VFD conveyor problem

hainter

Member
Join Date
Nov 2005
Location
Cypress, CA
Posts
10
Greetings

I have recently been faced with a problem of replacing an unreliable torque tamer on a conveyor. The conveyors are prone to getting stuck and currently the solution is to use a torque tamer (which is basically a clutch) in conjunction with a proximity sensor to sense the conveyor turning when it is supposed to be. The torque tamers are very unreliable in that when they slip they need to be re-adjusted and they are difficult to adjust in the first place.

The solution I have come up with is to monitor the current in the VFD and stop the conveyor if the current gets too high. This will either be done with an analog out from the VFD using "set points" in the VFD to allow for current spikes, or just simply having one of the outputs on the VFD be set to go on at a certain current level.

I have researched monitoring the shaft torque with a torque sensor, monitoring shaft speed, using a current sensing relay on the VFD output, and some other options. These options are either too expensive or not suitable.

My question is: Is there an easier way to solve this problem that i haven't discovered? Will the current sensing be accurate enough or will normal operation spikes make it difficult?

Thanks

Joe
 
Check into the options available for your VFD. Just about any drive will let you select frequency or current as the drive's analog output, but some will allow you to select additional values, including torque. You may also be able to configure one of the drive's discrete outputs to change state at a specified torque level. If the drive you're using doesn't give you the option you need, look into another brand.

For example, the Dura Pulse line from Automation Direct only lets you configure the analog output as frequency or current, but you can configure a relay output to be activated at a torque level.
 
Most drive today have a 0-10 volt output that is 0 to 200% current. Since current is dircetly proportional to torque it can be used to set a threshold in a monitor device.

Many drives also have one or more digital outputs set for monitoring almost any thing internal to the drive and then have a threshold programmed as a percentage.

Have you considered just putting a flux vector drive in place of all that other stuff? It is a fantastic torque control drive that can be configured for most any thing you want. Coupled with a Marathon Black Max motor, and you have smooth steady running. If you set the torque limit down below the motor's FLA the motor will stall and hold that level right nice.
 
Leadfoot said:
Since current is dircetly proportional to torque it can be used to set a threshold in a monitor device.

Be careful about such a universal statement. Current is absolutely NOT proportional to torque when using a typical, modern, vector AC drive.

In a DC motor, yes, Current has a direct relationship to torque, but not in an AC one.
 
you can add a watchdog timer in your program to stop the motor. it depends really on your conveyor configuration...i had used watchdog timers to stop my upstream conv if my downstream is empty and my sensor sense a jam condition for a period of time.
 
Many newer drives, notably such as ABB's ACS800 series feature PLC like programmability to their outputs, just. Of course the magnitude of the torque vector itself, is available as a parameter(32767 counts, 0-100% or even engg.units )to be used in the logic for disconnecting/connecting any of upto 6 digital outputs available on the drive. But there are also timers and counters as well. In my last drive order, I programmed the panel's space heater to switch on when the temperature fell below 10degs. Likewise there were 2 cooling fans, which were switched on at certain temperature set points. Please give this a try. you can overcome the problem of transient current spikes.
 
Maybe i missed something...The VFD controls the conveyor right?? So if the VFD hits an overload (this is programmable on most drives)..then it shuts down..this would stop the conveyor..now depending on the drive you could reset the fault from a PLC or from the drive..again i must of missed something
 
ok, darrenj, let's step thru this in bite-sized pieces.

First, torque in an induction motor is not measured accurately by current especially at light loads. The problem is that magnetizing current and torque-producing current are vectors 90 degrees from each other and the motor lead current is the vector sum of the two. The good news is that most drives, even the simple v/Hz drives internally calculate torque and usually can display it or send it to an output as an analog signal. This gives you a reasonably accurate measure of motor shaft torque that you can analyze and set your control system up to trip or whatever at a specific setpoint.

Second, many drives have the ability to set one of their output relays up as a torque level indicator. You program the relay to pick up at a specific torque level and then use it to stop the drive or signal your control system for further processing. Sometimes, you can set up pickup and release time delays on these relays too.

Third, for a simple solution without additional gadgetry, many drives can be set up to detect a torque limit level. You then have the option of setting the drive to hold or reduce speed once the torque limit is reached or, you can set the drive to fault instantly when the torque limit is reached. This is often called Electronic Shearpin. The option to reduce speed when the limit is reached usually includes the ability to back speed down all the way to stall, at no time exceeding the torque limit you have set. This is usually the preferred way to operate since it does not involve tripping with the necessary resetting.

I've used the relay detection system to protect very fragile stainless steel conveyors and the operation is very fast and consistent. It will be far better than any mechanical torque limiting means.
 
I knew i was missing something!!

Someone should let the OP know this becouse he was talking about putting CT's in the drives output..I just figured that if that was good enough then the drives internal CT's should do the job..

BTW.. DickDV Thats a damn good expliantion..are you in teaching?

I know you are the drive guru but always wondered what you do..




DickDV said:
ok, darrenj, let's step thru this in bite-sized pieces.

First, torque in an induction motor is not measured accurately by current especially at light loads. The problem is that magnetizing current and torque-producing current are vectors 90 degrees from each other and the motor lead current is the vector sum of the two. The good news is that most drives, even the simple v/Hz drives internally calculate torque and usually can display it or send it to an output as an analog signal. This gives you a reasonably accurate measure of motor shaft torque that you can analyze and set your control system up to trip or whatever at a specific setpoint.

Second, many drives have the ability to set one of their output relays up as a torque level indicator. You program the relay to pick up at a specific torque level and then use it to stop the drive or signal your control system for further processing. Sometimes, you can set up pickup and release time delays on these relays too.

Third, for a simple solution without additional gadgetry, many drives can be set up to detect a torque limit level. You then have the option of setting the drive to hold or reduce speed once the torque limit is reached or, you can set the drive to fault instantly when the torque limit is reached. This is often called Electronic Shearpin. The option to reduce speed when the limit is reached usually includes the ability to back speed down all the way to stall, at no time exceeding the torque limit you have set. This is usually the preferred way to operate since it does not involve tripping with the necessary resetting.

I've used the relay detection system to protect very fragile stainless steel conveyors and the operation is very fast and consistent. It will be far better than any mechanical torque limiting means.
 
As you can see, I live in Michigan so that means its really tough trying to make a living in the industrial realm. As a result, I really have three jobs.

First, I am a territory manager (sales rep) for ABB's Drive Group. That's AC and DC drives and motors.

Second, I do lots of fee-based consulting work commissioning and programming drives, not just ABB. This includes application engineering work and sometimes project management. I do this throughout the upper Midwest.

Third, you guessed it, I am a part-time instructor for National Technology Transfer Corporation based in Denver. One week a month I teach a two or three day seminar on Variable Speed Systems (my favorite, of course), Electric Motors, or Electrical Circuit TroubleShooting. For example, next week I'm doing a three day seminar in Houston.
 
Thanks for your feedback everyone. I have been looking at all these options for the drives. We were originally using a powerflex4 drive which had limited flexibility. We have decided to use a powerflex40 drive with one output indicating a set current level. We are then gonna take that discrete output into a PLC and filter it depending on what we observe.

I had a lengthy discussiont with AB tech support about the difference between flux current and regular current in their powerflex line and they seemed to think that regular current would work for what we are trying to do. The powerflex40 can only monitor total current so hopefully this will work for what we are trying to do.

Again, thanks for all the feedback

Joe
 
DickDV,
I just looked at the Powerflex 40 manual and apparently that drive is NOT capable of motor torque monitoring, or more specifically, what they "call" torque monitoring is in %FLA, so it really is current monitoring which as you said, is not going to be that useful in this application. It also has no ability to trip on torque or current other than Motor Thermal Overload.

hainter,
Given this situation, I suggest using the % of Drive Rated Power setting for your analog output by setting parameter A065 to 13 ("OutPowr 4-20"). This will be a more accurate representation of a jam condition than current alone because current will also fluctuate based on line voltage changes. Output Power will track along with any input line voltage fluctuations. Note however that this is % or DRIVE rated power, not MOTOR rated power, so it will take a little more trial and error to find the correct trip level.

Better yet, scrap that Powerflex and buy a better VFD. Almost all other drives in that range have output torque monitoring or tripping capabilities.
 
"Better yet, scrap that Powerflex and buy a better VFD. Almost all other drives in that range have output torque monitoring or tripping capabilities."---jraef

Couldn't have said it better myself!
 
Yes, but coming from a competitor your opinion would be tainted!;)

I have no axe to grind with AB and I still find it disappointing that they left that feature out of that drive series. It's not as if this is rocket science!
 
jraef said:
Yes, but coming from a competitor your opinion would be tainted!;)

I have no axe to grind with AB and I still find it disappointing that they left that feature out of that drive series. It's not as if this is rocket science!

Generally AB can make those things happen for a price. That or you have to read the manual VERY, VERY carefully and find the parameter that turns it on. Then you get to buy the interface board and programmer or software to actually be able do it yourself. Been there on 1395's and 1336's over the years.
 

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