Electric Motors for Marine Applications

 

A large number of vessels sail in more than one ocean, from warmer seas to arctic waters, so motors for marine applications must be versatile enough to operate in a variety of conditions. While the greater majority of my experience involves building motors for use in fin controls on mini submarines, robot arms on deep sea remotely operated vehicles (ROVs), deep ocean oil well tools, under ice scanners and scientific research support equipment, the same principles nevertheless apply to motors for use on any kind of workboat. For Arctic operators, there are myriad variables to consider, address and ultimately monitor.

 

The torque, speed and power requirements to accomplish a task are exponentially higher for motors that operate under water. Undersea movement is much slower than in air due to the power required to push through the water (viscous drag). Often, the power limits are not on the motor itself, but instead on the power supplies because batteries have limited capacity, especially at low temperatures. Battery life is a critical concern for ROVs, so it’s common to see performance specifications change as the math gets done to calculate the power needed to make a specific move.

 

Cables can supply power to undersea motors, but power loss between the drive electronics and the motor can be an issue. For example, a 2,500 foot cable is really a 5,000 foot cable on a roundtrip. A typical AWG 14 copper wire would present about 12.5 ohms of resistance. At 5 amps, and using ohm’s law, a 63-volt loss in the cable would be realized before it gets to the motor. 

 

Control of the system is another issue. While it’s desirable to have control topside, feedback delay and noise on the line are reasons to put the drive in proximity to the motor. A reasonable compromise is to place the drive electronics in a dry tank located near the motor; this way only power and high-level communications come down the cable. Fiber optics can be used to solve the noise and speed issue for more reliable communications.

 

Every marine application must address the effects of corrosion. For long-term submersion, the material of choice is 316 stainless steel. It is best if the steel is ‘passivated’ after machine work is done. Passivation maximizes the corrosion resistance of stainless steel by removing surface iron with an acid solution. While a number of boats use anodized aluminum to resist the effects of corrosion, when it comes to mechanical assemblies that are bolted together there are problems with the anodize coatings. To be specific, the coating is a hard anodize. This aluminum oxide coating is difficult to get into small holes, and the thickness of the ceramic layer is such that putting anodize over threads in a hole will typically make it impossible to install the bolts.  In addition, the actions of tightening the bolts will crack the protective layer exposing the aluminum to seawater corrosion.  

 

The corrosive effect of sea water is accelerated when there is a mixture of metals. Dissimilar metals in contact with seawater can form a kind of battery, resulting in rapid metal erosion known at galvanic corrosion. When necessary, such equipment can be protected by intentionally adding sacrificial metal blocks such as zinc. If one checks the anodic index of metals, it is possible to appraise the likelihood of such corrosion. For example, brass has an index of .4 while galvanized steel has an index of 1.2 – the 0.8 difference represents the voltage that will be generated between the two metals if they are in contact while submerged in an electrolyte such as sea water. In this example, the combination of brass and galvanized steel is almost certain to result in rapid erosion of one of the metals.

 

While corrosion to the exterior of a motor is to be mitigated, internal to the motor it is best to be avoided. The copper wire that is typically used for motor winding corrodes quickly,  but more  immediate is the fact that salt water is conductive,  so it will short out the electrical circuit. Sure, there are coatings, varnishes, and potting materials that are protective, however for long term submersion nearly all the organic insulation materials will absorb water. Hence, it is far better to avoid the problem by excluding the water from the assembly in the first place using oil filled and pressure equalized assemblies.

After taking all other variables into consideration, motors that perform in extreme cold do have unique issues. The viscosity of oil and grease increases at low temperatures. Since drag increases with viscosity or grease stiffness, it takes more power from the motors to overcome the stiff grease in the bearings or the viscous drag of an oil filled assembly.

 

A designer must consider the coefficient of thermal expansion (CTE) more carefully when making equipment for low temperatures. As materials get cold they shrink at different rates. If the CTE is not considered in the design, failures can result. A simple example is a bronze bushing being used as a bearing around a steel shaft. As the temperature drops, the bronze shrinks faster than the steel so the bushing clamps on to the shaft and prevents it from moving. 

 

Another issue facing Arctic operators is that steel and ferric metals become brittle at low temperatures, so gears and other mechanical assemblies subject to shock and high loads have to be significantly oversized to avoid breaking when temperatures plummet to -55 C.

 

Similarly, elastomeric materials become much less flexible, so dynamic frictional seals, typically used for shaft seals on motors are less reliable. Due to lower chemical activity at lower temperatures, battery output is degraded, and this can happen just as the demand for power is increasing. 

 

The end user or design engineer has to come to grips with general performance degradation at low temperatures. The choice is to specify performance when the system is cold – this significantly reduces performance at normal temperatures – or to provide performance versus temperature ranges.

 

Building an oil-filled, pressure compensated, waterproof motor is a complex task with several factors to consider. To do it right, one first needs to know the amount of oil in the assembly, the expected temperature range, the temperature at which the motor is to be filled, the final position of the cylinder after filling, and the total volume of displacement of the cylinder over its travel.

 

If the unit is completely filled with oil at room temperature, when the motor runs and the oil warms up, the oil will expand. If there is no pressure compensator (in this case, a piston in a cylinder), the hydraulic pressure will blow the seals out of the assembly.

 

On the other hand if the motor assembly is under filled at room temperature, when the unit gets cold, as in the Arctic Ocean, the piston will hit bottom, the oil will continue to shrink and the sea water under pressure will get inside the unit.

 

As industry and governments alike awaken to the new reality of expanding Arctic operations, many variables – unique conditions for properly built electric motors, for example – will come into play. In this case, ‘sweating the small stuff’ will pay off for stakeholders who hope to successes in a rapidly expanding, but still unfamiliar ocean environment.  

 

The Author

 

 

(As published in the October 2016 edition of Marine News)