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VFD,air clutch and enertia?

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  • VFD,air clutch and enertia?

    I am building a machine to pull rubberized fabric from the out feed side of a commercial sewing machine.The drive for this machine is a 1/2hp 1750 rpm 3~ motor and a 60:1 right angle gear reducer seperated by an air clutch.I'm using a VFD to vary the speed of the motor and the clutch allows for the feed to be dropped in and out without the VFD being shutdown and restarted each time.

    The VFD shows 1.1amps with the clutch out and 1.4amps with the clutch engaged.Even at 2x's full load on the puller the amp reading never climbs above 1.7.Motor and drive are rated at [email protected]

    Here is the problem,above 45hz when the clutch drops in the drive ,looses rpm and lags,at 60hz every second or third time it trips the over current limit.Setting the display to readout current draw when this event happens the current spikes at 4.3amps @ the 60hz setting.

    Some of the DC drives I have seen used a flywheel on the motor shaft for just that reason to compensate for the effect.

    So what I am thinking is I have three options-
    1 Experiment with a flywheel
    2 Get a larger drive/motor
    3 Change out the wormgear reducer for a planetary reducer and eliminate friction.

    I have tried resetting some of the drives parameters with no real improvement.I was also thinking of putting an orifice in the air line to the clutch to slow the engagement,but I am thinking that might make things worse.

    Any clues?
    I just need one more tool,just one!

  • #2
    Big Capacitor?

    Opps, AC motor. Never mind!

    Comment


    • #3
      Use a bigger reduction. A dinky 3ph motor like that can probably run over 100 hz no problem.
      Free software for calculating bolt circles and similar: Click Here

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      • #4
        Weird:
        Since the motor is well below full load steady state it sounds like the HP is adequate. Since the current goes well above FL when the clutch engages this implies that the problem is in accelerating a large inertia so the immediate question is: "Do you really need that fast an acceleration?" If not then perhaps your idea of an orifice in the air line might work although a lot of the slowdown would probably come before the clutch engagement so more time would be wasted. Another approach in this line would be to try reducing the air pressure feed to the clutch. This way the clutch would slip more after engaging while it accelerates the inertia at a lower acceleration rate and this would present a lower torque to the motor thus decreasing the current required.
        If you actually need the fast acceleration then a flywheel would probably be useful. If the flywheel inertia is large compared to the motor inertia you might have to lengthen the speed ramp in the motor drive. On any I've set up this is simply a software setting.
        Another approach would be to look through the software settings and see if you can add a speed droop function. This allows the speed to fall off with current so the acceleration would be lower. Somewhat in line with this is to look for a current limit setting and if possible set it to down around FL current or less for the motor. This would prevent tripping and would allow the speed to sag for a bit after clutch engagement.

        Evan:
        I don't see that setting the hertz to 100 would be much help. Doing this simply runs the speed up above base speed and the rules here are pretty much the same as for a dc drive in the field weakened range-- the hp cannot go above the rated value so the allowable torque must be decreased. Normally this is factored into the software and there's not much you can do about it.

        My experience was mostly on larger drives so this little one may be less flexible in settings

        Ken

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        • #5
          By using a higher RPM you can use greater torque multiplication even if the horsepower doesn't increase. Since it isn't near using full hp the increased torque should help.
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          • #6
            Evan:
            As is: motor speed=1750 rpm hp=.5
            speed after 60:1 gear reducer = 1750/60=29.167 rpm

            Tavailable=(5250*.5)/1750=1.5 lb ft
            after 60:1 gear reducer T=90 lbft

            Now run the motor at 100hz giving n=(100/60)*1750=2916.67 rpm

            Tavailable=(5250*.5)/2916.67=0.9 lbft
            Now put in a new gear reducer to give 29.167 rpm from 2916.67 motor speed
            GR=2916.167/29.167 = 100:1
            Now the torque available at the gear output = .9*100=90lbft

            so we are back where we started.

            Are you depending on some transient effect which happens when the clutch closes? If so I can't see it at the moment.

            Ken

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            • #7
              Off the top of my head if you double the rpm of the motor there should be about 4 times the stored energy in the rotor. The rotor is a pretty significant flywheel.
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              • #8
                I like your bigger flywheel idea wierd, very dependable, and minimum weight if you have the room for it, just a light flywheel that is much larger than the rotor, with all the weight on the outer parimeter of the flywheel.

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                • #9
                  Well I played with it some more today.I took the fan and shroud off the motor and popped on a heavy cast iron pulley I had on hand to act as a flywheel.It was roughly I think the same mass as the rotor and as large a diameter as the motor case.It did nothing,the drive fell flat on it's face just like before with no improvement.

                  The next experiment was redcuing the air flow to the clutch,this worked to a degree,but still no dice.I was able to pull off a start at 50hz with no problem,but even at 51hz it reverted to it's bad habits.Now,I also tried engaging the clutch at low pressure,this worked flawlessly,but the delay between stop and engage was a bit long.

                  The next test was to remove the drive from the system and cycle the clutch with the motor running full speed directly off shop power with an amp meter in place.I got the same readings as with the drive in circuit,less the spike.The amp spike was no more than 2.0amps during engagement.This tells me the drive is stumbling and can't compensate fast enough to the clutch being engaged.

                  I tried going over the parameters once again.It can be set either for constant speed or constant torque,niether made much difference.I reset the accel. limits lower from the default 30seconds,down to 5 seconds trying different settings in 5 second intervals.Performance did improve,but cycling the clutch produced the same result of an over current.

                  I am begining to suspect Marathon motors.The motor they sold me is an all-in-one mount 56C face motor that is supposed to be inverter rated.I wonder if the motor is actually designed for inverter service,or if the insulation is just rated for inverter use.

                  At any rate I have a meeting with the customer tommorow and I'll see what he thinks.Even if I can get this setup to work,I don't want to deliver something that is marginal at best.He may also need to up the feedrate from it's current max in the future,which I don't think this motor/drive combination will be capable of.I may be looking for a larger drive and motor.It will be over kill,but it will work.
                  I just need one more tool,just one!

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                  • #10
                    Weird:
                    It seems to me your experiments have pointed the finger straight at the drive so I would look at this or even consider changing to a better drive before blaming the motor. You've gone through the parameter settings and I've seen no mention of either current limit settings nor of any sort of "response" setting. On a well designed drive these should be available to set during setup and if you replace the drive make sure they are. The current limit setting sets the maximum current allowed to the motor and if it's working properly you should not get overcurrents. The "response" concerns regulator gain and lead or, in another language, proportional and integral gains. If these are not set right you will get current overshoot on a step change in load which is precisely what's happening--and they can be very difficult to get right. You really need the help of an application engineer from the drive supplier for this. You mentioned adjusting the accel and decel rates in the 5 to 30 second range but these affect only the rate of change of speed when going from one speed setpoint to another so I wouldn't expect much effect on this problem.
                    I've designed and started stop and go drives like this (usually dc)in larger sizes and usually there is no clutch; you simply accelerate and decelerate the drive. A good drive might allow you to do this.

                    Ken

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                    • #11
                      I too would tend to think that there may be a firmware setting tweak that may give you enough fudge factor to get by.

                      However, along the line of your orifice idea to limit clutch engagement, I would think that a small reservoir would have the desired effect. The reservoir (empty prior to the air being turned on) would take just a bit to fill and the pressure at the clutch would therefore rise a bit more slowly. I don't know if the clutch would then actuate smoothly and more slowly or not, but it might be worth a try. All it would take is a piece of iron pipe for the reservoir and a few fittings. You could vary the size of the iron pipe (either diameter or length) to tune it.

                      Paul
                      Paul Carpenter
                      Mapleton, IL

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                      • #12
                        I'm tending to think that slowing the engagement of the clutch would make things better. Obviously the clutch will be slipping more over its lifetime, but maybe that won't be an issue. A bigger issue will be the engagement characteristics of the clutch. It might be smooth, or maybe grabby. A test would be in order. I go with the orifice idea, maybe something with a needle valve adjustment.

                        I like Evan's idea of a higher speed motor with a larger gear reduction for the reasons he mentioned. I think there would be less variation reflected back to the VFD as the clutch is engaged.

                        Something else I wonder about is where the clutch is placed- between motor and gearbox, or after the gearbox-
                        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                        • #13
                          I've been into the over-current settings and the recovery settings.Over current allows for 5.5 amps max and recovery seems to be a repeat of accel/decel,niether helped.I did talk to the mfg today and the "plugging" load current limit is 3.0amps max,it is not recomended or warrantied beyond that.

                          I'm going to keep the drive and motor for a normal constant speed/load application.The other requirement here is the input voltage must be 115vac,that eliminates a VFD,since 1/2hp is the max I can find in the 1~115vac in 3~220vac out.

                          Talked to the customer today and he wants to checkout the puller to see where the speed band falls and see if we need to change the final reduction ratio before setting things in stone.

                          Darryl,the clutch is between the motor and gear reduction.It's an 1800 rpm nominal motor and a 60:1 box yielding an output of 30rpm more or less.From there it goes into a 5:1 split chain ratio.

                          I would have prefered to use a clutch directly on the traction roller shaft for obvious reasons,but at the torque requirement I needed there the clutch would have cost $850-1,000,bit steep.

                          Given the voltage reqirement a DC drive is the obviuos choice at this point.A 1hp drive can be had for $150,the motor though will probibly run $200 or better.I know for a fact the 1hp DC controller and motor will do the trick.

                          Plus I have the option of mounting the speed pot on the sewing machines clutch arm,so if the speed of the walking foot increases the traction roll follows suit.
                          I just need one more tool,just one!

                          Comment


                          • #14
                            It surely seems as if the flywheel should have done SOMETHING..... might not have been good...

                            The problem with the flywheel is that it makes speed changes pull more current, since a fast speed change requires more energy input.... and you HAVE speed changes.

                            When the motor is slightly slowed by engaging the drive, it has to speed back up. The flywheel makes that harder, so more current is pulled from the drive. if that goes over a current limit, you have a stumbling problem.

                            At higher speeds, you have more energy, but you also have to return it to the flywheel faster. With a faster rate of energy return, it is easier to over-current the drive.

                            I don't think this is really related to a "plugging" current, you are not throwing it directly across the line for reverse, nor apparently are you reversing it at all. Seems like the actual max current limit is your real issue. The only way that the plugging current limit would affect it is if your start-stop duty cycle ends up with as much heating as plugging the motor would... I don't see that, since plug -reversing is a much longer process than a speed recovery.


                            Most drives like that try to MINIMIZE the mass of any rotating parts that are started and stopped, etc.

                            Do you have any idea of the mass involved in the input of the reducer? That would have to include the "reflected" mass looking into the drive. All the gears, rollers etc reflect back according to the ratio. The higher the ratio the less "effective mass" is reflected (not rigorous engineer-speak, but you see the point).

                            How come this has to be slammed into gear so fast that a slower clutch engagement is not good enough?



                            Clearly the VFD is your limiting factor, based on what you seem to have found. So the DC drive may be best for now.

                            It doesn't seem reasonable that the AC drive would be that bad.... except that you have a limit since the largest size VFD with 115V input that you can get evidently does not have sufficient peak current.
                            1601

                            Keep eye on ball.
                            Hashim Khan

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                            • #15
                              Since this is a take up roll for fabric what's wrong with using a visco-elastic coupling along with the clutch? So what if it slips? That should reduce the momentary torque load.

                              [edit]

                              From what I understand here you don't need a set rpm, you need a set torque sufficient to reel in the fabric. A slip mechanism that will transmit enough torque while permitting the motor to overrun should work. It can be something like the coupling that I mentioned that works like a auto xmission torque converter or it could be a magnetic eddy current coupling.

                              See here:

                              http://www.motionsystemdesign.com/Zo...couplings.aspx
                              Last edited by Evan; 09-21-2006, 01:55 PM.
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