# Three-phase versus single-phase operation of a generator

My generator (a Fermont MEP-803A, 4 cyl diesel, 10kW rated) can run in three distinct modes:

• Single-phase 120V, in which one terminal is hot, and another is neutral
• Single-phase 120V/240V, in which two terminals are hot, and another is neutral. The difference between the terminals is 240V, and each one to neutral is 120V.
• Three-phase 208Y, in which three hot terminals are each 208V apart from each other, and each one is 120V from the neutral terminal.

I need to run a bunch of lights that all need 120V. So the obvious thing would be to just use the 120V mode. But I'm thinking it may be better (more efficient maybe? less vibration maybe?) to run it in three-phase mode, with the load balanced among the three hot terminals.

My thinking, based on my coarse understanding of generators, is that in three phase operation, the torque should be more homogeneously distributed over the course of a single turn of the engine, so there should be less vibration and more equality in effort among the cylinders. In short, I'm guessing that three-phase generator operation would have the same advantages three-phase motors have over single-phase motors.

Is my thinking correct here? If not, why not? Conversely, are there any disadvantages to three-phase operation over single-phase operation?

While the other answers here are compelling, they're only half the truth. If you consult your manual you'll find that the generator load windings are reconfigured when using the mode selection switch to one of three output modes.

In either mode, the full output of the generator is available, either as 104A@120V(1ph, one circuit), 52A@120/240(1ph/split, two circuits), or 34A@120/208(3ph, three circuits).

If you select position #2 (1ph @ 120V) then you can place your entire load between L3 and LO without worrying about balancing anything.

The only reason to select three-phase mode is if you actually have a three phase load to drive. It has no other effect on the available power of the generator, but using it in that mode does require you to manually balance your single phase loads. Choose the mode that fits your load. If you only need 120V 1ph service and you need all 104A for single phase 120V loads then choose that mode.

In fact, if you run single phase loads in 3-phase mode or in 120/240 split phase mode then you DO have to worry about keeping the phases reasonably balanced yourself by evenly distributing loads to the L1/L2/L3 terminals (in 3-phase mode), or to the L1/L3 terminals (in 120/240V split phase mode).

Operator's manual, 2-18

When using single phase connections, always attempt to balance loads between terminals(do not connect all loads between one terminal and LO). Failure to observe this caution can result in damage to generator set.

If you have a 3ph load you need to power, obviously you need to select three-phase mode, and a 3-phase load will naturally be balanced if you're not adding any single phase loads on top of it. If you're only driving single phase loads, save yourself the headache and just use mode position #2 (1ph, 120V). That way all your loads simply go between LO and L3 and you don't have to worry about balancing anything.

• As always, the lesson is - understand your manuals. (I haven't read them. Your link is back to this thread - not the manual.) What you have quoted describes a standard generator with direct connection from the stator windings to the output socket. The rotor is driven with DC to generate the rotating DC field. The stator output voltage is regulated by the AVR (automatic voltage regulator) which adjusts the rotor current to maintain output voltage. There is no "AC->DC->AC transformation via a full wave bridge rectifier" which would require an (unnecessary) inverter. No isolation either. – Transistor Mar 24 '20 at 18:34
• @Transistor Fixed the link. – J... Mar 24 '20 at 18:38
• @Transistor And you're right - there is a full wave rectifier in the design, but you're totally correct that it's not on the output side (it's used to convert the exciter AC output to DC to energize the generator rotor.) I read too quickly, and updated the answer. – J... Mar 24 '20 at 18:54
• Now if only you discussed the torque ripple of the three configurations of the reconnector switch a bit more explicitly ... – Phil Sweet Mar 24 '20 at 20:18
• @PhilSweet There's a huge flywheel - it's a military generator, it's designed to be used in either mode to its rated capacity. The rest is minutiae. OP seems interested only in practical concerns. – J... Mar 24 '20 at 20:20

But I'm thinking it may be better (more efficient maybe? less vibration maybe?) to run it in three-phase mode, with the load balanced among the three hot terminals.

Correct.

Figure 1. The load is constant through the generator cycle. Source: T. Davies - website not found.

My thinking, based on my coarse understanding of generators, is that in three phase operation, the torque should be more homogeneously distributed over the course of a single turn of the engine, so there should be less vibration and more equality among in effort among the cylinders.

You are correct. The beauty of three-phase is that the torque presented by a balanced three-phase load is constant through the full cycle.

In short, I'm guessing that three-phase generator operation would have the same advantages three-phase motors have over single-phase motors.

Figure 2. The three-phase voltage waveforms are 120° apart. (Graphic by author.)

Figure 3. The power produced by each phase will be given by P = I2R so the powers are all positive. (This assumes a resistive load.) Note that the sum is constant which means that the load on the generator is constant through the full cycle. (Graphic by author.)

Is my thinking correct here?

Yes.

Conversely, are there any disadvantages to three-phase operation over single-phase operation?

Three-phase is efficient in its use of copper. You will need four conductors rather than two but they will be lighter. Technically, if you balance the load well you can reduce the neutral to zero or size it for your worst case imbalance.

The only downside is a slight increase in complexity and the need for fusing on three phases instead of one.

• Nice answer. But the set is powered by a fourbanger Onan diesel. Even worst case scenario, the gen head torque pulses would be rather small compared to the power head torque pulses. And hypothetically at least, with a four pole exciter, you could clock the 1 phase or split phase to the diesel in a constructive manner. – Phil Sweet Mar 23 '20 at 23:58
• The generator itself is 3-phase 120v. Using it in 120v mode or 120/240v mode means loading 2 of windings with off-phase current that decreases efficiency and generates more heat in the stator. – fraxinus Mar 24 '20 at 13:33

@Transistor's answer is quite complete and correct, but I feel a bit more complex than what you require.

The only difference in how you pull power off of your generator is in how the output windings are connected to a plug. Each of the three windings will have the same rating whether you pull 1, 2, or all 3 phases. So if your total load (the bunch of lights) is less than the single phase rating it really doesn't matter at all.

The reason it doesn't matter is that your diesel prime mover will never notice how the power is being made off the generator. Three phase power is great and works very well, but in this case the diesel engine is the thing that makes power in strange pulses. Evening out the load in nice 120 degree phases won't matter a bit to an internal combustion engine.

• I think I see how the engine probably doesn't care. But what about the generator itself? In single-phase operation, if only one output winding is taking the load, then the generator's resistive torque is not homogenous, and I would think that would cause the whole genset to vibrate more, right? Also, I would think that in single-phase operation, all the heat would have to dissipate through one winding rather than 3, so I would guess that winding would get hotter and more resistive, and there would be more wear on it. To what extent is this thinking valid? – Andrew Cone Mar 24 '20 at 17:34
• @AndrewCone, the generator will vibrate because of the uneven pulsing of the diesel. I doubt the electrical vibration is measurable. – Tiger Guy Mar 25 '20 at 3:39