Solar Powered AC Pump Dry Running Protection

[abicash]

Hello

I have developed a Solar powered 3-Phase Inverter, and it is working fine.

it is basically an MPPT based VFD to alter motor frequency based on an MPPT point.

I want to introduce a Dry running mechanism now.

What i did till now...

I measured a current say Imotor. From this value i determine Idry
Idry = Istart/2 (factor is determined by actually measuring both currents)
Wait for some time and measure Imotor, if it is below Idry, confirm dry running state and shut down motor.
But the problem is that whenever Sunlight is low, Imotor is anyways low and false dry run current is recorded.

How is this done?
Can someone share some thought?

Thanks and regards

[Wy_White_Wolf]

Add a circuit that measures pump temperature.

WWW

[russ]

outlet pressure with a time delay for starting = a few second delay only

[abicash]

Thanks guys for replying.

I have known both concepts.
I am actually looking for a sensorless control.
Is there a way it is done by measuring motor current.
Can someone shed some light?

[russ]

You can do that - it is unreliable though so you never see it done on an industrial basis.

[inetdog]

Thanks guys for replying.

I have known both concepts.
I am actually looking for a sensorless control.
Is there a way it is done by measuring motor current.
Can someone shed some light?

There are standard run-dry controllers for AC well pumps (Pumptec is the kind sold by Franklin pumps). They basically detect a DECREASE in pump current as the load decreases and the motor speeds up. (The closer to zero slip rate below synchronous speed, the lower the current.)

Just Google pump run dry protection for a lot of solutions, including an article on the subject from Grundos, a major manufacturer of inverter driven DC submersible pumps.

In your case, since you will be building a VFD, you will have to take into account the back EMF, current, and speed. No external sensors required.

Just keep firmly in mind that the requirements for run-dry protection are very different between a pump which will be free pumping into a head pressure with no flow restrictions and a booster pump which will often be running at little or no load if the flow volume is restricted. Motor current sensing will not work in the latter case.