What Matters Most in Submersible Motors

The manufacture of specialty motors is a broad field – one type that BEN Buchele frequently produces is the submersible motor, which is used, for example, to drive pumps on suction dredgers.

Whether for dredging the navigation channel of the Main-Danube Canal or for wet extraction of quartz sand at a quartz sand plant – various customer projects have required submersible motors with special design specifications. The motors serve as drives for suction dredgers mounted on a ship’s pontoon. They drive an underwater pump on the lowerable suction structure, which is used to extract sediments and sand deposits so that waterways have the necessary width and depth for ship traffic – or so that quartz sand can be mined in a natural basin.

For underwater use, design considerations depend not only on standard key data such as power or operating mode but also on other factors. Since the motors operate completely submerged in water for all projects, specifications regarding water depth, temperature range, and immersion duration were critical. Whether the environment involves saltwater or freshwater – or even more aggressive liquids, such as in an underwater motor for a pump in a wastewater treatment plant – is also relevant for the motor’s design. The housing, paint finish, and seals must be correspondingly resistant.

Sealing is the be-all and end-all

The central challenge of an underwater motor lies in the sealing, which must be all the more reliable the deeper the motor is deployed. Absolutely no water must penetrate the interior of the motor. As a rule, protection class IP68 is therefore assumed. For less demanding applications, IP67 would also suffice (immersion depth approx. 1 m, for an immersion duration of approx. 2 minutes); however, BEN Buchele’s submersible motors are designed to IP68 for longer operations at greater depths. The motors are usually cylindrical with components that are as round as possible, as these are easier to seal. The classic terminal box at the rear of the motor is also cylindrical in shape and is secured with a particularly large number of screws to ensure a truly tight seal.

Application example: Main-Danube Canal

For the motors used to dredge the navigation channel of the Main-Danube Canal, cable connections were initially requested for the connection to the pump. However, so-called “external cables” are always problematic for submersible motors, as movement in the cables can create gaps, which in turn can allow water to enter. In addition, the size and weight were enormous due to the required power of the motor – 550 kilowatts – a size 400 motor weighing approximately 3 tons. With these dimensions, the cable diameter and thus the weight of the cables would also be extremely large (cables 30 meters in length were required). Because this did not seem practical, the company offered an alternative plug-in solution, which, although significantly more expensive, worked best logistically for the customer. For this purpose, a plug connection was provided on the terminal box, which was fully wired so that the customer received a plug-and-play-capable, securely sealed connection solution.

Risk of Condensation Openings

Every BEN Buchele submersible motor is equipped with a standby heater as standard. This ensures that no moisture forms inside the sealed housing after the motor is shut down – moisture that could otherwise accumulate and lead to condensation. Motors without a standby heater have condensation openings through which moisture can escape. Despite the standstill heater, the customer insisted on additional condensation vents; however, they forgot to install the sealing screws, allowing water to penetrate due to the 45-degree installation angle and rendering the motor inoperable. For this reason, BEN Buchele recommends limiting openings on submersible motors to a minimum and ensuring sufficient sealing.

For the quartz sand mining project, in which quartz sand is extracted via a suction pipe into a natural sedimentation basin, cable connections were requested. To prevent water from entering the motor here, a strain relief for the cables was provided. This consisted of a bracket that initially guided the first 30 cm of the cables rigidly, so that they could not bend around the seal and screw connection and thus create a gap. It should be noted here that the motor was significantly smaller than in the Main-Danube project and therefore required smaller and lighter cables.

The selection of cable quality is also important – depending on the immersion depth and duration, special submersible cables with robust insulation must be used so that the cables do not become saturated with water even during continuous underwater operation. The immersion depth and thus the water pressure are the primary factors in cable selection.

Water detector as a safety feature

A simple yet effective component that the electric motor specialist includes in all projects is a water detector that signals water ingress despite all precautions. This is not a highly technical system, but rather a two-bolt terminal block that is connected to the bearing shield on the drive side at the lowest point of the motor – depending on the installation orientation. If water accumulates between the two bolts, a contact is made, triggering, for example, an audible alarm, a warning light, or an emergency stop.

Don’t forget the details

As with all custom motor manufacturing, it is important to define all specifications early on. Typical design-specific parameters, such as power or speed, water depth, and temperatures (South Seas or Arctic), are common, but even seemingly minor details like the selection of connections matter. If cables can be used, is a plug connection more flexible? Does the plug-and-play option justify the higher investment costs? Subsequent changes are often possible and are readily implemented, but they can entail additional costs.