Inverter type generators for RV’s have been on the market since the 80’s. Everybody seems to like them but not many people know what goes on inside of them.
Why do we like them? They are marketed as quiet units that won’t disturb the neighbors while camping. They perform this by having controls that adjust the engine speed to the load on the generator. Coupled with an oversized muffler, this makes for a fairly quiet unit at partial load. At full load they are not as noisy as their constant speed counterparts but they are not absolutely quiet at all times.
The basic parts. Inverter type gensets have three major parts; an engine, a permanent magnet generator and an inverter. The engine provides the power to make electricity. The generator produces the electricity in a raw form. The inverter processes the electricity to make it usable for our use. There are some other (minor) parts like fuel tanks, control switches, output receptacles, housings, mufflers, etc. that are only necessary if you want the unit to run and/or produce power.
The engines in this type of genset are always high torque gasoline or diesel engines. Since they adjust their speed to the load on the generator, they have to have a wide torque range to be able to accelerate to accept the load we put on the units. The gasoline fueled units have a ‘stepper motor’ mounted on the carburetor that responds to signals from the inverter control section. The diesel units usually have basically the same setup only it is connected to the engine governor control or fuel injection pump.
The generators in these units are permanent magnet type. Mounted on the rotating flywheel of the engine is a metallic cylinder with a bunch of bar magnets attached to it. A stationary set of power coils are mounted around the rotating magnets and power is produced in the coils. Normally, the power coils are arranged in a Delta or Double Delta configuration and the resultant 3 or 6 phase power is sent to the inverter assembly for processing.
The inverter takes the delta voltage, changes it to dc and then chops the dc up into single phase ac that is crystal control at 60 Hertz and is then sent to the output receptacles. Along the way, the inverter checks the voltage level, if low, it speeds up the engine. If high, it slows the engine down. The current is measured and if too high, the inverter shuts down and the engine idles down or shuts off. Normally, the inverter assembly also contains the engine ignition control and the safety circuit for the entire genset.
These units can be expensive for their size but are very useful for camping due to the low noise and lowered fuel consumption of the units. Most units of the same manufacturer and the same wattage can be hooked together to provide additional power when needed. They cannot be connected to a shore power cord or a constant speed generator without being destroyed. Maintenance consists of changing oil, keeping the fuel and air filter clean, and periodic cleaning of the muffler. The inverters are usually potted and are very difficult to work on because the manufacturers don’t release the wiring diagrams. When an inverter part burns out, the rest of the inverter is usually overstressed and will fail again if repaired.
Generator Jim