How do I Monitor my Septic Pump?

[JimS]

I believe it is because the load of these types of pumps goes down when flow is blocked. The resistance to flow is from the acceleration of the fluid/gas as it passes through the blades. When flow is blocked, the substance runs around in a circle and doesn't load the motor. Listen to a vacuum cleaner for example. When you plug the inlet the rpms go up. A positive displacement pump is the opposite (piston type).

That's the way I think of it and a good way to explain it, IMHO. Centrifugal pumps and a lot of air blowers (such as in an HVAC system) behave the same. When the outlet is restricted the load goes down. If the inlet and outlet are wide open it can overload the motor. Here's a link talking about that effect. See particularly post #5.
http://forums.mikeholt.com/showthread.php?t=146363

You are correct that when the flow is blocked the load goes down allowing the motor to speed up but the current goes DOWN!

Here's a link that talks about current and blockage.

http://www.plctalk.net/qanda/archive/index.php/t-33130.html

You don't have to believe me - try it for yourself. Hook up the pump or blower (not positive displacement type!), put an ammeter on it, and run it blocked and open. Don't run it for too long blocked or just partially block it by pinching the pipe or whatever is fairly easy to do. If you run it for too long with no flow it may overheat even though the power and current are lower - there are still significant losses and heating.

 

[apostolakisl]

If the inlet and outlet are wide open it can overload the motor

This is why HVAC systems spec out plenum pressure range. You might think less back pressure is good. .. making life easier on the blower. . . but it is not. Too little and too much are both wrong.

I hooked up an old hvac unit in my workshop just to blow cold air for an hour here and there when I am out there. Despite not having any ducting (it just blows out) I still put a plenum on it and restricted flow.

 

[kurtmccaslin]

OK, so what is the best sensor to use to monitor amperage?

 

[sic0048]

As far as using the time between cycles... I originally thought this would be good but what if you go away for a day or two? The system won't see any pump cycling and trigger the failure alarm. I suppose you could suspend timing when the alarm system is armed in away mode... The run time should be fairly consistent though so this could be used to indicate that the pump is functioning properly. You could look at current but the motor could be running and not actually moving any sewage so timing the run time seems like a better method.

You could also monitor water coming into the house with a water meter. With that data, you could reliably know when the pump is suppose to run and not run. So if your pump doesn't run for a day, but there is no water coming into the house during that time period (because there is no one at the house), then the alarm is NOT set off. But if there is water coming into the house in large enough quantities, but the pump does not kick on after a certain period of time, then the alarm would be set off.

Another option would be if you don't have a water meter you can monitor, but you do have a water shut off valve that you can monitor. If the valve is shut off, then the alarm isn't sounded. That isn't as good as actually having a flow meter, but it could work OK too.

 

[maltesemanTS1]

OK, so what is the best sensor to use to monitor amperage?

Current doughnuts are cheap and easy to monitor. (http://www.omega.com/pptst/MFO_RCT.html)