Hello,
I would really appreciate your advice on the following:
I am making up my mind on how to design a 25 HP bowthruster system for my ship.
I will not bore you with all of the possibilities and limitations of the situation, but hydraulic power is not an option, the main engine cannot be used as a power source, and on board generators are too small. Direct diesel drive is problematic due to space constraints.
Performance specifications of the arrangement boil down to: 25 HP, 1600 nominal motor rpm +/- 10%, in practice 3 minute per hour max duty cycle. Cooling is not an issue. Batteries can be installed very close to the motor/motor controller. Re-charging of batteries is not a problem.
I am considering to use battery power to energise the bowthruster. It would be really nice if in the setup, we would have control over bowthruster rpm. Cost is an issue, but technical integrity (reliability/safety) of the solution is also important. I do realise that in some of the concepts below, we get to very high current levels, in the order of 600 amps peak.
I am getting the impression that it should be possible to use a VFD as commonly used to control AC motors. The idea would be to feed the DC bus that is part of common VFD design directly with battery power of a suitable voltage (say 192 V). This would bypass the rectifier arrangement of the VFD, but who cares? Combined with a more or less standard AC motor, Bob would be our uncle.
This seems possible, but there are a few snags: it is expensive, it requires rather a lot of batteries in a series arrangement, and it is an unproven concept.
I would see three alternative arrangements:
Reading up on the matter on various Electrical Vehicle sites, I have seen AC drives some of which run on voltages as low as 48, requiring specially wound motors (e.g. Curtis
Four 12 volt batteries in series, a DC-DC converter to transform 48 to 192 or higher voltage (feeding the DC bus as per base case above), a standard VFD and a standard AC motor.
Four 12 volt batteries in series, a PWM regulator, and a DC engine. This arrangement is common for lower power bowthruster applications, but relatively rare in the 25 HP range. See e.g.
Developments for some of these components are going fast these days. What was impossible yesterday, may be possible now.
I would be very grateful for any views / comments you may have