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Old April 3rd, 2003, 04:32 PM   #1
Tim
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Monitoring motor current

Motor control advice,
I've been programming a transfer system that transfers parts from one station to the next. The FWD and REV travel is controlled by a motor.
The problem I'm having is that sometimes a part will fall off the transfer and the transfer will smash or wedge the part against something as its transferring and causes damage. There are other situations that occur that also causes the transfer to smash a part into something. I know this is not a lot of information, but I have ruled out that I cannot do any more programming or sensor installations to keep this from happening.

This motor is controlled by a Mitsubishi E500 Frequency drive. There is a setting in the parameters that allows me to set up the output current monitor. This setting is for allowable output current from the VFD and how much time it can run above the set current before the VFD will shut down. I thought great, I can set this up so if the transfer runs into something the motor will draw more current, thus shutting down the VFD before any major damage. BTW, the transfer runs at a low speed. I thought I could set the allowable output current from the VFD just a little above what the motor was running at in normal operation. The problem is that when the transfer goes back to home position it hits a slow down limit switch, which slows down the transfer, so it doesn't overrun the home position limit switch. When it hits the slow down LS the VFD outputs a lower frequency. The motor at this point draws almost double the normal operating speed current. This is obviously a problem in the way I sat up the parameter. The VFD shuts down when it hits the slow down LS.

Is there something out there that will allow me to monitor the motor current at predetermined times of your choice? Basically in my situation, I don't really need to monitor the motor at the point where itís slowing down to reach home. This is a short distance and I don't think I need to worry about this point in the transfer process. What I do need to monitor is the danger areas of the transfer process. I'm looking for something that I can give an output from the PLC or through a relay to tell the monitoring device when I want it to monitor the motor current draw. Like I said this transfer moves slow, so I think I can stop it before something gets broke. This equipment is brand new and I already have had to replace six $700.00 SMC cylinders. Not to mention the countless hours of programming to help combat this problem.
I don't have the motor specs with me, but if this is something someone is going to need let me know.
Thanks in advance,
Tim
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Old April 3rd, 2003, 04:38 PM   #2
Tim
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Another idea that just dawned on me was that I might be able to put a clutch between the motor shaft and the worm screw thats driving the transfer.
Is this a better solution the the monitoring device?
Tim
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Old April 3rd, 2003, 04:52 PM   #3
controlsguy3
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If you use an overrunning clutch with a limit switch you can save your air cylinders and also let your machine know that there's a problem.
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Old April 3rd, 2003, 05:04 PM   #4
Tom Jenkins
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A couple of thoughts - FMC used to make a fluid fill clutch that you could use for torque limiting and soft sarts. Varying the fill volume varied the torque. Very accurate and neat system, probably more accurate than a pneumatic or mechanical clutch.

Second, most VFDs have an analog output that can be configured to provide a motor current or a kW signal. Some VFDs may require an option card to do this, or you can get a true RMS amp transmitter from NK Technologies or others. If you bring this signal into the PLC you can incorporate it as an alarm or control to a current limit, and interlock it with a time delay or various parts of the transfer cycle, and not have to limit motor torque for starting etc.

Last edited by Tom Jenkins; April 3rd, 2003 at 05:08 PM.
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Old April 3rd, 2003, 05:31 PM   #5
rsdoran
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I was wondering if you couldnt use a CT and connect it to an anolog input...or a CT monitoring relay that could provide a digital input and just use the input as a condition to stop the VFD/motor when it overcurrents in the forward motion? You said the motor was controlled by the VFD but does it get a signal now to state which direction to travel? or just an enable? or any at all?

I think after reading Tom's post he has stated the same thing.

I have used these units for something similar to what you are describing, they can be preset for a certain current and trigger the relay output to use as an input to the plc. Using this means just working in the cabinet not on the machine itself.
http://www.entrelecssac.com/cgi-bin/...ard/ff-sen.htm
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Old April 3rd, 2003, 06:38 PM   #6
Tim
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Rs,
It has a FWD and REV controlled by the plc. I'm just wondering if a mechanical clutch or something similar to what Tom and controlsguy3 mentioned would be better. Your thoughts on this would be helpfull. I hope if I choose to do it by "electronical" means it won't still cause some damage. IOW, if the plc can get the input on overcurrent and stop the motor in time before damaging things.

Tom,
The VFD has an analog input for frequency resolution, but no analog output. Wish it did, so I could use an analog module with the plc,..it's been a while!! I'm leaning towards a clutch with some sort of input when it trips to use with the plc. If someone can nudge me off this idea for any reason, please do. Right now its got some sort of aluminum coupler between the motor shaft and the worm shaft. Don't know the specific name for it, but it looks like one of them couplers you use on the end of an encoder, but a lot bigger. Going to try and find a company who might be able to help.
Thanks all,
Tim
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Old April 3rd, 2003, 06:54 PM   #7
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I am not familiar with Mitsubishi drives but a number of drives have a current or torque monitor function which can be programmed to a digital output. This output could be connected to a PLC input and the PLC could then decide if the drive should be stopped when the current is above the limit.
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Old April 3rd, 2003, 07:03 PM   #8
rsdoran
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What about this then, I always seem to have some kind of idea or maybe I just like ta ramble.

I dont see a problem with a clutch AND use something like that SSAC monitoring relay, then the plc would KNOW if there was an overcurrent while going forward, disengage the clutch and alarm.

The way the number crunchers are these days I thought you might want to try a low cost solution is why I mentioned using a CT relay. If you have an opening to using an analog input then maybe you might consider monitoring current for both directions with a CT to make sure that one in a million situation doesnt happen in REV.

Its a thought, I usually have to come up with inexpensive options because I work for a cheap company.
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Old April 3rd, 2003, 07:04 PM   #9
Tim
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Vic,
It does have a torque and current monitor setting. I can allocate on one of the terminals on the drive to close when this happens. I tried this approach taday, BUT the drive shuts down anytime this over current occurs. Thats everytime it gets near home position and slows down. I could not find anything to set, so it would just give me a signal and not shut down the drive.
Thanks
Tim
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Old April 3rd, 2003, 09:20 PM   #10
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Tim, I'm not personally familiar with all the details of the Mitsi drive you are using but, generally, you can set up one of the output relays for level detection. I would program it for torque, not current. Torque is much more accurate especially at low loads because the magnetizing amps dilute the torque amps severely at low loads. I would then place this relay in the drive's stop circuit or, if desired, in its external fault loop.

To avoid the false operation when engaging the slow speed limit switch, adjust your decel time so the current doesn't spike so high. If you can't do that, consider engaging a second decel ramp only when the slow speed is signaled and set that second decel out a little longer.

In my experience, if you are looking for a fast torque release or limit, electronic methods are generally preferred over mechanical.

One other thought, why not set the torque limit in the drive low enough so the conveyor just stalls when it jams. As long as the jam torque is higher than any running torque, this is probably the best method.
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Old April 3rd, 2003, 11:17 PM   #11
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DickDV,
Thanks, I will try tomorrow. I'll look at the manual again tonight on Mitsubishi's website for study. Obviously, if I can accomplish this task without having to buy anything would be great. Then again we've already spent over $4,000.00 in damage so money is not an issue. BTW, there are four of these transfers. I'm hoping I can follow your advice and get this thing so sensitive, I can keep the darn thing from moving by holding on to it with my hand. Under normal operation its only running around 3.5 amps, but when it hits the slow down, it spikes up to around 6.7 amps.

UPDATE>>>> I just went through the entire manual and there is no torque monitoring capability. I didn't even see the word torque but once in the entire manual. Another thing that I found is this drive has a RS485 port and I have a spare one on the HMI thats on the machine. I might be able to monitor the current value and write this into a register and send this to the plc. I would use the plc for connection with the VFD, but the HMI is taking up the port.
What do you think DICK? It says it will monitor current as low as 0.01 amps.

Thanks Tim
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Old April 3rd, 2003, 11:36 PM   #12
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Hello Tim,
sounds overvoltage problem with decelerating ? Have there braking resistors ?
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Old April 3rd, 2003, 11:45 PM   #13
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If you can't do it electricaly.You could do it on the motor mounting bracket. I have worked on equipment that have used somthing like that.It was a feed hopper to feed chickens.The motor was mounted kind of on a swing.When the motor overtorqed it moved and then it hit a limit switch witch shut the process down.This was a old system.
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Old April 4th, 2003, 12:45 AM   #14
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Yes there is a simple device. they are small donut shape coils mounted in panel. each leg of your power wires are routed thru the hole in each. a small DIN type module is set to signal overcurrent to plc. Just use that as a bit in the area of the program where you need to. On the Drawings they are identified as "CT" or something like that but I don't know what they are really Called. Should be able to find them in part catalog like Newark.
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Old April 4th, 2003, 02:00 AM   #15
Eric Nelson
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I assume you're referring to Parameter 156 as that "Current Monitor Setting"? According to the chart for this parameter, you have a lot of choices. I don't know this drive, but it seems that you have the choice to Continue or Not Continue operation when an overcurrent condition happens. Does it still fault if set to Continue? IOW, if set to Continue, will it only signal the condition via the output and NOT stop the drive?

You also seem to be able to select WHEN to looks for an overcurrent condition... During Acceleration, Deceleration, or at a Constant Speed, or any combination of the 3. You might try setting it to check at Constant Speed only to prevent tripping during decel.

Here's the chart I'm referring to:
[attachment]

Just a few ideas... I seem to suffer from that same "rambling syndrome" Ron has...

beerchug

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