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Unread December 3rd, 2019, 08:54 AM   #1
Ruskied
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VFD/PLC Vessel Tilt Constant Torque Application

This is more of a VFD question where a PLC is used to monitor/trend several datalinks. The application is to tilt a vessel with two AC motors driven by one VFD as the vessel is filled with molten steel. The application is said to be constant torque with Volts/Hz control mode (no encoders). At certain vessel weight, the VFD can't move the vessel anymore. VFD was sized based on motor (both motors) FLA and Volt nameplate data. At first glace, the VFD looks to be undersized. However, looking at the datalink trends, VFD never reached max. power, not even half. VFD appears to be overheating due to being ran at 200% of rated current. Motor FLA (both motors) is 82 Amps while the VFD overheated at 144 Amps (running for ~3 sec). It's a rockwell powerflex 753 heavy duty 77 amp drive (can operate at 115 Amps for 60 sec. which is more than it needs for this application).

Questions:
1. Would this be a constant torque application?
2. Drive current limit is currently set for 144 Amps, would lowering that help?
3. Are there any other options left before resizing the drive? (ex. boosts, other operating modes)

Any ideas or thoughts or links for further reading would be greatly appreciated.
Regards.
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Unread December 3rd, 2019, 10:13 AM   #2
dmargineau
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It does seem the VFD had been 'undersized', however, before making any replacement decisions you should try running it in 'Vector' Motor Control Mode (Speed Control with Torque Limits).

It might be just enough to satisfy the application; you will send a Speed Reference, however, you will limit the produced Torque Current to a setting which will reduce 'loading' to a satisfactory value; the VFD will 'try' maintaining the commanded speed AND the torque limit(s) by slightly varying the frequency output.
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Unread December 3rd, 2019, 10:58 AM   #3
thingstodo
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Quote:
Originally Posted by dmargineau View Post
It does seem the VFD had been 'undersized', however, before making any replacement decisions you should try running it in 'Vector' Motor Control Mode (Speed Control with Torque Limits).

It might be just enough to satisfy the application; you will send a Speed Reference, however, you will limit the produced Torque Current to a setting which will reduce 'loading' to a satisfactory value; the VFD will 'try' maintaining the commanded speed AND the torque limit(s) by slightly varying the frequency output.
You can't .. or at least you couldn't in the past .. run two motors from one VFD and have the VFD in vector mode. The VFD is measuring induced voltage from the motor to 'calculate' rotor position. Having 2 different rotors messes this up.

Boost helps to start the rotation, and tapers off .. normally above 20 Hz or so. I don't think that will help.

When you say 'tilt' ... do you mean 'rotate'?

What speed are the motors running to do the 'tilt'?
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Unread December 3rd, 2019, 11:38 AM   #4
dmargineau
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It's still worth trying before spending in excess of $10K.

If 'relatively accurate' is good enough then it might just work, multiple motors or not.
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Unread December 3rd, 2019, 12:20 PM   #5
GaryS
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I have never heard of running 2 motors on ne vfd in torque mode you can do it in straight v/f mode only
you state the load is only about half but the very next statement you say that it is running at 200% those 2 statements can't work together
why 2 motors are they sharing a common load
you really should have somebody come in and do a full evaluation of your system
Also you are playing a very dangerous game dealing with molten metals be very careful
with out a lot more information it's hard to get into details.
Each motor should have it's own vfd sized for that motor
with your application I would use a Flux Vector drive on each motor with encoder to get the best control over the moor
I am not sure that torque control is a good choice for this application and that may be part of you problem
I worked in foundries for the 10 years of my working life then moved on.
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Unread December 3rd, 2019, 04:01 PM   #6
jraef
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This is not the kind of application I would do with 2 motors on one VFD, but apparently that decision is moot at this point.

When you use simple Scalar (V/Hz) control, one of the problems is low speed torque capability is not the best it could be. The only recourse in Scalar control is to use the "Torque Boost" feature to try to overcome these shortcomings. Basically you tweak the V/Hz ratio at lower speeds to boost torque, but return to the standard V/Hz ratio as speed increases in order to avoid over saturating the motor continuously. This has nothing to do with the PLC programming, it's done in the VFD setup. In a PF750, this is called "Start/Acceleration Boost", found in parameter 60.

Short of that, your best performance option is to have separate drives running in tandem using SVC with one as the master and the other as a torque slave.

Last edited by jraef; December 3rd, 2019 at 04:06 PM.
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Unread December 3rd, 2019, 10:15 PM   #7
GaryS
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Are you a contractor they brought it to fix a problem ?
If you are then this is one project you should consider walking away from.

To help me understand what you have. This vessel is filled with molten metal from the melting furnace
The this drive and motors system is used to tilt the vessel to pour the molten metal l into smaller vessels / ladles to pour it into molds .
This vessel has 2 motors one on each side of the vessel to tilt it to pour the molten metal
One drive set in torque control mode to run both motor. The problem with that is that both motors will not share the load equally one motor may see an overload while the other see very little load. Not a good situation the drive system will overload or stall. I think this system was poorly thought out from the start.
I know some of you are going to think this is off the wall but think about it for a while.
It should be completely redone 2 drives and 2 motors. A master slave configuration if you will
For safety reasons both motors must have a motor brake is case of a failure so they will hold the load (Vessel)
The master drive should be a flux vector drive with an encoder feedback the slave drive should be set up as torque follower with the torque reference coming from the torque / load of the master. I know that this is different than you would expect. if you try to run both motors in flux vector mode the speed of each motor will be exactly the same but due to mechanical differences the loads will be different. The 2 drives will end up fighting each other and one or both will trip and shut down. One motor alone can’t handle the full load. The speed reference for the salve should come from the master speed and scaled just a little faster than the master ( master 30hz slave 33hz ) This to limit the no load speed of the slave
As the master is run with an increases in speed the current to the motor (torque ) will increase when it increases the current to the slave motor will increase to the same level so both motor are running at the same torque. As the slave pick up the load the load on the master will decrease until they equal.
All loads would be shared by both motors.
In this arrangement I would set up a common buss configuration with a braking module to handle the regenerative load. There will be a regenerative load when stop the tilt
On any drive fault or power failure both motor brakes will set and safely hold the load in place

I have worked on a few common load system with 2 flux vector drives on a common shaft, they always end up fighting each other and one or both will trip out by using the torque follower both will share the load equally this setup will work if the drive and motors are different size if set up correctly hey each will provide the same percentage torque for their given motor.
A standard vfd running open look or v/f will fight each other if they are trying to share a common load you can only see the drives fighting if you carefully for it. But because they don’t hold the speed accurately enough they will appear to share the load but they will not share well. I have observed this on a systems.

As I said I know this is kind of out of the box thinking but that’s what we are all about find solutions to problems.
The reason would use a flux vector drive for the maser it would give me absolute control of the load at all seeds even down to actually holding the load at zero speed even with the brake released . with the slave in torque mode it would share the load so the system would act a one drive .
It will require some careful planning for the wiring and drive sup.
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Unread December 4th, 2019, 01:04 PM   #8
thingstodo
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Quote:
Originally Posted by GaryS View Post
Are you a contractor they brought it to fix a problem ?
If you are then this is one project you should consider walking away from.
+1 on that!

If you are an employee looking to 'fix it' ... I would advise getting an engineering group that has experience in this to take it on and 'fix it right'

To answer the original question .. a bit at least .. if the motors are stalled, boost will help. The highest I've ever used in production is 4%. 200% current sounds like a stall condition!

On a small motor (that I was testing for curiousity) driving an over-sized load ... I went up as high as the 1333 drive would go. 40% I think was the limit. The motor behaved very poorly below 10 Hz and the motor shaft turned in fits and starts. This behavior started at about 10% boost. I don't recommend going anywhere near 10% boost.
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