PROPULSIONElectric Motors for Marine Applications: Empire Magnetics President Richard Halstead discusses the critical considerations necessary for engines operating in Arctic waters. By Richard HalsteadA large number of vessels sail in more C G M AONSIDERATIONS FOR ENERAL ARINE PPLICATIONSthan one ocean, from warmer seas to The torque, speed and power requirements to accom-arctic waters, so motors for marine plish a task are exponentially higher for motors that operate applications must be versatile enough under water. Undersea movement is much slower than in to operate in a variety of conditions. air due to the power required to push through the water While the greater majority of my ex- (viscous drag). Often, the power limits are not on the mo-perience involves building motors for tor itself, but instead on the power supplies because bat-use in ? n controls on mini submarines, teries have limited capacity, especially at low temperatures. robot arms on deep sea remotely oper- Battery life is a critical concern for ROVs, so it’s common Halsteadated vehicles (ROVs), deep ocean oil to see performance speci? cations change as the math gets well tools, under ice scanners and scienti? c research sup- done to calculate the power needed to make a speci? c move.port equipment, the same principles nevertheless apply to Cables can supply power to undersea motors, but power motors for use on any kind of workboat. For Arctic op- loss between the drive electronics and the motor can be erators, there are myriad variables to consider, address and an issue. For example, a 2,500 foot cable is really a 5,000 ultimately monitor. 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 desir-able 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 elec-tronics 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 cor-rosion. 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 corro-sion resistance of stainless steel by removing surface iron with an acid solution. While a number of boats use anod-ized 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 spe-A waterproof motor with stainless steel exteriorCredit: Empire MagneticsOctober 201644MN MN Oct16 Layout 32-49.indd 44 9/21/2016 3:27:28 PM