Speed Control

[ryani]

I have two drives that control two seperate pumps that need to control pressure for plant cleaning. The company wants one pump to be running and when required the second pump needs to start to help keep the pressure at the set point. What is the best approach to this job?

 

[parky]

It depends on what sort of control you want, for example are you expecting to control the pressure at a constant pressure i.e. PID control, or is it just if the pressure falls below a certain level start the second pump to keep it above the minimum setpoint.
The cost difference in the two aproaches is significant, as well as the control limits.
It could be that two pumps one duty & one boost if needed does not give good control of pressure there is also a danger of over pressurising the system or the standby pump may be switcing on/off above it's duty cycle.
The ideal way is to have the pumps controlled by VFD's (or at least one of them), then feedback the pressure to an analogue input.
But this requires the addition of VFD's & Analogue card whereas simple on/off control of the secondary pump does not need a VFD.
I think more detail is needed to give you answers that pertain to your application for example a diagram of the system as you see it, control type & feedback.

 

[ryani]

2 x vfds controlling two multistage pumps in parallel
1 x 4-20ma pressure sensor to an analogue input

At the moment it is programmed on 2 PIDE loops, when pump one falls below a pressure set point for a certain amount of time it brings on the second pump until it is no longer needed.

This has been bypassed in the plc because it wasn’t maintaining pressure and is now running on the first PIDE loop only and both pumps are running always

 

[MaxK]

Like a prosecutor in court
We have to coax every word

How did the system originally work?

Both pumps are off. Pressure = 0
Pump 2 turns on. The PID controller increases the output of pump 2.
If the capacity of pump 2 has reached its maximum, and the pressure has not reached the setpoint, then
Pump 1 turns on. The PID controller increases the performance of pump 1.

And in reverse

Right? Or did the system work differently?

Why wasn't it maintaining pressure? Or what does it mean?

Are the pumps the same capacity?

What else we have to know?

 

[Ken Moore]

Have one PID loop. Scale PID output 0-50% = 0-100% speed reference for pump-1. Scale PID output 50-100% = 0-100% speed reference for pump-2. When PID loop is > 55%(adjust as needed), start pump 2. If these are centrifugal pumps, then they will not pump very much until 40% or higher depending on pump. So may want to eliminate the scaling on pump-2, have it start at 55% speed. Then, when PID output drops to 45%? Stop pump-2.