The ability to synchronize your inverter to shore power or a genset can provide substantial benefits. For power voyagers, more than for sailing voyagers, alternating current (AC) is an important source of power. And sometimes you find you are using AC from different sources, such as shore power, an inverter and a generator. How can those different sources be synched so their output is in phase?
If an AC source is a clean one (such as the main power grid), the oscillations of the changing current form a series of smooth sine waves, but if the source is not a clean one (as is the case with most small onboard AC generators) the waveform is ragged, and may be constantly varying.
These waveforms are a function of changing magnetic fields as the rotors in generators spin inside their stators. In other words, the waveform produced by a generator is a function of the relationship between two heavy pieces of metal, one of which is rapidly spinning and as such has considerable momentum.
Let’s imagine two generators running independently. Let’s say that if we plot their waveforms on the same screen, they do not coincide. This is known as being out of phase. If we connect these two generators to the same output circuit, tying them together electrically, in effect the momentum of the two spinning rotors is counterposed, creating a violent shock load. Something is likely to break. I once worked on a 2,000-hp generator whose crankshaft was snapped in half through being brought on to a foundry’s power grid when in an out-of-phase state.
To prevent damage, if two AC sources are brought onto the same line, the waveforms must first be matched both in shape, and in time. This is known as synchronization. In the old days, we did this by manipulating the governors on the generators to slightly slow one down, or speed the other up, until the two waveforms coincided, at which point a switch was thrown to bring the new generator on line. Nowadays, synchronization is done with sophisticated electronics.
In 2003, Victron Energy, a Dutch company and a technical leader in the DC-to-AC inverter market, released its MultiPlus inverter. This has the ability to electronically synchronize its AC output waveform with that of shore power or an onboard AC generator. Although such a capability had previously been available in the off-the-grid home-power market (notably from Trace), this was the first time it had been available in the marine world.
Synchronization brings with it substantial potential benefits for power voyagers.
Boosting the shore-power cord
Let’s imagine your boat is at the end of a long dock with undersized AC cabling and many other boats plugged in. You will see a substantial voltage drop in the shoreside supply. If you plug in and add additional high loads (such as air conditioning) you will drag the voltage even lower to the point that this may cause much AC equipment on your boat to run poorly, may damage some, and can, in some circumstances, create a fire risk. For most people, it’s not a good idea to plug into low dockside voltage, but in your case you are the owner of a MultiPlus connected to Victron’s VE.Net display and control unit, or else have a similar inverter from another manufacturer (there are other companies entering this marketplace).
You plug into shore power. Within 10 to 20 seconds the MultiPlus has synchronized with the shore-power supply, and switched to its pass-through mode. Your AC systems are now running off shore power. You see on the VE.Net display that the extra load is dragging the shore-power voltage down to unacceptably low levels. If it goes below some user-settable minimum (the factory default is 91 volts on a 120-volt system and 180 volts on a 240-volt system), the inverter will automatically disconnect the shore-power cord and switch to invert mode, which is now likely to happen every time a heavy load comes on line (such as your air conditioning). This will put a heavy load on your batteries.
However, you have the option to limit the current level (amperage) that is taken from shore power, with the inverter supplying anything that exceeds this level, drawing its power from the boat’s batteries. By adjusting the current control knob on the panel, you can find a level at which the shore-power voltage is kept above an acceptable minimum, with the inverter picking up any additional load. Instead of being either on shore power at an unacceptably low voltage, or else on inverter power, which will rapidly drain the batteries, you can take as much power as possible from the shore-power cord and simultaneously supplement this from the boat’s batteries via the inverter.
Now let’s assume your load is quite variable. For example, the air conditioner cycles on and off. When it is on, its load exceeds the amp limit you have set on the shoreside supply, and as such the inverter is called upon to meet the additional load, discharging the batteries. But when the air conditioner cycles to off, the load is less than the amp limit. The inverter will switch into its battery-charging mode, adjusting the charge rate to load the shore-power cord to your preset amp limit until such time as the batteries are charged to the point that they cannot accept the available charging current. The battery-charging voltage regulator will now scale back the charging amps, reducing the load on the generator below the preset limit.
Finally, let’s assume another boat on the dock upstream of yours brings on a heavy load, dropping your shore-power voltage such that the amp setting you have in place is now too high to protect against low voltage. As the input voltage to the inverter falls towards the cut-off threshold, the inverter will scale back any battery-charging output to try and sustain the input voltage. If the voltage still falls below the low-voltage trip point, following a small delay (to protect against nuisance tripping when loads with high-inrush currents kick on) the inverter will seamlessly disconnect the shore-power cord and switch to invert mode.
The hard life of generators
These two functions — boosting an incoming AC source and maximizing battery-charging opportunities — have even greater benefits when applied to onboard generators instead of a shore-power cord.
The typical life of an onboard generator is a miserable one. Let’s assume, once again, that the primary reason we have a generator is to run air conditioning. Most air conditioners have high start-up or inrush loads that are up to six times the running load. If the generator is not to stall when the air conditioner first kicks on, it must be sized to handle this inrush load, but now it is grossly oversized for the running load. When the air conditioner has cycled to off, there is no load at all on the generator.
This combination of light running loads and no load is a terrible operating environment for the diesel engine driving most marine generators. It is extraordinarily inefficient from a fuel consumption point of view, creates a high level of exhaust pollutants, and results in soot formation in the engine that accelerates engine contamination, increases maintenance, and reduces engine life. Additionally, at light loads the generator is likely to be operating inefficiently.
Synchronizing inverters
If a MultiPlus or similar synchronizing inverter is added to the electrical system and wired downstream of the boat’s shore-power/generator selector switch, it will parallel itself with the generator’s output whenever the generator is on line. Now we can size the generator to handle the running load of the air conditioner (or other loads), rather than the peak load, and use the MultiPlus controller to set the current limit for the generator at a level that will keep it from overloading and stalling during inrush and other high-load events. In many cases, generator size can be reduced to as little as 25 percent of what would be used in a conventional system.
Any time the demand on the AC system exceeds the current limit setting (e.g., the air conditioner’s inrush current) the inverter will seamlessly meet the excess demand by pulling the necessary energy from the boat’s batteries. Any time the demand on the AC system is less than the current limit setting, the inverter will switch into battery-charging mode to recharge the batteries. During the off cycle with the air conditioner, so long as the batteries have the capacity to accept the charging current, and so long as the inverter has a battery-charging capability that can utilize the generator’s output, the inverter will keep the generator loaded to the preset amp limit. This will not only keep the diesel engine that is driving the generator operating at, or close to, its maximum efficiency whenever it is running, but it will also keep the generator operating at, or close to, its maximum efficiency in converting mechanical energy to electrical energy.
Victron’s testing has demonstrated that most diesel engines and generators run at peak efficiency at something less than full power (typically, around 80 percent of full power). This correlates well with the recommendation from generator manufacturers not to run generators at full continuous rated power. If the peak efficiency load can be determined, or is available from the maker, for optimum efficiency it can be used to set the current limit on the generator.
We have now substantially downsized our generator, with concomitant savings in cost, weight and volume, while also improving the generator’s operating efficiency by ensuring that it is loaded to its most efficient operating point. This will reduce fuel use, lessen maintenance, and extend the engine’s life.
At one level, synchronizing inverters are nothing more than an evolutionary development of existing technology. At another level, they represent a qualitative leap forward. In particular, the ability to boost what would conventionally be considered a grossly undersized generator, and to then keep it well loaded by charging a buffer battery bank at those times when AC loads are light, can transform the power equation on a power voyaging boat that uses an AC generator.
Contributing editor Nigel Calder is the author of numerous books, including The Boatowner’s Mechanical and Electrical Manual.
