Selecting a suitable motor for an application always means weighing up multiple factors. Safety requirements, local conditions, and installation costs had to be taken into account for a boat’s on-board pump motor, designed to pump water from the bilge in an emergency. The result was an air-cooled motor with a slipping clutch that disengages the fan when the motor is flooded in order to prevent motor failure.
A shipbuilder needed an electric motor for a pump that could remove water from the engine room of a yacht quickly. Other specifications included a power output of 36 kW and a speed of 3,600 revolutions. S1 continuous operation was specified as the operating mode, as the motor is in permanent use when large quantities of water need to be pumped out. In addition, the customer required protection class IP68, meaning that the motor had to be absolutely watertight, as the motor itself could be submerged in water and therefore had to be suitable for underwater use.
A powerful motor for emergencies
It is normal for water to penetrate the hull and collect at the lowest point in the bilge – including when waves crash over the deck in rougher waters. In the event of heavy flooding, a pump is needed to remove the water quickly. As such, the motor is not in constant use, but is only needed for emergencies. It is difficult to quantify how often such flooding can occur, but the motor must be able to run at full power in continuous operation if such an incident occurs.
Another customer specification was the installation position – not mounted horizontally on the pump itself, as is usual with pump motors, but vertically with the shaft projecting downwards. The key challenge in the design was preventing the motor itself from being flooded, as it needed to continue operating underwater. We proposed three options to the shipbuilder:
Safe, powerful, and compact: the water-cooled motor
The safest and most elegant solution was a water-cooled motor, which is very compact and requires little installation space, but has a high output and – thanks to the water cooling system – does not emit any heat into the environment. Such a motor is fully protected against water ingress and continues to operate reliably even under water. A water-cooled motor requires a cooling unit, which means a little more cost and effort. It was suggested that the motor could be integrated into an existing water circulation system on board, which would reduce the work involved somewhat.
High material usage with unventilated motor
Alternatively, there was the option of installing an unventilated motor, which dissipates heat through the surrounding air and moisture. However, unventilated motors are much larger. A size 280 would have been necessary for these requirements, which would have meant using a lot of materials and thus higher costs. In addition, cooling through the ambient air does not work as well as fan or water cooling, especially as engine rooms can be hot environments. Nevertheless, as the ambient temperature specified by the customer was 45 degrees Celsius, our designers did consider an unventilated motor.
Air-cooled motor with slipping clutch
Option three was a much more compact size 200 motor with an integrated fan. Overall, this variant was the most favorable solution for the pump motor. However, fan operation is risky if the water rises too high. In this particular case, the fan is located above the motor, so the water reaches it last. But in an emergency the fan has to work against the high resistance of the water, which introduce losses into the motor. The motor could overheat and fail despite the cold water surrounding it.
To prevent this scenario, we developed a motor model with a fan and a slipping clutch. The slipping clutch is a safety device used on products such as conveyor belts. If forces occur that prevent the motor from continuing to operate smoothly, the slipping clutch disengages and prevents the motor from overheating. With a fan-driven motor, the coupling is positioned between the shaft and the fan. It has friction surfaces that offer a certain resistance. If the counter-resistance caused by the fan trying to rotate in the water becomes too great and reaches a certain torque, the frictional surfaces slide apart and the fan disengages. The motor can continue to run without a fan, as the surrounding water has a cooling effect at this point. As slipping clutches are standard products, the air-cooled motor is attractively priced, not least due to its smaller size and lower material usage, even with the additional slipping clutch function.
Additional safety devices
Two additional safety monitoring tools are part of every motor of this protection class (IP56 and upward): a standstill heater and a thermal sensor. The standstill heater ensures that the motor is heated even when it is not running, thus preventing the formation of condensation. A thermal sensor is also integrated into each motor to protect the winding from overheating.
Decision: air-cooled motor with slipping clutch
After weighing up all the parameters and costs, the shipbuilder opted for our concept of a fan-cooled motor with integrated slipping clutch. Given that emergencies that necessitate the use of the motor are likely to be infrequent, this option best met the technical and economic requirements.