Low Voltage Effect On DC Motor

[Walt]

Hi All,

I have a 12 volt pump in a shallow water well with a battery and solar panel for power. The water tank has a reed switch float to shut off power to the pump when full. On several occasions the float has stuck and the pump ran the battery down. My question to ya'll: What effect will the low voltage have on the 12 DC pump in the well? I know there are electronic switches that will keep this from happening but was wondering how it will effect the pump motor.

Thanks,

Walt

[richhagen]

The situation is bad for both the motor and the battery.  The batteries life will be greatly shortened for basically every type the deeper you discharge it.  As for the motor, you could end up in a stall condition where the current is continually dumped into one set of coils and effectively cooks the insulation off of the windings.  Perhaps you could use a 12V relay with a bit of histerysis to turn on the power to the pump.  Run a diode or two or three in series with the coil to adjust the voltage at which it will click on, and loop a resistor from the output of the relay back to the coil input to prevent it from fluttering on and off when the pump kicks on, but not so low of resistance that it will allow the battery to discharge too deeply.  Of course, if your solar panels are adequate and your cistern is large enough, why not do away with the battery all together and use a similar setup with a large enough capacitance to keep it from fluttering too much. It will charge up the capacitors enough to turn the pump on for a bit, run them down and click off if the solar power is not adequate to keep the pump going continuously.  Of course leave the reed switch in series with the relay so it still shuts off when the tank is full.  

Rich Hagen

[Nando]

Depending on the motor you may not do damage to it if the motor was able to keep rotating for some time.

There is a solution to your problem and it is to add a low voltage relay if the Float switch does not work --

Also, you could use 2 float switches placed in series if one is stuck the other will operate.

Can you tell us what is the current of the pump motor, I may presume that is a brush motor type.

To be able to guess if you may have a winding problem due to the stuck float switch, please say the shallowness of the well and the total head that the pump operates on, from the well to the reservoir.

The ideal relay should be a step relay to save power -- a pulse for ON and another for OFF.

The best circuit is one that the float switch energizes and once it reads low voltage triggers the OFF pulse.

Let me know

Nando

[Opera House]

While this is a simple problem, the solution isn't.  Any voltage relay will continue to cycle in this condition.  Just a couple percent of recharge will raise battery voltage enough to trip a voltage relay before the battery is fully charged. If the voltage relay also has an interval timer to prevent turn on for several hours, that would help.  Without a battery, a diversion relay would have to be placed on the panel to simulate equivelant motor load.  Redundant switches is probably your best option.

[Walt]

Thanks for the responses.

Here is a little more information on my system. The well is 34 feet (10.4 meters) deep. It has a head of 12 feet (3.6 meters). The pump is set at a depth of 20 feet (6 meters). The cistern is small and hold 40 gallons (151 liters),used for wild game watering. Birds, squirrels, and deer.

The pump is a new LVM "Congo" submersible rated at 9.5 gallons (36 liter) per minute. At 12 volts it draws 6 amps. The head rating is 32 feet (9.7 meters) at 14 psi (.96 kg/cm2).

So far I have only been running the pump while I was there to keep an eye on it. It's located in my off grid part of heaven on earth that I plan to build a cabin for quite solitude.

Keep having fun.  

[Drives]

If you reduce the voltage to the DC motor it will reduce speed.  For a centrifugal pump this will give you reduced flow, and reduced current draw (less load) provided you are not at max head.  

The advertisement for the pump stated "The LVM111 is excellent as a fuel or water transfer pump, a marine bilge pump, or for a domestic holding tank water system. This is a continuously rated pump, and should not be run dry for long periods."

Therefore, provided you do not run the pump so slow that you can't overcome your head, and you keep some water flowing through it for cooling...you will be ok.  Actually, running the pump and motor at a slower speed will grant you an increase life span of the whole unit due to reduced current, reduced heat, and most importantly far fewer RPM's vs time.

[Argie]

Davies Craig automotive cooling water pumps, which replace the belt-driven mechanical ones,  are controlled by a setup that sends them only 3 volts at low temperatures (two seconds one three seconds off), and full voltage only when they reach the temperature you adjusted them for.

The brochures say 4 volts but I have seen between 2.86 and 2.98 volts, occasionally a minimum of 3.15 volts, as measured with two digital multimeters that seem to be very precise and I was wondering myself how much lower that voltage can get before doing any damage to the motor.