Posted by Steve Dashew (April 2, 2008)
When we designed the systems for Wind Horse, a key component was the theoretical ability of her Mastervolt inverters to work in phase with the generator so that if necessary, the inverters would handle excess load. In theory, this allowed a smaller genset (our is just 8kW) which would be run at load most of the time. After four visits by techs, the replacement of the initial pair of Mastervolt units, and a general recall on units which were still not functioning correctly, we gave up. At the suggestion of suggestion of Todd Rickard, now project manager for the FPB 64 program, we installed Victron inverter chargers. After a year of use we are happy to report that these units indeed work as advertised. Let’s go back a minute for some generator theory. Gensets typically are happiest at about 80% load. This is where they get the best efficiency from their diesel engines, and where they will give you the longest time between overhaul (TBO). The problem comes in matching the genset to the load. Take Wind Horse, for example. During a typical genset cycle, the washing machine is on (light load) and then the heating coil of the dryer will operate at different levels during the drying cycle (medium to high load). We are probably OK with the two 16,000 BTU salon air conditioners running as long as they are not starting and stopping. The starting current on a compressor can be three to six times the running current. If you total the wash/dry air con loads they will come in at just under 8kW. But in the real world this does not work because the loads are varying, and when you add the motor start current, the genset bogs down. On other hand, the genset is often loaded at just four or five kW if the drying element is off and one of the compressors are off – which happens often. And during this period the genset is underloaded. In the olden days you sized the genset for the largest load, but then it usually ran at light load most of the time. The Victrons solve this problem in two ways. First, they sense the genset load and at a point which we set, the inverter kicks on, matches the wave form of the genset AC power, and adds the needed extra electricity to the circuit. This way when a load temporarily comes onto the circuit above the preset point – in our case 28 amps (at 230 volts) – the Victrons supply the additional power from the batteries. But wait…it gets even better. The Victrons are smart enough to know we want to keep the genset loaded at 80%, or 28 amps. So, when the load falls below this level, the Victrons switch to battery charging mode, and use whatever is left of the gensets available generating power (to our preset 80% loading) to charge the batteries. This way the genset runs at an even load most of the time. The same thing works with shore power too. Take the temporary slip in which Wind Horse presently resides in Marina del Rey. The normal occupant has a 100 amp/380 volt circuit. So, we have to use our 80 foot 16 amp power cord and plug it into a 50 amp outlet (which is too far away for our 50 foot 50 amp power cord). We use the same inverter control to limit shore power to 16 amps, and then if more power is needed the Victrons supply it. Too good to be true? In the absence of proof it works, we’d say yes. But after the aforementioned year of experience, we can tell you the inverter revolution so for real.