Marine resistor design

DIVING INTO MARINE RESISTOR DESIGN

EV2 modular resistor for electric vehicles

DESIGN CONSIDERATIONS FOR OFFSHORE ELECTRICAL COMPONENTS

Covering over 70 per cent of the Earth’s surface, the oceans are a vital element of our planet’s ecosystem. However, for the millions of vessels that cross them, the aquatic environment can present a problem. Vessels are increasingly using electrical systems to power across oceans, but a component’s design must account for these extreme conditions.

Whether for main propulsion propellors, crane or lifting systems, or cable laying, electrical drives can be found at the heart of many marine operations, offering increased control, reliability and mechanical simplicity. Dynamic braking resistors (DBRs) are an essential part of an electric drive system that remove excess energy from the system when braking to either dissipate as heat if system is not receptive to regeneration or if system is receptive, but energy level goes beyond the system limits, so needs to be removed.

When designing electrical components for offshore applications, material selection is key from the start of the process to guarantee that equipment will perform under harsh conditions, including saline atmosphere, high wind loadings and corrosive sea water.

Engineers tasked with designing resistors for marine applications must consider material choice, structural stability and cooling method.

CORROSION-RESISTANT MATERIALS

Sea water and the saline atmosphere is corrosive, which could leave equipment inoperable. Due to this, stainless steel, combined with special paint systems, is typically used for the enclosure metalwork for resistor elements. With materials containing at least 10.5 per cent chromium, stainless steel reacts with oxygen in the air to produce a protective layer on its surface to prevent corrosion if not painted.

There are many grades of stainless steel that can offer high corrosion resistance, which can be further enhanced by the addition of extra elements. For below-deck applications, 316 and 304 stainless steel contain nickel to broaden the protective layer created by the chromium, and can be used in unpainted condition.

However, for above-deck components, 316 stainless steel has a higher nickel quantity and added molybdenum, so the resistor unit’s metalwork receives optimum protection against the marine atmosphere, but in some conditions, painting will also be required. Cressall’s resistor enclosures for the EV2 resistor terminal cover boast at least an IP56 ingress protection rating, certifying that sea water cannot enter the unit to cause harm.

In addition to the exterior, it is important that the resistor’s element can withstand the harsh conditions. For these applications, Alloy 825 sheathed mineral-insulated elements are less vulnerable to atmospheric corrosion. As the element in encased within the mineral insulated sheathing, the sheath is at earth potential, so if water or high humidity is present this will prevent accidental contact with the live element, making them a much safer choice for marine applications.

STRUCTURAL STABILITY

Weather at sea is unpredictable, so vessels must be able to withstand the large variance in wind and harsh sea conditions found worldwide. Many offshore structures such as wind turbines are located in areas with high winds, so if the system requires resistors to help provide stability to their electrical components these considerations must be considered within a resistor’s design.

Considering the impact of a vessel’s rotational motions — its side-to-side motion, or pitch, and its front-to-back motion, or roll, is crucial. Design engineers need to ensure that there is enough mechanical support in the structure to stabilise the resistors for safe operation when it is subjected to these forces.

Cressall can conduct finite element analysis (FEA) to help ensure structural stability. FEA allows design engineers to predict a product’s performance in the real world, then see the impact of forces and make changes accordingly. This ensures the resistor performs well in the potentially extreme weather conditions.

It’s also important to consider the size constraints of marine applications. In contrast to onshore units, offshore electrical components must fit into a compact area, so the size of the unit’s support structures must be minimised without compromising durability.

COOLING METHOD

An essential part of a resistor is its cooling system. Since the resistor dissipates excess energy as heat, the cooling system is responsible for cooling the resistor element to ensure continued operation. Depending on the layout and resources of the system, resistors can be naturally or forced air or water-cooled.

Air-cooled resistors come in two types — forced and naturally cooled systems. Forced cooling systems use a fan to dissipate heat in a compact space. These units are suitable for deck mounting and can be secured using anti-vibration mounts. Natural cooling is the most common in marine applications, offering a higher power rating and can be mounted in machinery spaces, protected environments or on deck. For machinery spaces or protected areas, consideration should be given to how the hot air released from the resistors should be evacuated to ensure other equipment mounted locally does not overheat.

Alternatively, the cooling system can use the vessel’s chilled water system, which circulates cool water for air conditioning and equipment cooling. If the chilled system uses sea water, titanium-sheathed elements with super duplex steel metalwork can be incorporated, for continuous use in acidic, tropical sea water and downgraded to 316 stainless steel for freshwater systems.

The ocean is a valuable asset for energy, transport and trade. Ongoing development of electric drives for marine applications can be challenging, but taking these conditions and energy savings into account makes them a viable and advantageous option for powering vessel and for use in offshore structures.

CR470

Used to the unusual

DESIGNING BESPOKE POWER SOLUTIONS FOR DEMANDING APPLICATIONS

Entrepreneur Henry Ford’s automotive legacy may seem everlasting, but his words on customisation certainly belong in the past. Credited with once saying “you can have any colour you want as long as it is black,” customers nowadays no longer seek a one-size-fits-all solution. The wealth of applications that require power solutions means that product design often comes in a variety of shapes, sizes and power demands. But what must we bear in mind in order to achieve a bespoke product range?

INDUSTRY AND APPLICATION

Whether the resistor is destined for an automotive application or a marine setting, its environment is an important consideration.

In marine and offshore applications, a design could use a range of suitably rated resistor elements such as Incoloy-sheathed mineral insulated elements that are highly resilient to physical damage and safer to use in harsher, corrosive environments. Designing enclosures with a suitable Ingress Protection (IP) rating is also an important factor when supplying to customers in harsh environments.

On board ships, space is often particularly restricted in machine and engine rooms where resistors are usually installed, because they are tightly packed with equipment. In this case, resistor manufacturers may need to design a more compact solution so that the equipment can fit safely on board without taking up a great deal of space and weight allowance.

COMPUTATIONAL FLUID DYNAMICS

Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and algorithms to visualise how gas or liquid flows in certain applications. CFD uses equations that describe how the velocity, pressure, temperature and density of a fluid are interconnected.

Design engineers can use CFD to help them make the most out of their equipment’s unique surroundings, and use them to their advantage. Returning to the offshore example, engineers can assess the wind or wave force that an enclosure used to house electrical equipment is subjected to without needing to physically build it.

Taking things one step further, CFD can also be used to analyse water flow inside water-cooled resistors and better understand the natural air convection of enclosures and to deliver a solution that is bespoke to these unique elements.

THEM’S THE BREAKS

Dynamic braking resistors (DBRs) are an essential component in elevator operations. Without them, the lift wouldn’t slow down in the time determined by the drive. It is therefore critical that the system works every time, without fail.

An elevator in a local supermarket wouldn’t be tasked with the same load as one carrying passengers to the top floor of The Shard. Therefore, custom resistors must exactly match the elevator manufacturer’s design specifications.

Before providing the right resistor, Cressall first evaluates the energy per stop, the duty cycle and the ohmic value. The first two are typically considered as a single variable — the required power of the resistor. The energy per stop is the sum of the kinetic, rotational and potential energies, minus any frictional losses and any electrical losses in the motor or inverter system.

Because all the energy produced by the braking process is used in heating the resistor, the characteristics of the duty cycle are critical before specifying the right size for the DBR in order to reduce heating. With these calculations, we can be sure that we are providing a DBR that is bespoke to the individual elevator, helping to deliver unprecedented security where safety is a top priority.

Customisation extends far beyond having the latest car in a stand-out colour. For some industries, their unique demands mean that an off-the-shelf model simply won’t suffice. In these cases, building a relationship with a resistor manufacturer that has over 100 years’ experience in designing and manufacturing resistors can help make sure the size, shape and power demands of the finished product are as unusual as required.

CRE389