Generators – Single or Three Phase?

Should you choose a single phase or a three phase generator for home backup power?

As power loads increase, many think of choosing a three phase generator.

In many aspects this makes sense. Three-phase generators are designed to produce three separate alternating currents, each phase offset by 120 degrees. This design makes them more efficient and capable of providing a stable and consistent power supply.

While they are primarily used for higher power three-phase applications, they can also provide single-phase power output by using a single-phase outlet (which produces one-third of the generator’s power capacity).

However, consideration should be given to the short-term (240V AC) backup power systems and the potential for voltage imbalance.

What is a Three Phase System Voltage Imbalance?

A three-phase system is considered balanced when the phase voltages have the same amplitude and are separated by a phase angle of 120 degrees.  In this ideal scenario, the voltage magnitudes and phase angle differences between the phases are equal.

Voltage imbalance occurs when the three-phase voltages deviate from this balanced state.

It can manifest in different ways, such as a variation in voltage magnitudes (unequal voltages), or a phase angle differences between the phases (unequal phase angles), or a combination of this.

Large single-phase 240V loads are a common cause of a generator’s voltage imbalance.

This is where there is a load on only one phase and neutral (not appearing on the other two phases) or between two phases (current drawn on only two out of the three). An uneven spread of 240V single phase power has the loads drawing current that can lead to slight voltage imbalance.

Considerations in the implications of voltage imbalance.

Unbalanced voltages can cause a phase current imbalance of many times more than the percentage of the voltage imbalance for a fully loaded generator.

Generators operating on imbalanced supplies should be de-rated, resulting in significant reductions in available loading.

Increased losses in power cables due to voltage imbalance may also necessitate cable de-rating.

Single Phase vs Three Phase kVA Generator Sizing

Kilowatts (kW) represent the actual active power used for doing useful work.

Kilovolt-Amperes (kVA) refers to the apparent power. That is, the working power plus the reactive power factor — which does not do the useful work but is necessary to maintain the voltage levels in the system.

The calculation or finding the Required Capacity kVA Rating or Amperage Capacity for Single and Three Phase generator is slightly different.

For a Single Phase 240V unit:

kVA = (V * I) /1000 where V is the voltage as 240V for the Australian household, and I the maximum load current.

As an example: 240V appliances with a 40 maximum amperage load (I), kVA = ( 240 * 40 ) / 1000

This calculates to 9.6 kVA, which we could round up to 10kVA.  A generator such as the Able GEN FP11YS may be suitable in this example.

For a Three Phase unit (powering the same 240V appliances):

kVA = (V * I * 1.732) / 1000 where 1.732 is the simple numerical value for the square root of 3 for the Three Phase distribution, where V is the voltage as 240V for Australian household, and I the maximum load phase current (not current per phase).

Using the same example loading: 240V appliances with a 40 maximum amperage load (I), kVA = ( 240 * 40 *1.732 ) / 1000

This calculates to 16.7 kVA, which we could round up to 17.5kVA.  A generator such as the Able GEN FP19YS may be suitable in this example if loading is constant and well balanced.

In this example, the FP19YS has a prime rating of 25A per phase.  Loading 40A on one phase can’t and shouldn’t happen.  As commented on before regarding imbalance issues: The phase loading needs to be as evenly distributed as possible for example: Phase 1 as 13A, Phase 2 as 13A and Phase 3 as 14A.

IF the load can be balanced in this example, a generator such as the FP19YS can be used.  However, it is important to note that most 240V loads in a general household setting are frequently switch on and off.  Not many turn on and stay on which makes it near impossible to balance these loads within a range of avoiding phase imbalance as described before.

A Three Phase generator such as the LG30IS which is rated at 42A per phase will which on paper will cope with the above example if loaded onto one phase.  However, significant voltage fluctuations on unused phases will occur when loading reaches over 50% of the phase in use if the other phases are not loaded.  The imbalance resulting in a usable power output of 21A from one phase with no loading on other phases.

Keep It Simple!

In summary, maintaining balanced phase voltages is crucial for reliable power, especially those with three-phase generators.

To try and use a Three Phase generator for Single Phase load application has significant complications.

The juggling act of trying to balance phases if using all 3 Phase generator is almost impossible in a household 240V setting and just isn’t worth it!

Voltage imbalance can have significant consequences, affecting motor performance and overall system reliability … if you can keep the loading simple, consider an Able GEN Single Phase 240V generator as the small home or farmhouse power backup system.