Parallel Generators: What Can Go Wrong

At this point we delve into theory and unproven concepts. The author can only report on what the electrical components will see and how they will likely respond assuming unknown components are participating in the game.

Running Out of Gas

Consider what happens when one generator runs out of gasoline and dies. The author found very little discussion on this topic on the internet and has not yet attempted such a consideration. He can only report on theory so anything goes on this.

Consider an imaginary combination motor generator. In its motor mode, it is powered by a battery. In its generator mode, it is producing electric potential above that of the supply battery. Consider the device operating in a motor mode producing torque rather than electricity. In this mode, the armature is developing a back EMF which is the agent limiting the velocity of its armature rotation. The back EMF, in a sense, battles with the supply voltage. If you increase the battery voltage the armature velocity will increase as well as the internal back EMF but the back EMF remains proportionally less than the supply battery voltage.

For a fuller discussion of such a bi-modality, please look at another web page we have put up on this subject.

Now consider that a new, previously unnamed, mechanical torque begins to act on the shaft turning the armature faster. The armature velocity has increased causing a corresponding increase in the back EMF of the armature. This new and higher back EMF is in opposition to the supply battery and reverses the current flow direction. Current is now flowing into the battery changing the battery from being a source of energy and to a load. The battery is being charged.

Back to the original question: At this point, we can address what happens when one of the two generators runs out of gasoline. The armature velocity begins a transient decrease causing its back EMF to decrease in a transient operation. This second generator (now out of gas) competes with the household grid as a load demanding power from the remaining generator.

Do you recognize what has theoretically happened? The generator that ran out of gas has now become an electric motor. The remaining generator will attempt to keep the “dead” generator turning. As a newly commissioned motor (former generator), it will continue to develop its own EMF in opposition to the remaining generator and will continue to turn under the power delivered by the remaining generator. The remaining generator will supply the difference between this “dead” generator’s back EMF and what voltage it has been designed to produce. The remaining generator services this newly created “motor” and will attempt to meet its demand by keeping it “powered” and at least have an appearance of running.

In case the reader is wondering what to consider about this, having one generator run out of gasoline may properly be designated as uncool. You don’t what to be doing it.

What happens next will depend on specific configurations. The remaining generator could become over-taxed and trip its output breaker canceling all power generation. One or both phases of the “dead” generator could trip (unlikely) eliminating the problem.

We, at the Chelsea Amateur Radio Club are not interested in what could happen. What we are interested in is trying to prevent one generator from running out of gas while unattended. We urge you to adopt the same policy.

Generators of Different Wattage Ratings

Consider the above discussion but in light of two or more parallel different generators of different capacity or wattage ratings. One of the generators reaches its capacity and labors to keep the lights on. This labor slows its armature velocity and therefore its phases fall behind and drag its companion phases from the other generator along with it slowing that aramature. This means that the total wattage capacity of the two or more generators in parallel is equal to the multiple of the number of generators times the capacity of the smallest generator.

Tripping One Phase Circuit Breaker

Another variation (applicable to both parallel and single generator connections) is one generator running out of gas is one phase of one generator being tripped. This is applicable for the generators that have three circuit breakers instead of just one: one each for the two phases and a third for the combination of phases A and B. One of the generators (producing phase A and phase B) has a demand such that the circuit breaker for phase A is tripped. Any of the 240 VAC loads within the house will now only see 120 VAC. Loads in the house connected to this side of the service entrance will simply lose power. The loads malevolently affected are motors such as the well or hot tub pumps. Depending on the motor itself, it may still try to run with only 120 VAC burning itself out.

For a parallel configuration, when one generator’s phase is tripped, it will likely cascade a trip onto the other generator’s matching phase. A parallel operation is therefore no help from this problem except that a parallel configuration is less likely to facilitate such a problem happening in the first place.