Power resistors, able to withstand and dissipate large amounts of power, have a wide range of applications within the marine sector. Resistors are required for many purposes including dynamic braking, ship stabilisation 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.
As power resistors dissipate energy as heat, marine requirements such as ingress protection (IP) ratings for enclosures can reduce airflow, modulating the resistors’ ability to work effectively. However, water and oil cooling systems are already present on the majority of large ships. Some resistors on the market including Cressalls’ water-cooled range are able to use these cooling systems, but careful planning is required prior to installation.
Resistance and power rating
Each application that requires a power resistor will have unique requirements, including electrical resistance and power rating.
The resistance, measured in ohms (Ω), determines how effectively the resistor transforms the electricity into heat energy, whereas the power rating, measured in watts (W), is the amount of power that can safely pass through the resistor without risking damage.
While their practical applications are distinctly separate, the properties are intrinsically linked as power is equal to the current squared, multiplied by resistance. As the power rating is measured and defined at standard conditions and is extremely dependent upon the final operating conditions, changes should be expected when used in the marine sector. Therefore, it may be wise to install a resistor with a power rating in excess of what is expected.