mjrx

Temperature Control Question

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I have been tasked with cooling a large motor. It has a VFD controlled cooling fan. There is an RTD at the intake area and another RTD after the airstream passes the motor. There is also a heat exchanger in the airstream that has chilled water passing through it. It also has an RTD at the water inlet and another RTD at the water outlet. The water flow through the exchanger is controlled by a modulating valve. My question is what type of control algorithm might be best for this arrangement? Should it be some type of cascade control, a single PID, differential, ??. It seems seperate PID loops may interact with each other and be hard to control. All 4 RTD's and analog to the VFD and valve are in place in my PLC and ready to go, I'm just not sure which way to go. Thanks in advance.

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This sounds like a classroom assignment question. Have you read the assignment?

If I'm incorrect please hire a competent programmer so you don't destroy that motor.

 

 

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I always start a new task by flowcharting the system.  A thorough flowchart should flush out fault scenarios and account for all modes of operation.  There are many free flowcharting utilities, or shareware programs like Visio.  Good old pencil and paper is my favorite, though.

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"Temperature Control Question"

Where have you been have you not heard that ever since "global climate change" we can't control temperature without government help and authorization?  ;-)

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Temperature control is designed for productions that depend on temperature, such as furnace, food and so on. Here's a real life application of controlling temperatures of the retort and the container of the product: http://www.icpdas-usa.com/kingviewreactionapplication.php?r=dora

In this application, the SCADA software did PID adjustment and the users can read real time trends of the temperatures.

Hope it can be a reference to you.

 

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Cascade control is complex and will probably cause more head aches than its worth. In your scenario there are two separate control loops one to control the water temperature through the heat exchanger and the other to control the volume of air flowing over the motor.

If it were me I would just use two separate PID loops by first determining the range of control over the water then determine the optimal temperature for air cooling and use Loop 1 to maintain that optimal cooling temperature .

In my opinion the second loop for the VFD is just to make sure you have good volume of cool air flowing over the motor. I think it would be very difficult to control air temperature with the VFD because of the temperature lag, the heat exchanger control loop will yield much better response for temperature control.

I have attached a schematic

 

 

cooling control.pdf

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Very nice cbvance, thank you. That was the general direction I was heading, after some simulations I have run I have ruled out cascade control. I also was thinking about a single loop to control both with a seperate gain value to give more "weight" to the water over the air temp, kind of like making the vfd a slave follower to the water temp PID loop.

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On the assumption that chilled water is more expensive than electricity, my first approach would be to modulate the speed of the fan in response to the exit air temperature. If the fan is unable to maintain the temperature setpoint at full speed, then modulate the flow of chilled water.

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