Page 45: of Marine News Magazine (July 2015)

Propulsion Technology

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run to stall, the motor life will be signi? cantly shortened. hauling in slack line faster than possible with an across the

The ability of the VFD to slowly ramp the motor during line starter. starting and stopping also adds a level of safety to capstan Lastly, the ? exibility with which you can con? gure a VFD operation. Capstans can be inherently dangerous because system compared to an across-the-line starter can make cus- the operator must control the loose end of rope that is tomizing the system to match operations much easier. A va- spooling off the capstan head. If the capstan starts or stops riety of operators can easily be added such as footswitches, abruptly, it can lead to a loss of balance by the operator. handheld pendants with multi-speed operation, or radio

Ramping functions can eliminate this safety concern from remote control. Many drives have outputs with the ability operating capstans. to monitor the status of the drive from a remote location.

Another speci? c side effect on vessels occurs when start- Also, con? gurations are password protected to add another ing an across-the-line electric motor, the generator and layer of security to the system from unauthorized personnel other electrical systems can be affected by the high inrush attempting to change limits in the VFD. current during starting or stalling. If the generator is al- If you have an application where adding ? exibility and ready running near capacity, the high current can cause equipment protection is necessary then a variable frequen- unacceptable voltage sag. This can lead to brownouts and cy drive may be the right solution. While a more costly also cause other sensitive equipment to fault or malfunc- initial purchase than an across-the-line starter, the payback tion during motor starting. This effect is easily removed period will be brief with the increase in equipment life, with the ramped start and torque limiting of a VFD. safety, and ef? ciency.

The slightly more obvious advantage of utilizing a vari- able frequency drive is the ability to vary the speed to suit

David Hughes has worked as a design the conditions required. With deck equipment, it may be engineer in the commercial marine industry advantageous to be able to slowly haul in line when ? nesse since 2011 at Superior Lidgerwood Mundy. is required. If the motor is rated to handle operating at a

Here he has worked to apply modern controls speed higher than 60 Hz, a drive can also operate in a con- technology to proven capstan, winch, and stant horsepower fashion often to 90 or 120 Hz depending hoist designs to improve ef? ciency, safety, on the drive; meaning that as the speed increases above 60 and user friendliness.

Hz, the torque is reduced proportionately. This allows for 45 MN

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