Flying sheer

[redeemer]

Hi, Can anyone direct me to some literature about the physics concerning the relation between an inverter and a motor. I'm trying to determine if a certain setup (inverter+motor) would be sufficient for a flying sheer application. The main question is: will the motor be able to drive the system within the given time limits or not. Thanks.

[panic mode]

Literature? Don't know of anything worth recommending but someone else could answer that. Typically, you choose motor to match load and then you choose inverter to match motor etc. One thing to keep in mind is motor cooling because specially at lower speed, motors don't get enough cooling. If you need to match the speed or torque, you can use gearbox etc. They are often integrated with the motor to make compact assembly. To do any sizing you have to come up with mechanical concept and some numbers...

[JimRowell]

You havn't really said much about how difficult you expect it to be to meet your needs but if it will be close to the edge then you may need a tested and matched package consisting of motor, gearbox, inverter where the details of how they work together are all known. Both electrical and mechanical. I find the best way is to simply give your requirements in full to your local rep and work with him to develop a combination. A good rep knows the limits of his products and can recomend something that will not disappoint you or will admit that they can't help you. You can meet with a couple and see which one that best meets your needs.

[redeemer]

Someone has calculated that a torque of 223 Nm would be required to achieve a required acceleration of the system at a certain stage. A 30kW motor will be used with the apropriate inverter. The thing that I'm not sure of is: will the inverter be able to supply the required current during startup, (the inverter maximum current is 57A). The problem is that if the inverter limited the current, the startup time will be longer that the available time. Did that make much sense?

[JimRowell]

Is 57A the full-load-amp rating of the invertor or a maximum limit? If it's a maximum limit then it sounds too low for a 30KW motor. If it's a nominal FLA rating then it's nicely oversized. Whether you are buying the invertor new or used or have it already, why not just consult with the invertor manufacturer? By phone or by email, I've always had a good experience talking with invertor manufacturers. They're your best source in specific situations. Also, they can talk from the standpoint of their particular product with possible strengths/weaknesses rather than general theory. You are missing out on your most valuable resource if you ignore them. If you phone them, ask to speak with an application engineer. Jim Rowell

[redeemer]

57A is the maximum rated operational current. Fadi Mansour

[JimRowell]

Okay, sounds good. How many seconds do you have to complete the accel?

[redeemer]

The system should be able to accelerate in 0.5s. Fadi Mansour

[JimRowell]

Without calculating anything, I'd say piece of cake but then again, I never really do any motor sizing myself. Not my area at all. Like I said, I rely on suppliers and manufacturers for this (and then I get to hold then to it ). If you are bound and determined to plough through the calculations then here's a good link: http://www.orientalmotor.com/support/mtr_sizing.htm Follow through all of the links on the page esp the example pdf. It's pretty good. You'll still need to look at the particular drives response times. For faster accel under load there may be an adjustment for "accel boost" which will increase the voltage more than normal during accel. For tighter speed regulation under varying loads, there may be an adjustment for "slip compensation" which will increase the frequency more than normal in response to current increases. Jim Rowell Edited March 7, 2004 by JimRowell

[redeemer]

Thank you very much, I really appreciate your help. Fadi Mansour

[JimRowell]

I forgot to ask you what voltage the motor is. I was assuming 600 volt which is what I usually work with. I suppose it's actually 400 or 480 right? That would make the invertor not so big.

[redeemer]

Actually it's a 380V motor.

[JimRowell]

Well, 30KW is 40HP which would be around 60 to 65 amps on 380 volt, right? So you'll be somewhere in the area of 5 to 15% over the rating of the invertor. What's the actual nameplate current rating on the motor? You're gonna want to make sure you have lots of cooling for the invertor. Why are you not going with a larger one?

[redeemer]

The nameplate says 58A. Anyway we were considering to use a 37kW inverter, but I was trying to determine if using an inverter of the same rating will be sufficient.

[JimRowell]

I'd go with the 37kw if I were you. With the 30 you'd have to worry more about ambient temperature, cooling around the inverter, duty cycle, heavy demand, etc. Even sudden failure of the motor is more likely to damage the invertor if it's closely matched rather oversized a bit. What if down the road, someone changes the motor and the new one is slightly less efficient. I'm still a little fuzzy on what the actual nominal Euro voltage is (when measured with a real-life meter) since they standardized on 400 volt. Is the supply now 400? or is it still 380? or what? Why are 380 volt motors still being sold? Anyway, if the nominal is really 400 and you are using a 380 rated motor then your real current will be higher than the nameplate. If this is true then it's one more reason not to match the invertor too closely. Jim Rowell

[redeemer]

I don't really know what they've got in Europe, because I'm not there, I'm actually in Damascus, Syria. The motor that we are planning to use is of Italian make, and the inverter is German (moeller). One of the things that I'm still not sure about in the application is what heppen on startup. From what I gathered, the inverter can provide 100% torque probably starting from less than 6Hz (I'm not sure of the actual number). So what happens when I give it a command to start with a brief startup time (0.5s)? Will the motor stay intert till the minimum torque required is achieved? which should happen near this "6Hz" limit? And what will the current be then? Can I determine it by calculating the voltage that will be applied at that frequency (the voltage will be less than the nominal, following a linear profile), and from this I can calculate the current? Does this sound Ok? I'm not so sure. And thank you again.

[Gerry]

Pardon me for butting in, but I think you're headed for trouble. My understanding of a flying sheer is a carriage that will move along a conveyor, match the conveyor speed, and pass a blade through the material on the conveyor without disturbing the material flow. The carriage needs to start from standstill, match speed, make the cut, and stop within the limit of its travel, then reverse to its starting point and stop in time to do it all again and make the next cut. Worst case conditions are determined by the combination of cut length and conveyor speed. This is a servo application and there are several considerations for the sizing of your drive and motor. You need to plot the complete motion cycle for the worst case - accel, constant speed, decel, accel (reverse), decel (reverse), idle. When you apply this to the mechanical data - friction, carriage mass, drive train efficiency, drive train ratio - you will get figures for peak motor rpm, peak motor torque, and RMS torque & rpm. The RMS values must be within the continuous ratings of the motor and drive. AND the motor and drive must be capable of the peaks. Finally, you need to consider the ratio of the reflected load inertia to the motor's rotor inertia. If this ratio is too high, you will have difficulty tuning the system. A ratio of 1:1 is ideal, less than 3:1 is very good, less than 6:1 is good, greater than 10:1 is bad. Inertia 'seen' by the motor is load inertia divided by the square of the drive train ratio plus the inertia of the drive train (e.g. gearbox). 'Someone' calculated a requirement for 223 Nm for accel. You'll need a similar torque for decel (less friction contribution) - definitely need brake resistors or a regen drive. Is that torque required at the load or at the motor? And, of course, you will need a motion controller - what will you be using?

[JimRowell]

Paul makes good points and that's why I originally mentioned about talking with your reps. It depends on so many things. 1/2 second is not much time if you are having to reach a high speed. I didn't realize you wanted to go from dead stop to fast in that 1/2 second figure. Your figure of 223 doesn't seem too high but how much can you expect from a regular motor and drive in that short a period of time?. I don't know much about flying shears but I would imagine that in at least some cases (maybe?), you can perform the cut while moving past the target without actually matching speed exactly. If the cut is short and the material is thin and pushable and doing that doesn't disturb the material feed. I was envisioning an oscillating punch press type of affair (the only style of flying shear I've seen). In this setup, you don't have to start from zero nor do you ever accelerate very far nor do you ever reverse direction. You merely have to correct for changes, delays, etc in the system. I did a quick google on flying shears just now and the back-and-forth style ones seem to be the rule. They're using either linear motors or servos combined with dedicated controllers. They seem to be quite concerned with the overall inertia of the system since it's a very high demand type of system. Has to be very accurate and has very high accel rates. I'd be a little too concerned to just fly (shear or otherwise) by the seat of my pants if I were you. Jim Rowell

[Gerry]

I wonder if this ended up in the 'too hard' basket? Who's Paul?

[redeemer]

So, is it time to panic! Now seriously, the material that is being cut is a steel profile, so it's absolutly NOT a punch press. Cutting should take about one second. Of course the requirements can be met given sufficient power, but what I couldn't do is get a reasonable figure for the power needed, and answer the question: Will the system reach the required speed in time. And this is of course what made me ask for the "physics" of the system. Thank you all for helping out. Maybe I should take a closer look at flying sheer controllers.

[JimRowell]

I have no idea who Paul is. That's not a good thing considering that I wrote it. I meant to say Gerry. Gerry, Paul... they do sound alike, no? Or how about, in my family, we call everyone Paul just to avoid confusion. Or Paul is short for Gerry? Okay, maybe I've been working a little too hard. (I can't wait to put my current project to bed).

[Guest Guest_Peter Nachtwey]

First, do the engineering. Many motion controllers can do flying shears. I have never heard of a motor flying shear in a steel mill application, they have been hydraulic due to the size and mass considerations. http://www.precision-strip.com/ These guys can shear steel quickly and accurately.

[Guest Guest_LIP]

You really cannot take an application like a flying saw(shear) the way you are doing now. You will be heading for serious trouble. As someone has mentioned before, you need to contact a drive vendor for this application. I would recommend SEW Eurodrive as you could utilize their MOVIDRIVE VFD to do all the work. They actually have a module in the drive for these applications and you will only need to enter the mechanical data and speeds required in a windows application. The benefit of SEW is also that they will size the drive for your completely. If the material is coming out continuously and you have to cut it with some sort of blade, you will need to have a starting point to synchronize angularly to the linear position of the material, this is the easy part if you have a VFD like the Movidrive with an angular sync option. Otherwise, you need a motion controller of some sort. Once synchronized with the speed of the material, the bridge can then tell the cutter to start cutting. Once finished, the cutter must tell the bridge to move back to "home" and the bridge must wait for the correct amount of material to pass through to make the next cut (unless there are cut mark sensors or something similar). What you must realize is that when you are finished cutting, you now have to accelerate the load (bridge with saw on it) back to the "home" position extremely fast as the material is still going in the OPPOSITE direction to your travel back home. All this must be calculated (dynamic torque, static torque) before you go any further. Good luck, LIP

[Guest Guest_LIP]

Forgot to give you a link: http://www.sew-eurodrive.com/03_produkte/i...ex_produkte.htm Go to the bottom selection on that page "drive applications" and then from the pull down select flying saw. Cheers, LIP

[redeemer]

Hi, I was just browsing my old topics and came across this. It seems that I have stopped the conversation, I must have been pretty busy at that time. What happened is that we delivered the application. After contacting a local supplier we followed his advice in using a servo motor (From SEM, UK) with a position control drive (from QUIN, UK). Quin helped us with an skeleton for a flying shear application using (in their lingo) the soft-cam functionality of their drive. The application was successful, it has been running for about three years now.