Drawing water from a deep well requires a special motor that operates underwater, meters below the surface: the submersible pump motor. Unlike surface pumps, this motor is lowered into the well casing together with the pump and runs continuously under water. This operating condition determines critical design choices such as how the motor's winding is insulated, how it is cooled and whether it can be rewound if it fails. The most fundamental distinction is whether the winding is wet (water-filled) or dry (oil- or resin-filled). In this article we examine the wet vs dry winding difference, rewindability, cooling, the thrust bearing, the NEMA connection standard and supply criteria of the deep-well submersible pump motor from an engineering perspective.

Deep-well submersible pump motor and its wet/dry winding structure

Basic Structure and Dimensions of the Submersible Motor

Deep-well submersible motors have a thin, long body designed to fit the diameter of the well casing. Common diameters are standardized as 4 inch, 6 inch and 8 inch (and larger); the inner diameter of the well directly limits the motor selection. The motor is located below the pump; the shaft is vertical and drives the pump stages, and water is pushed up by the pump through the well casing to the surface.

These motors are designed to run vertically and operate for long periods in continuous (S1) duty. For general deep-well pump motor selection logic see deep-well pump motor selection guide and for vertical high-pressure applications multistage vertical pump motor.

Wet (Water-Filled) Winding

In a water-filled submersible motor, the inside of the motor is filled with water (or a water-glycol mixture). The winding wires are coated with a special insulation resistant to operating underwater; these wires are usually PE/PE2 or PVC waterproof-insulated wires (submersible winding wire). In this design:

  • The water inside helps cooling by carrying heat from the winding to the body wall.
  • The shaft bearings and sealing are designed for the water environment.
  • The biggest advantage is that the motor is rewindable: when the winding burns out, the motor can be disassembled, rewound with submersible winding wire and reused.

Wet winding is common especially in large submersible motors of 6 inch and above, and provides an operating-cost advantage in the long run thanks to the rewind option. For the efficiency and cost aspect of rewinding, our rewind vs new buy article offers a general framework.

Dry (Oil- or Resin-Filled) Winding

In dry-winding submersible motors the winding does not directly contact water; there are two main sub-types:

  • Oil-filled: the inside of the motor is filled with a special oil that conducts heat and provides insulation. The oil serves both cooling and lubrication.
  • Resin-filled / encapsulated: the winding is fully embedded in resin and isolated from water. This type is usually small-diameter (4 inch) and not rewindable; it is replaced as a whole when it fails.

Dry-winding (especially encapsulated) motors are a compact and economical solution at small powers; however, their rewindability is limited or none. Oil-filled types provide good cooling and lubrication but require attention regarding oil leakage and environmental sensitivity.

Wet or Dry? Decision Criteria

The choice depends on the balance of power/diameter, rewind expectation, water quality and cost: where high power and long life/rewind are priorities, the wet water-filled winding stands out; where small power, compactness and initial cost are priorities, the dry/encapsulated winding stands out. This decision affects the motor's total cost of ownership (TCO); for general TCO logic see our total cost of ownership (TCO) article.

The thrust bearing of the submersible motor and cooling by water flow

Cooling: The Importance of Water Flow

Cooling of the submersible motor is fundamentally different from surface motors. Since the motor is underwater, it cools by the water flow surrounding its body. As the pump draws water, water flows up around the motor body and carries away the motor's heat. Therefore it is critical that the water around the motor moves at a certain minimum flow velocity.

If the well diameter is much wider than the motor, or in an arrangement where the pump is above the motor and water rises without bathing the motor, the motor may not cool sufficiently and overheats. In this case a flow sleeve / cooling shroud is used to make water pass around the motor body. Dry running (running without water) is the most dangerous condition for a submersible motor; when the water level drops, the motor is left without cooling and burns out quickly. Therefore dry-run protection (level sensor, flow/pressure protection) is recommended. For temperature and heating management see our temperature rise class article.

Thrust Bearing

In a vertically running submersible pump, a large downward axial thrust is created as the pump stages push water upward. This force is transferred from the pump shaft to the motor and is taken up by the thrust bearing at the bottom of the motor. This bearing is one of the most critical mechanical components of the motor; an inadequate or damaged thrust bearing shortens motor life.

The thrust bearing is usually of the sliding (Kingsbury-type segmented) or large-ball type and is lubricated by water/oil. Correct power and head selection directly affect the thrust force and therefore the bearing load. Therefore the pump-motor match must be made with flow and head calculation; see centrifugal pump motor selection (flow-head). For general bearing and bearing life see our bearing types and life article.

NEMA Connection Standard and Pump Compatibility

The connection between deep-well submersible motors and pumps is designed according to the internationally accepted NEMA submersible motor connection standard. NEMA standardizes the shaft, flange and connection dimensions for 4 inch and 6 inch submersible motors; this allows pumps and motors from different manufacturers to be matched compatibly. This standard connection facilitates replacing the motor with an equivalent in case of failure and its compatibility with the pump.

For equivalent and direct replacement logic see replacing an old-brand motor directly and for correct matching from the nameplate booster pump motor replacement (selection from the nameplate). Wastewater and drainage submersible applications are a different class; for these see our submersible drainage and sewage pump motor article.

Supply and Commissioning Notes

The main points to consider when supplying a submersible pump motor: compatibility of well inner diameter and motor diameter (4/6/8 inch), required power and head, voltage/phase (single-phase or three-phase), cable cross-section and underwater splice, dry-run protection and starting method. Since the starting current matters in high-power submersible motors, a soft starter or star-delta may be preferred; see star-delta vs softstarter. Submersible motors are common in agricultural and irrigation applications; see irrigation and agricultural pump motors.

At first start, insulation (megger) measurement, rotation direction and checking dry-run protection are important; see commissioning checklist and insulation resistance and megger test. To explore the product family, see our pump, fan and blower motors category, and for a general start the electric motors section.

Water Quality, Sand and Sealing

Deep-well water is often not pure; it may contain sand, silt and minerals. This directly affects the submersible motor's sealing and bearings. The shaft seal (sand guard) and bearings of a motor running in sandy water must be protected against wear; otherwise sand enters between the shaft and bearing, causing rapid wear and leakage. In highly mineralized or salty water, the corrosion risk increases; in that case stainless steel bodies or specially coated motors are preferred. For salty and seawater applications, our seawater and saltwater pump motors article details corrosion protection.

In wet-winding motors, keeping the internal fill water clean and replacing it when needed is important for winding life. Before installing the motor, the internal water level and fill should be checked according to the manufacturer's instructions; a motor with trapped air may not cool sufficiently. Therefore submersible motors undergo a filling/venting procedure in the vertical position before being lowered into the well.

Cable, Underwater Splice and Electrical Connection

The supply to the submersible motor is made with a special submersible motor cable running meters long down the well. This cable has insulation resistant to operating underwater and is connected to the motor leads with a watertight underwater splice. This splice is the most critical electrical weak point of the submersible system; a poorly made splice takes on water, lowers insulation and causes ground leakage that stops the motor. Therefore the splice must be made with the manufacturer's recommended heat-shrink or resin underwater splice kit, in dry and clean conditions.

The cable cross-section must be selected based on motor power and cable length (voltage drop); in long wells voltage drop is not negligible and a thicker cable is needed. An insufficient cable cross-section causes low voltage at the motor terminal and therefore excessive current and heating. For cable, fuse and contactor selection based on rated current, see our rated current: cable, fuse and contactor selection article. For voltage drop and grid tolerance, our voltage tolerance and grid fluctuation article is useful.

Starting and Frequent Start-Stop

In submersible pump motors, the starting method matters for both power and water hammer. While direct starting is sufficient at small powers, a soft starter is preferred for high-power submersible motors; soft starting both lowers the starting current and reduces the sudden pressure surge, protecting the pipe and pump. Submersible motors are also damaged by frequent start-stop; each start produces high current and heat, and very frequent operation overheats the motor. Therefore the number of starts per hour should be limited, and the pressure tank/control system should be set to prevent the motor from running too frequently. For the heating effect of frequent start-stop see starts-per-hour limit and for soft starting star-delta vs softstarter.

Maintenance, Failure and the Rewind Decision

Since the submersible motor is deep in the well, removing it in case of failure (lifting it with the pipes by crane) is costly and time-consuming. Therefore correct initial selection and protection are very valuable in submersible motors; dry-run protection, an overcurrent/unbalance relay and correct cable cross-section reduce the need to pull the motor from the well early. When it does fail, wet-winding motors can be rewound; this should be evaluated against the cost of a new motor. For the general rewind-vs-new decision see rewind vs new buy and for submersible drainage applications grinder submersible sewage pump motor. For the seasonal heavy use of submersible motors in irrigation and agriculture, our irrigation and agricultural pump motors article helps with supply planning.

Frequently Asked Questions

Can a wet-winding submersible motor be rewound?

Yes. Wet (water-filled) winding motors can be rewound with submersible winding wire; this provides an operating-cost advantage especially in large motors of 6 inch and above. Dry/encapsulated (resin) small motors usually cannot be rewound and are replaced as a whole.

Why should a submersible motor not be run dry (without water)?

Because the submersible motor cools by the water flow around it. If the water level drops and the motor is left without water, cooling stops and the motor quickly overheats and burns out. Therefore a level sensor or flow/pressure-based dry-run protection should be used.

What is the difference between a 4 inch and a 6 inch submersible motor?

The main difference is diameter and power range. 4 inch motors are used for small wells and low-to-medium power, while 6 inch and larger motors are for wider wells and high power. Motors of 6 inch and above are mostly wet-winding and rewindable. The connection is matched according to the NEMA standard.

Get a Quote

We support you in selecting a deep-well submersible pump motor suited to your well diameter and your flow and head needs. For wet/dry winding choice, dry-run protection and fast supply, request a quote on +90 (532) 345 49 86 or via our contact page.

Deep-Well Submersible Motor Selection Checklist

  • Determine the motor diameter (4/6/8 inch) based on the well inner diameter.
  • Calculate the power and number of stages based on required flow and head.
  • Choose wet (water-filled) winding if rewinding is a priority; dry/encapsulated winding if compactness and initial cost are priorities.
  • Make sure the water flow cools the motor sufficiently; use a flow sleeve if needed.
  • Add dry-run protection (level/flow sensor).
  • Confirm the thrust bearing suits the application load.
  • Verify pump-motor compatibility according to the NEMA connection standard.
  • Define voltage/phase, cable cross-section, underwater splice and starting method.