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Old December 22nd, 2013, 05:57 AM   #1
alexhk
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PowerFlex 400 PID Loop for DryCooler Fan / Temperature / Modbus

Hello

I am a newbie so apologies in advance if I am not making sense or miss important information. I am trying to setup a PID Loop on a PowerFlex 400 to control the fan speed of a Dry Cooler to stay at 40°C. The system is up 24/7, unfortunatley I can't just play around myself at this moment and I need to schedule down time after I understand more. Here are my questions:

(1) Does that sounds like something I can use the PowerFlex internal PID Loop?

I am just starting to understand PID but I am not sure if there are many different implementations and if there are known good and bad ones.

(2) I believe the most common way of using the PID loop is by using the two analog inputs, but at this moment I don't have temperature and setpoint available via analog signal. So I am trying to see if I could still use the RS485/Comm Port for now.

- Both, A152 [PID Ref Sel] and A153 [PID Feedback Sel] can be set to Comm Port. What I don't understand with this variation is how I could write both of these values. I believe I would write the PID Feedback (ie. current temperature?) to P038 [Speed Reference], but I am not clear in what format (certainly not Hertz?) and how/where would I write the PID Setpoint?

- Or I could set A152 [PID Ref Sel] to PID Setpoint so I would use A157 [PID Setpoint] to write the setpoint manually on the PowerFlex via keypad. Then I would still use P038 to write the PID Feedback (current temperature). But what format, and does that sound correct?

- If all of this fails/results in no feedback here on the forum, I would go the analog input route. That means I would wire a temp sensor to a 4mA-20mA input, and the same for the set point. Does that sound correct?

Again, thanks for your pointers in advance and enjoy your holidays!

Further info in case that was not enough:

I run two PowerFlex 400 drives. One for a 7.5kW water pump and the other one for fans on a DryCooler (air cooled heat exchanger). We use 400V 3-phase + neutral here in Hong Kong.

I run a simple program on a server that controls fan speeds via RS485/Modbus. The program reads the water temperature from a remote sensor and controls to fan speed. It does that by writing the Hz to the speed reference register via modbus (8193). Works fine, expect the program is not perfect and the server needs to go sooner or later.

Now I stumbled upon "a magic thing" called PID Loop ;-) Even better, it's a built in function in the PowerFlex drives. I did some reading about PID online, read the PowerFlex documentation and checked the parameters, and soon I want to give it a try.

I hope you would have a little bit of info for an IT head like me, as we don't have PLC people at my location (we have people selling equipment to us, but that's about it). I also hope that in return I can help with some other stuff, as I have been lurking on PLCtalk for a year or two.

Alex
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Old December 22nd, 2013, 09:36 AM   #2
Lancie1
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Quote:
(1) Does that sounds like something I can use the PowerFlex internal PID Loop?
Yes.

Quote:
Or I could set A152 [PID Ref Sel] to PID Setpoint so I would use A157 [PID Setpoint] to write the setpoint manually on the PowerFlex via keypad. Then I would still use P038 to write the PID Feedback (current temperature). But what format, and does that sound correct?
Not exactly. PID Ref Sel simply activates or turns on the PID function. Your temperature feedback should really be an input to the drive, either via analog 4-20 mA input, or by the RS485 Comm Port. If you use P038 as your feedback, then the speed reference signal would have to be scaled to an equivalent temperature (probably impossible to do because it may not be linear).

Quote:
- If all of this fails/results in no feedback here on the forum, I would go the analog input route. That means I would wire a temp sensor to a 4mA-20mA input, and the same for the set point. Does that sound correct?
Yes, that is probably the best method.
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