vary speed on small motor

[nusslt]

I am working on a control project, trying to tie together a 4-20 mA signal from a pH controller and a dc operated gearmotor of about 20 " lbs of torque. What is the best, and least expensive method of controlling the gearmotor to vary speed in conjunction with the varying mA signal from the controller. The main power supply is 115 VAC.

 

[itsmoked]

What kind of DC gear motor?
What kind of speeds?
How fast does the speed need to vary?
What kind of controller are you required to use?
What is the motor driving?
..

Come on "engineer up" a little here.. So we can help you.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Look for a DC motor controller that accepts a 4-20ma control signal.
Look for a VFD that accepts a 4-20ma control signal and use an AC motor.
If you need more help, follow itsmoked's suggestion.
respectfully

 

[nusslt]

The motor is geared down to run at about 20 rpm. I will need it to go from zero to full speed over the course of 7 to 0 pH values. It will be driving a peristaltic pump, which will be pumping fairly thin acid into a waste stream. Not a lot of torque required, but need to keep up with the demand of pH adjustment.

 

[Compositepro]

You better study-up on pH control. It sounds like you are trying to do continuous-flow, in-line control over the entire pH range. Although theoretically this is possible it is virtually impossible in practice. Each change in pH units represents a factor of ten in the amount of additive. Combine that with time for mixing to occur and for analyzer response and you will find that you need a tank of adequate volume with a mixer. Then you can use standard metering pumps and on-off control.

 

[itsmoked]

No kidding, every time I've tangled with ph..

There always seems to be a big chunk of 'time' in the control loop that needs to be handled along with wild control values.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[richs]

Hi-

I don't know about the tank issue in this particular case. The key word here is "waste". If, it is indeed a waste pH balancing act, wild "swings" of pH can usually be tolerated for short periods of time, as long as the aggreate effulient is pH netural.

It is a second order partial differential equasion that needs to be solved, added to the integral over time of the netural balance. This is indeed not a "linear relationship" solution.

What I find "interesting" is that there is no mention of "flow rates" in the specification. Especially in regards to the more alkaline "waste" to be treated.

What can seem like such a "simple problem" can rapidly turn into a nightmare.

Maybe a little more info would continue to help. Ranges, desired tolerances, flow rates, etc. With that, *MAYBE* we can help you.

As an "offhand" guess, I would expect that a PLC will typically not have the "math" requirements to do the control loops required. As far as "cheapest" well, a cast off PC running linux with a little a/d and a d/a with a servo amp to drive the motor *MIGHT* be a possible path for your efforts. BTW, the linux portion is optional, just this author's bias showing up. Sorry.

Again, more information is suggested.

Cheers,

Rich S.