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I have a submersible pump and it's clearly stated in the user manual that it should not be operated without water.
But what will happen if it's not adhered to? The obvious answer is that it will malfunction. But I want to know what causes the malfunction to occur. I am trying to use it for some other purpose that does not require it to be submerged.
The motor will spin much faster than it does normally when loaded by the pump moving water. This higher speed may wear out something much quicker than during normal operation.
The system should be able to handle the higher speed for a few seconds, but most likely it was not designed to operate at that speed long term. The extra robustness probably wasn't built into the friction points to be able to handle it, especially when there is no water flowing and thereby removing heat. Obvious failure points include the packing that seals around the shaft to keep the water out of the motor, the bearings, and possibly the motor brushes if it has any. The system may also not be balanced for the high speed, putting additional stress onto the bearings.
One thing that is NOT a likely failure cause is the motor overheating. Since it is spinning faster, it will draw less current, so the heating in the windings will be less. Unless the water flow is designed to cool the motor, this is not the issue. The ones I've seen simply have a motor sitting atop a pump, with no special water flow around the motor.
As with any device, when you violate the specs, you can't be surprised when it stops working, perhaps in spectacular ways. If the specs say the unit is intended to work only when water is present, then that's what it's intended to do. You can speculate on why that might be, but ultimately you don't really known. Running it without water is a Bad Idea.
There are good answers here, but just wanted to clarify some points. It's important to consider what the liquid (in this case water) is used for in the pump.
To begin with, it helps to see that the majority of pumps are made of two main components - an electric motor, and the actual pump (impeller and volute) itself.
Starting with the pump itself, the water is used to lubricate the seal (normally a mechanical seal these days not gland packing).
The electric motor will require some form of cooling. On most motors a fan is fitted to the NDE (non-drive end) to blow air along the cooling fins on the side of the motor. Where the motor is required to be fully submerged, the motor is fully sealed (to IP66 or above) and the liquid is required to transfer heat away from the motor. In general you can ascertain whether a motor is air-cooled or liquid-cooled by the presence of a fan - if it has one it is probably air cooled. Therefore, a submerged motor should not be run for more than a few seconds without being immersed to prevent over-heating.
The point made earlier that a dry pump will over-run and cause damage to the rotating elements (bearings, etc) is also valid.
The pump can overheat and the electric motor can burn out.
There are pretty good answers but I feel like non of them are complete answers. 99% of electric pumps are Induction motor. So for rest of the discussion I will be referring to induction motor.
As @Fred said above yes, electric motor will eventually burn out. But as @ Olin Lathrop said yes, it will take lower power while running at high speed (under no load conditions). Then the Question is how can an induction motor burn out while consuming low power ?
Culprit is rotational friction load and properties of wire. On just starting electric motor, rotor speed keeps on increasing because there is no load/water. At rotating high speeds bearings will fail much quicker than expected leading to rapid increase in rotational friction. So load keep increases and motor speed slows down until if finally stops but power consumption keeps on increasing. Leading to higher currents. Higher the currents and higher the wire heating. Higher the wire heating, higher resistance again higher heating. At some point in time and some point on copper wire, copper might get hot enough to react with oxygen or water and we call it burn out. So its both mechanical properties of bearing and electrical properties of the wire leading eventual failure of induction motor.
The worth to know how a system fails so that worst situations can be avoided in design level. In above induction motor failure, its an series of events happening. If we could stop at least one event, we can save the motor from burning out.
The noryl impellers of the pump to add to the load after the bearings and pump body gets hot. They expand, rub to the walls get over heated then melt. So not only the motor gets burnt out, but the pump too gets damaged.
As the submersible pump is designed to be located inside water, the water will be natural cooler to the pump. If it is not located inside the water, clearly it will be overheat immediately just after started. Unless you modify the pump by adding cooling oil inside the tube/body, it will be just fine. But keep in mind that adding oil inside the body will add more drag. Drag of oil is probably thousand time of drag of air (check viscosity's ratio). But if you only intend to use the motor, then just open the casing and give external fan to cool the rotor, and give the suitable load to substitute the water load (i.e: fan with longer and heavier blades). I myself are very common to just take a motor from one purpose and use it to another purpose.
