3 Phase Motor Speed Control

[mikeruth]

I am working on a project that requires a Micrologix 1200 PLC that will control the speed of a booster pump station water pump using a Schneider VFD. I am using a analog card with the PLC. The speed of the pump will be determined by the suction and discharge pressure of the system. I can get the suction and discharge pressures in to the PLC. I need help with the code to actually communicate and change speeds of the VFD.

 

[Tom Jenkins]

You can't control the suction pressure on a booster pump, just the discharge pressure. You can use proportional control or another algorithm to do this.

 

[leitmotif]

Taking Tom one step further.
You want to ensure Net Positive Suction Pressure (NPSH) at centrifugal pump suction. If you dont you will create air bubbles or steam in the impeller and cause cavitation which erodes it - worst case fill pump with steam - they do NOT pump steam.

Ensure suction piping is at least same size as pump inlet - larger will not hurt. With RARE exception NEVER throttle a pump suction. Try to design to ensure as much suction head as possible.

Dan Bentler

 

[mebc3_2]

Does the output of the PLC analog card match the input of the Speed Feedback for teh VFD? (eg 4-20mA or 0-10VDC)

 

[JHarbin]

You can't control the suction pressure on a booster pump, just the discharge pressure. You can use proportional control or another algorithm to do this.

Maybe the OP wants to control the DP across the Pump so he will subtract the suction pressure from the discharge and make this the PV of this control loop. A DP Transmitter is simpler, though.

 

[Lancie1]

I need help with the code to actually communicate and change speeds of the VFD.

Use your Analog Output Expansion Module attached to your MicroLogix 1200 to send the VFD a speed setting from the Control Variable Output of a PID block with your differential pressure as the Process Variable Input.

 

[Tom Jenkins]

Maybe the OP wants to control the DP across the Pump so he will subtract the suction pressure from the discharge and make this the PV of this control loop. A DP Transmitter is simpler, though.

Possibly, in which case you are right about the DP. However the booster pumps I've been familiar with generally sit in a water distribution network and boost the pressure up to a set level in the downstream distribution network. They are used to offset the friction losses in the distribution system upstream of the pump.

Because of the unique application booster pumps don't usually have cavitation problems because the upstream pressure is usually 30 or 40 psig.