The post explains an automatic submersible pump start, stop circuit with dry run protection in order to implement an automatic ON/OFF switching of the motor in response to the high/low water levels of the overhead tank.
In one of the previous posts we learned a similar concept which also dealt with an automatic start/stop function of the submersible pump contactor button, however since here the sensors involved float switches, the design looked a bit complex and not suitable for everyone.
Moreover, the dry run protection included in the design relied on the temperature change of the motor for executing the required protection of the motor. This feature too was not too desirable for a layman since installing the heat sensor over the underground motor was not easy.
In this post I have tried to eliminate all these hassles and designed a circuit that is featured to sense the water presence solely through metal sensors immersed in the relevant water sources.
Let's understand the proposed Automatic submersible pump start, stop circuit with dry run protection.
A single IC 4049 can be seen engaged for the entire sensing, start stop actions and the dry run protection execution.
The gates involved here are 6 NOT gates from the IC 4049 which are basically rigged as inverters (for inverting the polarity of the fed voltage at its input).
Let's assume the water inside the over head tank goes below the desired lower threshold, as indicated in the above diagram.
The situation removes the positive potential that ws supplied through the water to the input of N1. N1 responds to this by causing a positive to appear at its output pin, which instantly causes C1 to begin charging via R2.
The above condition also allows the positive from the output of N1 to reach the input of N2, which in turn produces a low or a negative at the base of T1 via R3....the associated relay now toggles ON and activates the "START" button of the contactor....however the relay activation is sustained only for a second or so until C1 is fully charged, this length may be set by appropriately tweaking the values of C1/R2.
For the moment let's forget about N5/N6 stage which are positioned for the dry run protection implementation.
Let's assume the pump is running and pouring water into the shown OH tank.
The water now begins filling inside the tank, until the level reaches the brim of the tank "kissing" the sensor corresponding to the N3 input.
This allow a positive through the water to feed the input of N3, enabling its output to go low (negative), which instantly causes C2 to begin charging via R5, but in the process the input of N4 also becomes low and its output inverts to a high prompting the relay driver to activate the relay.
The upper relay instantly activates but only for a second, toggling the "STOP" button of the contactor, and halting the pump motor. The relay timing may be set by appropriately tweaking the values of C2/R5.
The above explanation takes care of the automatic water level control by toggling the submersible start/stop button through the circuit's relays. Now it may be interesting to learn how the dry run protection is designed to prevent a dry run hazard in the absence of water inside the borewell or a underground tank.
Let's go back to the initial situation when the water in the OHT has fallen below the lower threshold and rendered a low at the input of N1....which also renders a low at the input N5.
N5 output turns high due to this and provides a positive supply for C3 so that it can begin charging.
However since the process is also supposed to start the motor, if water is present, the pump may start pouring water in the OHT which is supposed to be detected by the input of N6, causing its output to go low.
With N6 output at low, C3 is inhibited from charging, and the situation stays stalemate...and the motor continues to pump water with no change in the previously explained procedures.
But, suppose the motor experiences a dry run due to an absence of water in the well....as stated above C3 begins charging and the output of N6 never turns negative to stop C3 from charging fully....therefore C3 is able to complete its charging within a predetermined span of time (decided by C3/R8) and finally producing a high (positive) at the input N3.
N3 responds to this in the same way as it would do when the water in the tank is detected at the uppermost threshold....prompting the switching of the upper relay and stopping the motor from running any further.
The dry run protection for the discussed submersible pump start, stop circuit is thus executed.
R1,R4,R9 = 6M8
R3,R7,R6 = 10K
R8 = 100K
R2,R5,C1,C2,C3 = to be dteremined with experimentation
N1------N6 = IC 4049
ALL DIODES = 1N4007
RELAYS = 12V, 10AMP
T1 = BC557
T2 = BC547