Pump Hz minimum where to limit

Hardware
1756-L72
RSLogix 5000 V20
PIDE instruction
PowerFlex 400vfd
Grundfos submersible pump

The pump manufacture states that the pump should reach 30Hz in 3seconds and then operate at or above 30Hz. What would you guys consider good practice.
1) Limit the Hz using the CV Min of the PIDE instruction block and set it to 50% which with my scaling would be 12mA or 30Hz.
or
2) Limit the minimum Hz with the VFD using parameter P034[Minimum Freq] and keep the PIDE block CV Min at 0% so then 4mA would equal 30Hz.
Any pros or cons to either option?

Thanks

at 30 Hz is your flow less than what it takes to maintain your process?

maxketcham said:

at 30 Hz is your flow less than what it takes to maintain your process?

Click to expand...

There is plenty of flow at 30Hz. I suspect under normal conditions the pump will operate between 40-50Hz

I'd use the first option then

As a point of info, pump manufacturers may limit speed because below a minimum the pump isn't capable of enough head to overcome system resistance and so there would be no flow. That can result in vibration, overheating, and pump destruction. That minimum Hz is a function of system curve and pump curves and for the same pump may be different for different systems. A value like 30 Hz suggests to me the pump manufacturer is concerned about motor heating, which is also a valid concern.

I usually set the minimum Hz in the VFD parameters, as it keeps the amateurs from messing it up. To make life easy and make scaling consistent if the min Hz changes during commissioning or later on I keep 4 mA as 0 Hz and 20 mA as 60 Hz. Otherwise you keep having to re-think the speed/signal relationship during testing and servicing. Having 4 mA = 30 Hz and 12 mA = 45 Hz is going to confuse everyone and really has no benefit.

Thanks for the input guys
I agree the pump manufacture is concerned about heat and the back pressure on the thrust bearing or lack of. I think I will set Hz limit in the VFD

Agreed with Tom.. set in vfd. You can always limit an input to 30 and above. Keeping your relationship with speed control constant.

The only benefit to setting 30hz to equal 4ma is the increased resolution available when the analog to digital converter does its conversion. In a precision speed system, a/d resolution can be really important but, in this case, for a fan or pump, it is of no consequence at all. So, in this case, I'd agree with Tom that its best to do what's most understandable.

forgot to mention the input I was referring to was the HMI.