# VFD "surging"



## RICHGONZO1 (Mar 5, 2012)

ibew415 said:


> I was called out to a quarry to replace a Benshaw VFD on a 5hp motor.
> 
> plant manager told me the drive was bad....so I pretty much said, "whatever you say sir!"
> 
> ...


I'm not sure about the Schneider drives, but what is your min max freq set at? 

Also can you look in the Plc program? I had a similar issue with an apron feeder that would do the exact same thing out of the blue because on the belt and scale had logic to keep the belt from burying itself. Ours was ultra conservative and maybe they were mining a "lighter" section of rock which threw the tph off. 

It could be the program and the process variables or the min max freq is off. Hard to troubleshoot from here. I'm leaning towards freq setting because you are correct, 12ma should produce 30hz, I'd start there.


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## RICHGONZO1 (Mar 5, 2012)

Also make sure the analog in is set to 4-20 not 0-20


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## Cow (Jan 16, 2008)

Like Rich said, check your min/max and analog input range. 

Just yesterday I noticed a customers PLC controlled drive was running 66 hz or so and current limiting out. They hadn't even noticed. I told him that can't be right. Turns out the max freq was set to 120hz....whoops.


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## ibew415 (Mar 23, 2014)

RICHGONZO1 said:


> I'm not sure about the Schneider drives, but what is your min max freq set at?
> 
> Also can you look in the Plc program? I had a similar issue with an apron feeder that would do the exact same thing out of the blue because on the belt and scale had logic to keep the belt from burying itself. Ours was ultra conservative and maybe they were mining a "lighter" section of rock which threw the tph off.
> 
> It could be the program and the process variables or the min max freq is off. Hard to troubleshoot from here. I'm leaning towards freq setting because you are correct, 12ma should produce 30hz, I'd start there.


I can't look into the PLC, I asked the programmer...and he grumbled and asked me to check speed reference....i'll bring up the logic to him again..just rephrase my question, ha

I noticed the min freq is 0 and max is 72hz as i was poking around the parameters today. 

I guess i don't understand why this would cause a surge out of the blue...i'll adjust the Max to 60Hz and see is that fixes things


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## ibew415 (Mar 23, 2014)

RICHGONZO1 said:


> Also make sure the analog in is set to 4-20 not 0-20


I'm pretty positive its 4 - 20, i was thinking the same thing...then i had the operator set the speed ref. at 50% Then I monitored the analog input which came out to be 11.69mA


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## micromind (Aug 11, 2007)

I'd look at mechanical issues as well. Belts slip, maybe the headpulley is worn to the point it doesn't always run the belt at the proper speed. Also, the v-belts from the motor to the gearbox could be loose or glazed. 

Is the VFD running at or close to current limit? If so, the belt will run at whatever speed the rock will allow. 

If possible, look at the algorithm of the scale output vs. the VFD input. If the Integral part of the PID is too fast or too slow, it'll make the VFD overshoot. My guess is it's too slow. 

In any case, look at the VFD settings, it sounds like the min. speed is not set properly and the scaling might be off a bit too. 

Also, look at the ramp rates in the VFD, they might need to be set a bit lower so it responds quicker.

trying to make a belt follow a scale is not all that easy, considerable tinkering may be needed.


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## RICHGONZO1 (Mar 5, 2012)

ibew415 said:


> I can't look into the PLC, I asked the programmer...and he grumbled and asked me to check speed reference....i'll bring up the logic to him again..just rephrase my question, ha
> 
> I noticed the min freq is 0 and max is 72hz as i was poking around the parameters today.
> 
> I guess i don't understand why this would cause a surge out of the blue...i'll adjust the Max to 60Hz and see is that fixes things


0-72 hz could be causing that issue of surging, but lighter tph is the variable that is sticking out to me. I dont like to make a bunch of changes and wonder what solved it. Hopefully that will do the trick, I hate having "black boxes" i cant look at in my way, so frustrating. 

You never know, the programmer might see something and say oops, but never tell a soul, "it's in the field!" 90% of the time it is in the field and programmers know that. It's there go to explanation.


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## ibew415 (Mar 23, 2014)

micromind said:


> I'd look at mechanical issues as well. Belts slip, maybe the headpulley is worn to the point it doesn't always run the belt at the proper speed. Also, the v-belts from the motor to the gearbox could be loose or glazed.
> 
> Is the VFD running at or close to current limit? If so, the belt will run at whatever speed the rock will allow.
> 
> ...


All the mechanical things you mentioned is EXACTLY what the plant manager told me. Maintenance is going to take apart the feeder tonight/tomorrow. There was glazing, but "no smell" or "chunks of rubber lying around"

the ramp rate is at 3 seconds...I'll dial that down monday morning. 

As far as Min speed, and Scaling...matching the output Hz to my analog input should do, eh?

50% = 30hz

75% = 45hz

100% = 60hz?


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## RICHGONZO1 (Mar 5, 2012)

ibew415 said:


> All the mechanical things you mentioned is EXACTLY what the plant manager told me. Maintenance is going to take apart the feeder tonight/tomorrow. There was glazing, but "no smell" or "chunks of rubber lying around"
> 
> the ramp rate is at 3 seconds...I'll dial that down monday morning.
> 
> ...


Yes, the scaling should be that on a typical motor.

Total plant manager move, knows what the problem is, but throws parts at it to make it go away. 

I understand his philosophy, production numbers affect him on a day to day, maintenance costs affect him quarterly or yearly. Besides, there is probably a maintenance manager that will eat the cost on his budget. A year from now he will be yelling at a maintenance manager for spending too much for decisions he partly made. The circle of life, it all rolls down hill!


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## ibew415 (Mar 23, 2014)

RICHGONZO1 said:


> Yes, the scaling should be that on a typical motor.
> 
> Total plant manager move, knows what the problem is, but throws parts at it to make it go away.
> 
> I understand his philosophy, production numbers affect him on a day to day, maintenance costs affect him quarterly or yearly. Besides, there is probably a maintenance manager that will eat the cost on his budget. A year from now he will be yelling at a maintenance manager for spending too much for decisions he partly made. The circle of life, it all rolls down hill!


ha! not only diagnoses the drive, but the BS politics too! thank you sir!!

somewhere I remember someone saying that your minimum Hz should be at the point where the motor barely moves...ie put the drive in HMI mode and dial up the Hz until you can see the shaft barely turn...then come down from that value a tad? does this sound correct?


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## RICHGONZO1 (Mar 5, 2012)

ibew415 said:


> ha! not only diagnoses the drive, but the BS politics too! thank you sir!!
> 
> somewhere I remember someone saying that your minimum Hz should be at the point where the motor barely moves...ie put the drive in HMI mode and dial up the Hz until you can see the shaft barely turn...then come down from that value a tad? does this sound correct?


I dont like to set the minimum too low unless it has aux cooling. Just my .02.


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## ibew415 (Mar 23, 2014)

RICHGONZO1 said:


> I dont like to set the minimum too low unless it has aux cooling. Just my .02.


ah, brilliant!


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## RICHGONZO1 (Mar 5, 2012)

The dead band will prevent it from throwing off the scaling, my usual number is around 20hz if you know they run the motor close to 60hz. From there I would discuss aux cooling if they want to run at lower hz. Just broad numbers, I don't like risking burning up a motor and it being my fault. 

If I get it in writing then I'll do whatever they want as long as its code and morally sound. I have had many arguments over the term "code."


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## Jhellwig (Jun 18, 2014)

Another thing to check would be the scale. Loose connections on load cells can make the readings drift causing surging in a pid loop. Make sure all connections at any summing blocks or junction boxes are snug.

20hz is a good low end is good for most applications. Some drives can vary though in what that lower frequency setting means. Some drives have a minimum and maximum speed that are independently set from a speed reference min and max. If you set the min drive speed to 20 but the speed reference min is 0 you will get 30 hz at half scale. This is not the case for all drives though. Just something to be aware of. Some Allen Bradley drives operate this way.


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## ibew415 (Mar 23, 2014)

Load cells could be part of it....when I was watching the drive the TPH was almost "floating" 315-269 TPH and there was no change in the drive speed. 

I agree I have a scaling issue in the speed of the new drive. but the old drive was having surging issues too


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## John Valdes (May 17, 2007)

Altivar in a aggregate setting?
I hope this drive is capable of full torque at all speeds as a standard VFD is not designed for high torque loads at anything less than full speed.

Do you have a feedback device? What is the speed range you are working with?


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## ibew415 (Mar 23, 2014)

an Altivar is Schneider's name for the drive....an Altivar 71 is a constant torque drive.

I have an output Hz on the HMI...but no other feedback. 

as far as speed range....4mA = 0 20mA = 60hz


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## varmit (Apr 19, 2009)

If your TPH feedback is changing and the drive speed is not changing, I would check the PID tuning. If the tuning response is too slow or too fast, there can be oscillations of flow rate. Also, I would think that rock flow rate, on a conveyor, would not be uniform (constantly varying due to material distribution on the belt(s). I would think that some variation of flow would be normal. Maybe the TPH readout should be averaged over a longer period of time?

I would check the VFD load to determine if the VFD is operating in current limit sometimes. This would cause the drive output (speed) to decrease as the belt loading increased, giving the opposite of the desired response. Check the scaling on the system components to see that every thing matches: Max flow rate = max VFD output = max TPH. The normal test, if the process allows, is at 0, 25%, 50%, 75% and 100%.

Always remember when doing maintenance work: You will never get the whole story on any problem. What has changed with the equipment, conditions, product or personnel? Is there any noticeable voltage drop at the VFD line side terminals?

Good luck! some problems can be a real challenge.


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## JRaef (Mar 23, 2009)

micromind said:


> I'd look at mechanical issues as well. Belts slip, maybe the headpulley is worn to the point it doesn't always run the belt at the proper speed. Also, the v-belts from the motor to the gearbox could be loose or glazed.
> 
> Is the VFD running at or close to current limit? If so, the belt will run at whatever speed the rock will allow.
> 
> ...


I like everything said here, follows my experiences with rock conveyors as well, especially the current limit issue. People very frequently misjudge the size of conveyor motor they need, because they used to use mechanical VS drives, line Reeves Drives. What they failed to remember is that as you lower speed on a mechanical drive, you are INCREASING the torque, and conveyors are very torque dependant control systems. So they use the same 5HP motor that they used to with the mechanical VSD, but get the SAME torque when speed is reduced. With a VFD, the way it deals with current limit, which equates to torque limit, is to artificially lower the speed output regardless of what the command is telling it to run at. That would, by the way, explain the anomaly that you are seeing. 

Added to that the 72Hz top speed issue. Above the motor base speed design, 60Hz in this case, the VFD can no longer maintain the proper V/Hz ratio to the motor, because if you get to 460V at 60 Hz, but continue up to 72Hz, there are no more volts to go with it. So the motor will lose torque at that point, which if the other issue above is already a problem, only makes it worse.

Last issue, alluded to by others later, is the belt weighing system. A REALLY common problem in this industry is that people wire up the motors with SO cord or DLO cable, something flexible because the equipment moves. But the output of a VFD is like a really powerful radio transmitter ((FM radio means Frequency Modulation, what does a VFD do?), so if the output conductors are not in grounded steel conduit, it bleeds RFI like crazy all around those wires. Belt weighing transducers are notoriously sensitive to EMI /RFI interference, so it's a match made in hell... If the cables from the VFDs are using flex cord, change it out with shielded VFD flex cord, and ground the shields at BOTH ends, not one (different reason for the shielding). If they used shielded cord, or steel conduit, make sure there is a ground connection at both ends of either one.


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## DriveGuru (Jul 29, 2012)

Listen to JRaef, he's got you covered 😊


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## JRaef (Mar 23, 2009)

By the way if you want a simple way to test the current /torque limit speed override theory, find the parameter associated with that and change the response to "Trip" instead of adjust frequency. You might be surprised as to how often it is happening.


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## ibew415 (Mar 23, 2014)

JRaef said:


> By the way if you want a simple way to test the current /torque limit speed override theory, find the parameter associated with that and change the response to "Trip" instead of adjust frequency. You might be surprised as to how often it is happening.


Interesting! I am fairly positive that this motor is not reaching current limit, they only run it between 35 and 50% speed...output current was about 2.9 amps on a motor with an FLA of 6.5

its one of 3 feeders that feed rock onto a conveyor, each feeder is loaded with different material, and they run them at different speeds to make their "blend"


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## garfield (Jul 30, 2009)

There should be a screen to see the error rate so you can tell whether it's a pid tuning issue.


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## ibew415 (Mar 23, 2014)

garfield said:


> There should be a screen to see the error rate so you can tell whether it's a pid tuning issue.


I'll ask about that tomorrow, thanks!


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## Jhellwig (Jun 18, 2014)

Unless someone had changed something in the pid tuning for some dumb reason I doubt it is that unless it was never tuned right in the first place in which case the bad tune would severley amplify the a disturbance caused by the real issue.

Like stated before you could be hitting the current limit in the drive causing it to slow. That is easy to check but I think you have ruled it out already.


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## ibew415 (Mar 23, 2014)

I FINALLY found a cause for the "Surging" problem. The V belt was loose! That's it!


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## Semi-Ret Electrician (Nov 10, 2011)

Jraef, agree with everything you said except grounding at each end, which just creates a loop antenna.


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## Jlarson (Jun 28, 2009)

People underestimate v-belts as a potential problem source sometimes. 

Shielded drive cable must have the shield bonded at each end. Ground loops are not a concern like with instrument cables.


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