Project method

Hello
I have: 2 motors, A and B
1 Micrologix 1200 PLC
1 IF2OF2 Analog module
1 Variable frequency drive
Motor A is 3ph 480v 30hp whose starter is controlled by the PLC
Motor B is 3ph 480v 1hp whose run/stop is controlled by the VFD

What I would like to do is:
1) Control the speed of motor B as a function of the current drawn by motor A. I would like the relationship to be inversly proportional.
2) I'd like to initiate some actions with the PLC based on different discrete values of motor A's current.
I think this can be accomplished by:
1) Buying a current transducer with a 4-20ma or 0-10v outputsignal that goes to the analog module.
2) Take take the current or voltage signal to the input that controls the frequency of the VFD.
My questions are:
1) Would this equipment setup work.
2) Well, to be Honest I don't know enough to even formulate any reasonable questions! Any input and/or pointers to information I could use to educate myself in this area would be appreciated.
3) Why 4-20ma and 0-10v. Whats the difference? What I'm trying to say is, isn't there a potential difference any time you have current? So why the two methods?
Thank you
Danny

Answer to (1). Yes, provided your transducer has the capability to be programmed for conversion, inversion, centering and ranging, otherwise you will have to develop those features within your program.
Answer to (3). 4-20ma control is somewhat more noise immume than 0-10v and the length of the signal cable is self compensating on 4-20ma. It is not on 0-10v. Long runs of 0-10v can cause noise pickup and voltage drop to an unacceptable level.

have you checked this?

Danny,

some VFD's have the ability to send an analog output signal (4-20mA, etc.) to "report" the motor's current, speed, torque, etc. - if yours has this capability - and the signal is not already being used for some other purpose - then you might not have to buy the current transducer - also, some types of transducers don't give very good results on VFD current readings because the output waveform is usually not a true sine wave - how critical is the current signal's accuracy going to be in your particular application?

also - is there any reason why you can't give us some more information about WHY you want to do this? many of the guys on this forum are VERY good at coming up with suggestions on "how-to" or "how-not-to" go about accomplishing a project like this - i'd suggest that you give them as much information to work with as possible -

Ron,

You are right, we need more information. I'm guessing about the following:

I believe from reading the original post that Motor A will be the lead or Master motor. Motor B (the VFD one) will be the Slave motor.The Motor A current appears to me to be used to control the speed of Motor B. Maybe he is going to use Motor B as a feeder device of some sort to put more or less product (Load) on Motor A. Therefore, it would seem to me that the waveform on the current transducer would be near true sine wave because Motor A is being powered from the standard line input power and not from the VFD as Motor B is.
See this link:
http://www.api-usa.com/
I am not affiliated with this company in any way.
This company makes all kinds of conversion modules to condition the signal inputs and outputs including current, voltage, frequency and pulse. They can be inverting (reverse acting) or non-inverting (direct acting). The gain, range, and offsets can be adjusted to center up the control point. This really reduces the amount of programming for me but others might want the PLC to do all the processing.

JR and Ron, Thanks for your replies. JR, your explanation of the setup is exactly right. I'm not trying to be secretive just wanted to keep the questions short and concise as I'm able.
The 30hp motor drives a saw blade and is wired through a plain old fashioned starter. The 1hp saw (wired through a VFD) controls the rate, that product to be sawn, is fed through the 30 hp saw. The issue here is that density of the product varies. So, with a constant product feed rate we have two undesireable scenarios.
1) When the product becomes less dense: The blade could be cutting more, hence its production rate suffers.
2) When the density of the product increases: Blade life is adversly affected.
As far as off the shelf hardware is concerned I think I'd prefer to go the PLC programming route. I understand this may be more difficult but I'd really like to learn how to do it. I did some searching this afternoon and found a couple of articles on the subject of PID and analog. Thanks again. Any further input would be welcomed.
Regards
Danny