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Motors on tough assignments

09 July 2012

Building motors that not only have to operate under extreme physical conditions, but which may also have to comply with strict output noise limitations, requires a specialist technology. David Melder highlights the challenges posed by naval and land-based military hardware design projects, and describes various types of motor that his company has developed for these technically demanding applications.

At a depth of 400m, the pressure on a submarine’s hull is in excess of 300 tonnes per square metre. When it dives below its operating depth towards its crushing depth, the bulkheads bulge and partition doors are routinely ordered to be opened to avoid jamming as the superstructure is squeezed under these phenomenal loads. Clearly, any motor that is expected to operate under such conditions is going to be rather special.

Designed to withstand the rigours of ocean-going surface and submarine environments, such motors need to operate faultlessly under conditions of extended immersion, exposure to salt spray, hostile weather, flammable atmospheres and temperature extremes, as well as resisting the effects of shock waves resulting from onboard weapons deployment and/or incoming enemy fire.

The author’s company has developed special ac motors to power a wide range of marine, naval and submarine services, including sonar and radar systems, fire pumps and mission-critical services. Motor construction and housings are optimised to meet the various levels of shock protection required for operational vessels (such as the Astute class submarine and Type 45 destroyer shown here), which might range from ‘mild’ shock (30g in all planes) to extremes of 340g.

Given that these motors are designed specifically for naval applications, their performance parameters may also be suitable for particularly demanding civil and industrial duties such as those calling for submersible operation, weather resistance, shock tolerance and operation at temperature extremes. Military standards pertaining to noise, shock and vibration, and extreme ambient conditions will also have some relevance to civil activities such as offshore drilling.

Two case studies
Motors onboard ships dedicated to the detection and disposal of mines have special design requirements. In particular, the use of standard ferrous materials is prohibited as these would risk detonation when attempting to locate magnetic mines, for example.

The author’s company has developed a special motor, designated Type 53, featuring a low magnetic material construction and electrical configuration as well as integrated strayfield compensation and degaussing to minimise their magnetic signatures. In addition, Type 54 motors feature stray field compensation coils, which automatically track the motor load to achieve significantly reduced strayfield signature with the use of ferromagnetic materials or complex electronics.

These low magnetic motors can be supplied in protection classes IP54 to IP68 with standard shock protection up to 60g. Such motors are currently in operation on board Royal Navy minehunters, such as those recently deployed off the coast of Libya during UN sanctioned operations by Britain and France.

Turning to land-based applications, motors required to power the hydraulics that position the plough on a minefield clearance vehicle need to be both rugged and reliable as well as having specific characteristics. A permanent magnet dc motor was developed for this application that is capable of operation under hostile conditions, in hot, confined environments.

A relatively high power output specification of 6.2kW meant that effective heat dissipation was a key consideration of the motor design. This was achieved by fitting a metal cooling fan and introducing a slotted body in the area of the brush gear. A thermal sensor and thermal cut-out were also incorporated. The low losses associated with this type of motor construction (rare earth permanent magnets and low-loss steel lamination) together with the motor’s low input current, also contributed to minimising heat output.

The motor, which is close-coupled to the hydraulic pump via a hollow shaft, operates from an 18-32Vdc supply, and is thus easily powered via a motor vehicle electrical system.

David Melder is with Astrosyn International Technology

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