Paralleling Generators: Mixing Inverters

What’s the Difference between Generators and Inverters?

The above discussion is wholly and absolutely with regard to paralleling fossil fuel-driven generators. This is to say, generators that develop potential by means of an armature that rotates through permanent magnetic fields. These are distinguished from power conversion units.

Power conversion describes a process of converting one form of potential to another. For example, there are inverters that accept an AC potential, and output DC. A generator, however, describes a process where an electric potential is created from a non-electrical potential such as chemical.

To be clearly understood, a case could be made whereby a generator is also a form of power conversion in that gasoline comes to us with so many Joules per cubic centimeter of “potential” energy and therefore has inherent potential. We use that fossil fuel potential to develop electrical potential and thus it is not qualified to call it electrical power conversion. We can call that process the conversion of chemical potential to electrical potential but it is not understood as power conversion.

Mixing Inverters with Generators

While special provisions can be made to mix inversion technologies with generators, such does not apply to the above discussion. You cannot add an inverter (even with the same amplitude and approximate frequency) to a parallel architecture as described above and expect it to work as described above. However, it can be done with special characteristics recognized.

We already understand the technology of the generator with its rotating armature driven by a gasoline or diesel-driven small engine. Power conversion inverters are completely and wholly different. Their frequency cannot be altered even to the smallest degree from outside forces as can be done with the generator.

Consumer-grade inverters operate from the principle of accepting a DC potential and then electronically chopping it to produce what is commonly called a “modified sine wave” output. The output shares characteristics with generators in terms of amplitude and frequency out. This may seem enough to mix the two but hold on to your hats.

DISCLAIMER: The following is theory and has not been proven by experiment. The author is not foolish enough to try this stuff. Further, different inverters have different topologies with differing modes of modified sine wave output.

A generator’s frequency will be pulled down slightly as it attempts to deliver the ever-changing power demands of the load. Generally, inverters will hold steady on their frequencies, irrelevant of load demands. Hold that thought.

An inverter sharing load responsibilities with a generator(s) will by default become the head boss or master of the whole operation. The inverter will become the master and the generator(s) will become the slave(s). This is because the generator’s frequency changes slightly as a function of meeting load demands. But here’s the problem: A generator sees a transient load demand and wants to struggle to meet that demand and in so doing, its frequency is pulled down slightly. But the generator must obey the master inverter which sets the pace. The generator will therefore not be able to struggle to meet the demand and only heaven knows what happens then.

Do not be fooled by “pure sine wave” generators that run on batteries. These are inverters that may look like generators.

MY SUGGESTION: Do not even consider an inverter mixed in with generators. There are too many ways to go wrong and you don’t want to be burning out your well pump when the power has been out for 4 months and you haven’t seen a policeman in 3 months.