OT: Load Sharing with DC Drives

steve708

Member
Join Date
Sep 2003
Location
Ypsilanti, MI
Posts
30
I have several long conveyor systems (6" chain) powered by multiple 7.5 HP DC motors. DC drives are AB 1370 Series B and are very old and no longer supported. Each motor armature is wired in parallel to the armature circuit of the drive, and each motor has a resistor in series to balance the effects of different wire lengths. Each motor field is wired in parallel to a fixed 150VDC field supply and each motor has a variable resistor in the field circuit to allow for tweaking the speed of individual motors to pull slack chain out between motors. There is an external feedback loop based on chain movement that creates a trim signal to combine with the main speed control pot, and armature feedback in the DC drive is sufficient.

It is time to upgrade these DC drives to something that is supported, and PowerFlex DC is the preferred option. It seems though that a fixed field supply is no longer available and the drive wants to precisely control field current which takes away the ability to individually adjust motor speeds. I am also being told that the way to do things now is to put in a DC drive for each motor individually and network them together to accomplish the load sharing.

It is cost prohibitive to change to AC motors, and also to install multiple DC drives in existing control panels while absolutely not causing one second of downtime due to the upgrade. Also, the conveyors cannot be run during non-production time for testing.

What I really need is a new up to date DC drive which can directly replace my old system with no hassles - any suggestions?
 
Let's see here. A complete changeout of motor controls without a minute of downtime and no way to test the system before going on line!

Is it ok to laugh out loud?!!! You wouldn't possibly be a production manager or CPA now, would you?

All humor aside, you really need to make room for some downtime and testing if you want a successful system. Nobody is good enough to do this stuff instantly and with no need for fine tuning.

In my view, the advice you got is probably right on. With today's digital drives, you almost certainly would run each motor from its own little drive with one drive set up as a lead speed regulator and the rest as torque followers or, better, as speed followers with torque trim.

Properly done, you will have a very reliable sharing of load throughout the conveyor's length and will probably see reduced wear on the conveyor components too.

If you really want to minimize downtime and headaches, I would suggest contracting with a system integrator that has done this several times. With you being in Flint MI, you are very close to many such integrators in Automation Alley just south of you down I-75. I've worked with several that have integrated ABB drives into conveyor systems and know that the skills are out there to make your upgrade easy and fast.
 
That may have sounded a little unrealistic, I am actually an engineer, not a CPA!

This is a car assembly production line, so I need a few dozen people there to build cars if I want to run it, but when they are building it can't be stopped for testing.

I have a main/standby pair of DC drives existing so I could conceivably install the individual motor array of drives and keep one of the old drives to switch in and out to have a safe backup while testing, but this would get real messy and half of my panel space would not be available.

If I were to do the multiple drive installation it would have to wait until plant downtime with all product removed from the system, and in this case we would call upon one of our reliable system integrators to do the job, but the cost would be high.

This is why I would like to find a new "old" drive, it's looking like they don't make them anymore.
 
That may have sounded a little unrealistic, I am actually an engineer, not a CPA!

This is a car assembly production line, so I need a few dozen people there to build cars if I want to run it, but when they are building it can't be stopped for testing.

I have a main/standby pair of DC drives existing so I could conceivably install the individual motor array of drives and keep one of the old drives to switch in and out to have a safe backup while testing, but this would get real messy and half of my panel space would not be available.

If I were to do the multiple drive installation it would have to wait until plant downtime with all product removed from the system, and in this case we would call upon one of our reliable system integrators to do the job, but the cost would be high.

This is why I would like to find a new "old" drive, it's looking like they don't make them anymore.

Just do this. You will save bigtime with some timeline..
 
I think you are underestimating the difficulty of doing good loadsharing, especially in a chain-type conveyor with slack. I venture to say that it simply couldn't be done piecemeal.

Better wait until a shutdown and do it all at once.

Located where I am, I've seen plenty of auto industry automation. I probably don't have to tell you that if you screw up their production, you won't just feel uncomfortable---you'll be looking for another job. (I don't need to tell you that there are NONE in Flint!).

Go to your supervision with a thoughtful, company-oriented case for waiting and doing it right. You'll be glad you did.
 

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