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AB VFD 700H Speed Control

AB VFD 700H Speed Control

AB VFD 700H Speed Control

(OP)
Good Day all:

A mechie coming over for some help.  

We have two identical AB 700H VFD controlling 300hp motors operating in parallel in our material flow. I would like to reduce the speed, one will allow the change and the other will not. Is there a way to merge the good "modifiable" settings file to the other VFD drive?

Thanks

Mike

RE: AB VFD 700H Speed Control

There is a setting that "Prevents tampering".  Turn that off.

Keith Cress
kcress - http://www.flaminsystems.com

RE: AB VFD 700H Speed Control

mccmike, how is the speed being controlled?  From the keypad, an analog input, fieldbus, or digital inputs using MOP or floating point control?

Are the motors "stuck" running full speed or something else?  What controls that on the one you can change?

RE: AB VFD 700H Speed Control

(OP)
This is my first foray into VFD's so please forgive stupid questions.
As far as I know the speed is being controlled by frequency selection on the HIM. Currently the speed is set at 60Hz on both, one can go up and down but the other one cannot be changed.

We have a PLC that can reverse the motors direction but it does not control the speed; too much variation for the guys in the plant will cause chaos.

itsmoked: Can you tell me the parameter number of the "prevents tampering" setting?

Thanks

Mike
 

RE: AB VFD 700H Speed Control

If one can do it and the other disallows it, chances are there was some thought that went into it and someone knows that for some reason, the other one should not be allowed to be changed. I would suggest this is not something to be done by novices.

For example, you said "material flow" without any further explanation, which could mean any number of technologies. But let's say it is a conveyor belt with two 350HP motors at each end. It could very well be that one is allowed to vary the speed, the second one follows the first one strictly by using a torque reference sent out by the first. This is a very common application and if you mess with it, you end up with the two drives trying to fight the stretching of the belt, it will start to act like a rubber band and you will have chaos!

Get to the engineer that designed the system and ask ask ask...


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
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RE: AB VFD 700H Speed Control

(OP)
jraef:

Thanks for your concern but I am an experienced mechanical engineer and I mentioned that the two motors operate in parallel but without any feedback or closed loop controls.

Slowing down the motors is a valid process control parameter that has frustrated management for a little while now. Also the designing engineer is no longer available for questions.

Mike
 

RE: AB VFD 700H Speed Control

The drive that doesn't change speed is probably programmed to look somewhere other than the HIM for the speed signal.

I would take a look at the analog inputs to see if there are any wires connected there.  If there are, I suspect that that is where the drive is looking for its speed signal.  Now, the challenge is to discover where those wires come from.  Knowing that, you will probably know where the speed control resides and can probably deduce how to change it.

However, as jraef wisely cautions, if the speed signals are from two different places, it is likely that some kind of linkage or logic with something else is involved.  At these horsepowers, you will need to acquire the "big picture" before making any changes.

Don't you just love it when you inherit a system with no documentation or reference people to consult!!

RE: AB VFD 700H Speed Control

Jraef

The two motor conveyor application that you referenced is a very interesting one.  With the 1 motor controlling the speed does the drive of the first motor control the speed and aquire a torque feedback using this torque feedback to send a torque reference to the second motor to pick up the slack of the remaining torque required to turn the conveyor? How does the first drive know how much additional torque is required?

In this application are there typically two VFD's one for each motor, or does on VFD handle each motor?

RE: AB VFD 700H Speed Control

Often, the helper motor is arranged simply to match the torque of the lead motor.  The lead drive knows what the torque in the motor shaft is either by estimation or by calculation from the frequency and shaft encoder.

When the helper motor is so configured, the drives automatically divide the load torque between the two of them and you have "load sharing".

RE: AB VFD 700H Speed Control

Thanks Dick. Couldn't have said it better, in fact I probably would have droned on and on about it...


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

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