Key features of underwater motors
The production of special motors is a broad field – one of our most common models is the underwater motor, which is used, for example, as a pump drive on suction dredgers. In this blog post, we explain what is important when designing underwater motors.
Whether for dredging the navigation channel of the Main-Danube Canal, dredging work in canals in the Netherlands, or the wet extraction of quartz sand in the quartz sand works – various customer projects required underwater motors with special design requirements. The motors are used to drive suction dredgers that are located on a ship’s pontoon. They drive an underwater pump on the lowerable suction structure, which is used to extract sediment and sand deposits so that waterways have the necessary width and depth for shipping traffic – or so that quartz sand can be extracted in a natural basin.
As with all electric motors, the key data such as power, speed, or operating mode (always S1 mode in the projects mentioned) are queried first. However, other factors are also important for underwater use. As the motors run entirely in water in all projects, information on water depth, temperature range, and immersion time was crucial. The liquid involved, be it salt water or fresh water or even more aggressive liquids, as is the case with an underwater motor for a pump in a sewage treatment plant, is also relevant for the design of the motor. The housing, paintwork, and seals must be correspondingly resistant.
Sealing is the be-all and end-all
The central challenge of an underwater motor is sealing, which must be all the more reliable the deeper the motor is used. Water must not be allowed to penetrate the inside of the motor. Protection class IP68 is therefore generally assumed. For low requirements, IP67 would also be sufficient (immersion depth approx. 1 m, with an immersion time of approx. 2 minutes), but our underwater motors are designed for longer use at greater depths in accordance with IP68. The motors are usually cylindrical with components that are as round as possible because they are easier to seal. The classic terminal box at the rear of the engine is also shaped like a cylinder and is fitted with a very high number of screws to ensure that it is really tight.
Main-Danube Canal application example
Cable connections were initially requested for the motors used to dredge the navigation channel of the Main-Danube Canal for the connection to the pump. However, so-called “routed cables” are always critical for underwater motors, as movement occurs in cables, which in turn can cause gaps and thus favour the ingress of water. In addition, the size and weight were enormous due to the required power of the motor of 550 kilowatts – a Type 400 with a weight of approx. 3 tonnes. With these dimensions, the cable diameter and thus the weight of the cables would also be extremely large (cables of 30 metres in length were required). As this did not seem practicable, we offered a plug-in solution as an alternative, which was significantly more expensive, but worked best for the customer in terms of logistics. For this purpose, a plug connection was provided on the terminal box, which was fully wired by us so that the customer received a plug-and-play-capable, securely sealed connection solution.
Risk from a condensation opening
This dredging project also showed what can happen if openings are not properly sealed. Every underwater motor is equipped with a standstill heater by default. This ensures that no moisture can form in the sealed interior after the engine is switched off without heating and lead to condensation. Motors without standby heating have condensation openings through which moisture can escape. Despite the standby heating, the customer insisted on additional condensation water openings, but forgot the screw plugs so that water was able to penetrate due to the 45-degree installation position and render the motor inoperable. For this reason, we recommend minimising openings on underwater motors and ensuring sufficient tightness.
Cable connections were requested for the quartz sand mining project, in which quartz sand is extracted from a natural sedimentation basin using a suction pipe. To prevent water from entering the motor, strain relief was provided for the cables. 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 gland, thereby leading to a gap. It should be noted here that the engine was significantly smaller compared to the Main-Danube project and required correspondingly smaller and lighter cables.
The choice of cable quality is also important – depending on the immersion depth and duration, special underwater cables should be used that have robust insulation so that the cables do not become saturated with water, even when used continuously underwater. First and foremost, the immersion depth and therefore the water pressure are decisive when selecting the cable.
Water detector as a safety function
A simple but effective element that we include in all projects is a water detector that indicates the ingress of water despite all precautionary measures. This is no high-tech system, simply a two-bolt terminal board that is connected to the drive side at the lowest point of the motor – depending on the installation position – in the end shield. If there is water between the two bolts, a contact is made and an acoustic signal, a warning light, or an emergency stop is triggered, for example.
Logistical challenges
It wasn’t so much the design, the special requests for connections and cables, or the sealing which were the most challenging aspects of our underwater motor orders, but rather the logistics surrounding them. At a weight of three tonnes, the Type 400 is not easy to handle. Everything starts with the production of certain large steel parts, for which, in some cases, extra-large machines were purchased. Transport was also difficult – the weight was too heavy for the company’s own lift, so the motors had to be manufactured entirely on the ground floor and all process steps had to be relocated here. In addition, there was no suitably dimensioned immersion tank available for the necessary leak test. Normally, motors are suspended in an immersion tank, pressurised from the outside inwards, and then monitored to see if air bubbles emerge. Instead, our oversized underwater motors were suspended in the yard, pressurised with compressed air, and then sprayed with a leakage spray. The formation of bubbles at these points also indicates a leak – however, our motors passed this leak test without any abnormalities.
Paying attention to detail
As with all customised motors, it is important to define all specifications at an early stage. Typical design-specific parameters, such as power or speed, water depth and temperatures (South Sea or Arctic), are familiar, but supposedly minor details, such as the choice of connections, are also important. Can cables be used, is a plug connection more flexible, does the plug-and-play variant justify the higher investment costs? We work with customers to analyse the entire application and also advise on the details. Subsequent changes are often possible and we are happy to implement them, but these may incur additional costs.