How Three-Phase Generators Work

How Single-Phase and Three-Phase Generators Work

Both single- and three-phase generators connect to and produce AC power. Just as the name indicates, single-phase generators deliver one wave of power. That wave, however, varies with the source of the electrical power connected to the generator. The wave’s power level crests and drops to zero at intervals once each cycle. Those drops are so quick that most people don’t notice them when they hit electrical devices — which is why single-phase generator power is usually sufficient for residential use, including household appliances, electronics like TVs and computers, and lighting.

But even a quick drop in power can be a big problem for large facilities or machines that need a significant amount of power to function. Three-phase generators produce power via three waves that overlap and are evenly spaced apart – when one wave rises, the other falls, and so forth. Those successive waves provide an uninterrupted current of power that never dips to zero. And in cases when an exceptionally high-powered load is required, the three phases can be synced to provide concentrated power to one piece of equipment. Three-phase generators can be used for either primary or back-up power in the heavy-duty industrial, agricultural and commercial sectors, as well as for data centers that require enormous power to run scores of servers.

The Benefits of Three-Phase Generators

As mentioned above, the situation for which the generator will provide power often determines whether a single- or three-phase generator is the right option. Just because a three-phase generator is more powerful doesn’t mean it’s the right generator to power your home. Similarly, just because single-phase generators have lower up-front costs than three-phase generators doesn’t mean that, over time, they’re less expensive.

Some specific advantages of a three-phase generator over a single-phase generator include:

•     Stronger, more durable and more resilient
•     Lower maintenance costs due to less damage from torque and vibration
•     Less space required for installation while providing more power
•     Fewer conductor materials used in construction
•     Less risk of system downtime or power interruption because of an outage

The Importance of Balancing Loads on a Three-Phase Generator

Three-phase generators offer many benefits and advantages. But to function properly, the generator’s connected load must be evenly balanced across all of the generator’s coils. That’s not always easy to do — but the consequences of failing to do so can be great.

An unbalanced system can cause significant problems, including:

•     Overload – when one single leg pulls more than a third of the total power produced by the generator
•     Interference with voltage regulation – unbalanced loads can impact the automatic voltage regulator’s ability to sense voltage changes and increase or decrease current to maintain the level of voltage within acceptable limits.
•   Interference with wave form and harmonics – AC generators produce electrical output in a sine wave that operates best when it’s smooth and transient harmonics are kept within limits. Unbalanced loads can bring those harmonics outside the acceptable limits and negatively impact the wave.
•   Unbalanced single-phase loads affecting connected three-phase loads – If a generator is connected to a three-phase load and a single-phase load, any unbalance can cause electrical supply problems to the motor. Motors should be derated by 25% for every 5% unbalanced load.

You can mitigate the effects of unbalanced loads by:

•     Rebalancing the connected load – Determine if existing loads can be redistributed. This is easiest to do when a system is still under construction.
•     Oversizing the generator – Select a generator size where the connected load across any individual coils does not exceed the designed output for each coil.
•     Derating the size of the motor – Load any motor to 80% or less of its full load rating.

To learn more about balancing loads on a three-phase generator, download the information sheet below.