Page 5 - Industrial Technology EXTRA 8th June 2020

braking, protecting electrical equipment and
improving power quality using harmonic filters to
protect delicate machinery on board.
With so many applications for resistors, there
are some important considerations to bear in mind
when choosing which resistor is suitable for the job
at hand.
The vessel’s function plays a large role in
dictating which resistors are suitable. In addition to
the ship’s size constraints and extra tests, such as
the specialist shock and vibration tests required for
the defence sector, permanent equipment such as
cranes may require additional resistors.

Using the elements
As power resistors dissipate energy as heat, marine
requirements such as ingress protection (IP) ratings
for enclosures can reduce airflow, modulating the
resistor’s ability to work effectively.
Challenges such as this provide the need for
temperature control. Resistors are typically cooled
by one of two elements — water or air. Air cooling
is traditionally a common cooling method, with its
robust fan technology and ability to withstand a bulk of the cables means that the drive motor has
significant amount of wear and tear without to work in reverse so that the cable reel can brake.
malfunctioning. In water-cooled resistors, pipes As its function switches from motor to generator, air-
containing a liquid coolant circulate around an cooled resistors are an advantageous addition to the
enclosure and out of the device to help keep process in order to safely dissipate this excess
components cool using the simple principal of heat energy. With heating elements enclosed inside a
exchange. fan-cooled cabinet, air-cooled resistors are suitable
Excess heat is thermally conducted by the water for deck mounting on the unit’s included anti-
in the pipes, which is transferred out of the device vibration mounts.
and regenerated for other purposes. Unlike air While air-cooled resistors are common in
cooling methods, the water can be stored in pipes applications such as lifts, hoists, cranes and
at temperatures below the application’s ambient conveyors, they are not always the best selection for
level and the different physical characteristics of the the job. Cressall has found that, in low voltage up
two coolant types result in faster cooling with to 1000V, high power applications where space is a
greater reliability. premium, liquid cooling takes the lead.
In applications such as cable-laying, which can Water cooling systems are already present on
involve laying anything up to a mile of cable over the majority of large ships. Some resistors on the
the side of the ship during passage, the substantial market including Cressall’s marine water-cooled

June 2020 • INDUSTRIAL TECHNOLOGY EXTRA! • p5