A low magnetic electric motor (or non magnetic electric motor) is an AC or DC motor with a reduced magnetic stray field signature.
Low magnetic or nonmagnetic electric [1] motors are used on board mine countermeasures vessels, minehunters, minesweepers and specific types of submarines. For about a hundred years sea mines or naval mines have been an established element in naval warfare. Modern naval mines nowadays react on magnetic, acoustic and pressure signals. By far the most important step in their development was the invention of the magnetic influence firing principle, which utilizes the magnetic interference field of passing ships. Naval mines with magnetic fuzes are effective, inexpensive and easy to deploy. An electric motor produces two types of magnetic fields: a dynamic alternating stray field, whilst running and a static remnant or residual magnetic during standstill. To avoid detection it is necessary to reduce [2] [3] [4] [5] these signatures at the source as much as possible.
Electric motors generate internal magnetic fields to rotate the rotor in the stator. A significant part of these magnetic stray fields emanate to the outside of the electric motor housing and can be measured and/or detected. [6] During running an electric motor also produce air borne and structure borne noises. The primary goal to avoid detection is to reduce these signatures as much as possible, to be able to operate in stealth mode. The measurements of ships' signature as a whole are usually performed at a magnetic ranging facility.
Low magnetic motors or nonmagnetic electric motors are designed to provide a reduced emanating magnetic stray field signature. [7] [8] These electric motors are manufactured from as little magnetic material as possible. The parts and components used to manufacture these electric motors are selected from materials with a low magnetic permeability. There are three major means of reducing the emanating magnetic stray fields of an electric motor: a dedicated electric and magnetic layout, electromagnetic shielding and additional compensating coils. [9] [10] Reduction and ways to achieve this are described in the American standard DOD-STD-2146, [11] the British Defence-Standard [12] 02-717 [13] and the German Standard BV3013. [14] Further reduction to the electric motors' magnetic signature, as well as the reduction of the ships' magnetic signature can be obtained by additional degaussing coils.
The primary source of air borne noise from an electric motor is the cooling fan to provide air to the electric motor. Reduction of air borne noise can be achieved by reducing the cooling air speed. An alternative is to use water or oil cooled electric motors. Air borne noise levels for different type of equipment on board Navy Vessels are laid down in the American military standard [15] MIL-STD-1474D, [16] the British Defence Standard 02-813 [17] or the Indian Naval Engineering Standard NES 847.
Structure borne noise in an electric motor is the result of roller bearing inaccuracies, rotor bar pass frequencies, magnetic unbalance, non-matching rotor and stator slot combinations, providing acoustic signals. Reducing structure borne noise can be done by various means. A correct rotor/stator slot combination, selected precision roller bearings, reduced magnetic saturation in the stator can help to reduce the structure borne noise levels. Methods of measurements and acceptance criteria are defined in the American military MIL-STD-740-2(SH) [18] and the British Defence Standard 02-813 [19]
Mechanical unbalance of the rotor will generate vibration. The vibration will result in structure borne noise, as well as unwanted acceleration forces or resonance, which eventually will harm or damage parts of the electric motor. To reduce the unbalance forces, precision balancing, as described in the American military standard MIL-STD-167-1A, [20] type I (internally exited) will be necessary.
Electric motors on board naval vessel may experience shock and vibration. A dedicated rigid design will allow electric motors to withstand these environmental impacts and provide sufficient active hardening. Passive hardening is done by installation of shock and vibration mounts onto the motor and/or system. Test method and limits are described in the American standard MIL-STD-167-1A, type II (environmental).
Due to the nature of naval applications, equipment on board of mine countermeasures vessels and submarines may be subject to underwater explosions. Therefore, low magnetic electric motors need to be shock proof or shock resistant. [21] Naval shock requirements on ships are specified in standards, such as the American standard NAVSEA-908-LP-000-3010 (Rev 1). [22] Equipment on board of naval vessels is specified in different standards, such as the American military standard MIL-S-901D, the British and Indian [23] [24] naval standard BR3021 or the German standard BV 0230. A large part of the environmental tests and measurements are stipulated in the American standard MIL-STD-810. Shock testing on equipment is performed by specialist institutes, such as TNO, [25] NTS Navy, Thales-ECC or QinitiQ.
An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate force in the form of torque applied on the motor's shaft. An electric generator is mechanically identical to an electric motor, but operates with a reversed flow of power, converting mechanical energy into electrical energy.
A mechanical or physical shock is a sudden acceleration caused, for example, by impact, drop, kick, earthquake, or explosion. Shock is a transient physical excitation.
A minesweeper is a small warship designed to remove or detonate naval mines. Using various mechanisms intended to counter the threat posed by naval mines, minesweepers keep waterways clear for safe shipping.
A magnetic bearing is a type of bearing that supports a load using magnetic levitation. Magnetic bearings support moving parts without physical contact. For instance, they are able to levitate a rotating shaft and permit relative motion with very low friction and no mechanical wear. Magnetic bearings support the highest speeds of any kind of bearing and have no maximum relative speed.
A brushless DC electric motor, also known as an electronically commutated motor or synchronous DC motor, is a synchronous motor using a direct current (DC) electric power supply. It uses an electronic controller to switch DC currents to the motor windings producing magnetic fields which effectively rotate in space and which the permanent magnet rotor follows. The controller adjusts the phase and amplitude of the DC current pulses to control the speed and torque of the motor. This control system is an alternative to the mechanical commutator (brushes) used in many conventional electric motors.
A minehunter is a naval vessel that seeks, detects, and destroys individual naval mines. Minesweepers, on the other hand, clear mined areas as a whole, without prior detection of mines. A vessel that combines both of these roles is known as a mine countermeasures vessel (MCMV).
The Gotland-class submarines of the Swedish Navy are modern diesel-electric submarines, which were designed and built by the Kockums shipyard in Sweden. They are the first submarines in the world to feature a Stirling engine air-independent propulsion (AIP) system, which extends their underwater endurance from a few days to weeks. This capability had previously only been available with nuclear-powered submarines.
U.S. MIL-STD-810 is a United States Military Standard that emphasizes tailoring an equipment's environmental design and test limits to the conditions that it will experience throughout its service life, and establishing chamber test methods that replicate the effects of environments on the equipment rather than imitating the environments themselves. Although prepared specifically for U.S. military applications, the standard is often applied for commercial products as well.
A rugged computer or ruggedized computer is a computer specifically designed to operate reliably in harsh usage environments and conditions, such as strong vibrations, extreme temperatures and wet or dusty conditions. They are designed from inception for the type of rough use typified by these conditions, not just in the external housing but in the internal components and cooling arrangements as well.
A reluctance motor is a type of electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. The rotor does not have any windings. It generates torque through magnetic reluctance.
HMS Pembroke is a Sandown-class minehunter of the Royal Navy. She was the second ship launched of the second batch of the class, which had several improvements over the first five ships built. The ship was posted for three years to the Persian Gulf between 2009 and 2012. Pembroke has since been deployed in international exercises and in historic ordnance detection in home waters. Pembroke was the first of the Royal Navy’s Mine Countermeasures Vessels to be fitted with the Oceanographic Reconnaissance Combat Architecture combat system to replace the previous NAUTIS combat system in early 2020.
The term unit load refers to the size of an assemblage into which a number of individual items are combined for ease of storage and handling, for example a pallet load represents a unit load which can be moved easily with a pallet jack or forklift truck, or a container load represents a unit for shipping purposes. A unit load can be packed tightly into a warehouse rack, intermodal container, truck or boxcars, yet can be easily broken apart at a distribution point, usually a distribution center, wholesaler, or retail store for sale to consumers or for use.
Acoustic quieting is the process of making machinery quieter by damping vibrations to prevent them from reaching the observer. Machinery vibrates, causing sound waves in air, hydroacoustic waves in water, and mechanical stresses in solid matter. Quieting is achieved by absorbing the vibrational energy or minimizing the source of the vibration. It may also be redirected away from the observer.
The Natya class, Soviet designation Project 266M Akvamarin, were a group of minesweepers built for the Soviet Navy and export customers during the 1970s and 1980s. The ships were used for ocean minesweeping.
Noise and vibration on maritime vessels are not the same but they have the same origin and come in many forms. The methods to handle the related problems are similar, to a certain level, where most shipboard noise problems are reduced by controlling vibration.
The Double Eagle is a remotely operated vehicle (ROV) built by the Swedish defence company Saab Underwater Systems AB and used for the disposal of naval mines.
Military Specification MIL-S-901D is for high-impact mechanical shock which applies to equipment mounted on ships. Its publication date was Mar 17, 1989. Two levels apply: Grade A items are items which are essential to the safety and continued combat capability of the ship; Grade B items are items whose operation is not essential to the safety and combat capability of the ship but which could become a hazard to personnel, to Grade A items, or to the ship as a whole as a result of exposure to shock. "Grade C", signifying that no shock qualification is required, is also sometimes referenced in acquisition documents even though the term has no official standing in the specification document. Qualification testing is performed on a specified machine or on a barge floating in a pond where an explosive charge is detonated at various distances and depths in the pond to impart shock to the barges.
Integrated electric propulsion (IEP) or full electric propulsion (FEP) or integrated full electric propulsion (IFEP) is an arrangement of marine propulsion systems such that gas turbines or diesel generators or both generate three-phase electricity which is then used to power electric motors turning either propellers or waterjet impellors. It is a modification of the combined diesel-electric and gas propulsion system for ships which eliminates the need for clutches and reduces or eliminates the need for gearboxes by using electrical transmission rather than mechanical transmission of energy, so it is a series hybrid electric propulsion, instead of parallel.
Electromagnetically induced acoustic noise , electromagnetically excited acoustic noise, or more commonly known as coil whine, is audible sound directly produced by materials vibrating under the excitation of electromagnetic forces. Some examples of this noise include the mains hum, hum of transformers, the whine of some rotating electric machines, or the buzz of fluorescent lamps. The hissing of high voltage transmission lines is due to corona discharge, not magnetism.
COMBIMAC is a manufacturer of special electric motors and centrifugal fans located in the town Emmen, the Netherlands.
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