Request

Our customer HollandMT was looking for a high-performance, air-filled submersible motor for its own client, Spaansen Marine Aggregates. Though this is a routine job for EN Motors, the accompanying requirements made for a complex assignment. Our restrictions:

• The motor must be as small as possible
• The motor is powered by a drive system with two 690 V frequency drives
• Operating point: 2000 kW at 300 rpm
• Overload capacity: 10% for 5 minutes per hour maximum
• The motor bearings must withstand high loads, as the impeller is mounted directly to the motor shaft.

Challenge

Our greatest challenge was to build a high-performance motor as compactly as possible. This violates several scientific laws, after all. Furthermore, due to the design of the frequency drives we were forced to “split” the motor internally to avoid exceeding the maximum current per drive. And because of the high loads generated by the pump, the bearings and shaft required careful consideration as well.
The design and high loads meant the bearings should be lubricated with oil and not grease, but as the motor is air-filled, the bearing system also needed to be hermetically sealed.

From IEC size 800 to size 710

We designed as efficient a laminated core with winding as we could. This allowed for good internal dissipation of the heat generated by losses within the motor. We accomplished that partially by developing a housing with a high cooling capacity. This way, we achieved the desired capacity while keeping the motor as compact as possible. Instead of IEC size 800, our motor’s size is 710.

Solution

The desired compactness of the motor is relative, of course. This type of motor, with such an impressive operating point, will always be quite sizeable. Going down by a single IEC frame size has a huge impact on both the weight and the dimensions, however. The weight affects the construction of the remainder of the vessel as well. A heavier motor would mean the vessel required a beefier hoisting installation, after all.

Two windings

The two frequency drives limited the drive system to 2x 1175 A. However, the motor’s nominal rating at full-load is 2120 A, climbing to as much as 2350 A at 10% overload.
To resolve the issue, we equipped the motor with two windings, each drawing 1060 A at nominal rating. This keeps our motor within the frequency drives’ maximum available current.

Cavitation

To create a more compact pump package, the impeller is mounted directly to the motor shaft. In practice, this means all the forces generated by the pump transfer to the motor’s bearings. We made our calculations based on the specifications in the customer’s loading diagram indicating radial loads of up to 140 kN and axial loads of 250 kN. Under certain conditions, however, so-called cavitation can occur in the pump during dredging, briefly generating an opposing axial load of approximately 1250 kN. This, too, we took into consideration. To withstand those forces, we designed a special bearing system with a double-row spherical roller bearing and an axial spherical roller thrust bearing. Both the bearings on the drive and non-drive ends of the motor are oil-filled and hermetically sealed. Using simulation software, we applied the Finite Element Method to the bearing system and motor shaft to calculate the required dimensions for the bearings and shaft. During this stage we again kept a strict eye on the size.

Shaft seals

The motor’s shaft seals have to keep the oil in and water out of the bearing system. This is achieved by means of a dual-action mechanical seal (with a shaft size of Ø 340 mm) that creates a seal based on an oil compensator with an over-pressure of 0.5 bar over the water pressure.

Result

That’s how we built the world’s most compact, largest air-filled submersible motor yet!
After subjecting it to extensive tests, the motor is now in operation at Spaansen Marine Aggregates.

Facts and figures:

• This motor weighs 22,000 kg. While still not exactly small, it’s a lightweight in its class.
• The submersible motor has a continuous output of 2000 kW at 300 rpm and can even reach 2200 kW at 300 rpm when overloaded.
• The motor has a nominal torque of nearly 64,000 Nm, 70,000 Nm when overloaded.
• The bearing system has a minimum life expectancy of 50,000 working hours.
• The high yield of 96.3% makes for a very energy-efficient motor.
• Furthermore, the desired power and torque have been “squeezed” out of a motor frame size 710.