Max Power delivered by an electrogen group

[Jicastro]

Hi, I have a question about an generator group. It is a generator of 50 KVA with a power factor of 0.8. I know this mean I can deliver a maximum of 40 KW. My question is: Is the generator of deliver al its power in the same phase, or there is a limitation of the power it can deliver per phase. I am asking this for any generator, not a specific model cause I want to know if there is a limitation in the physics, not in the width of the wires or anything like that. If it can be done, what are the repercussions it may have in the machine?

Thanks

 

[ScottyUK]

Most machines are rated based on a balanced 3-phase load. If you draw power from a single phase then the machine can deliver significantly less power than in the 3-phase state. The exact limitations depend on the winding connection of the generator.

 

[waross]

A 50 KVA generator will produce 50/3 or 16.67 kW per phase, but the total kW must not exceed 40 kW.
Be warned that small unbalances and short term unbalances are allowed, but continued use with widely unbalanced phase currents will lead to shortened rotor life.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Hoxton]

To clarify, the ac generator is the limit not the engine.

You could generate 40 kW single phase, but you would need a different (larger) generator. This change is usually impractical on such a small generating set.

 

[waross]

I must state some assumptions which are mostly but not always true.
Gen Set limits:
KVA- Limited by the current capacity of the generator winding and the rated voltage.
eg- A single phase winding rated at 40 KVA at 120 volts will safely carry (40,000 Volt Amps / 120 Volts) = 333 Amps.
333 Amps is the limit. If this set is used for 110 Volts the new KVA rating will be 110 Volts x 333 Amps = 36630 Volt Amps or 36.63 KVA

Kilo Watts. Rated kW is typically 80% of KVA rating.
kW is limited by the power of the Prime Mover.
The Prime mover will supply any combination of kW loads up to its maximum capacity.

Now the exceptions.
Prime rated sets are expected to run 24/7. I have seen prime rated sets able to supply rated load after 15,000 Hours of service.
Standby sets are expected to run much less. The most hours of many standby sets is the monthly exercise runs.
Often the same basic set will be marketed for both standby and prime power use, but with different ratings.
For example, a 50 KVA prime rated set may be marketed and rated as a 55 KVA standby set.
A prime rated set is allowed to produce 110% KVA and or 110% kW for one hour out of twelve. Greater use at elevated output may adversely affect the projected life expectancy of the set.

Now for single phase sets.
Single phase sets are not generally manufactured in sizes greater than 15 KVa for 1800 RPM or 25 KVA for 3600 RPM sets.
Larger sizes are reconnected and re-rated three phase sets.
When a three phase set is reconnected for single phase use, it loses at least 1/3 of its capacity.
The set may be connected in double delta, zig-zag, or with a Bar-Diamond or Collins connection. The Collins connection is used to re-connect ten lead generators for single phase use.
Occasionally only two phases will be used for single phase loads. eg: 120/208 Volts. In this case the capacity will be 67% for all 120 Volt loads but only 58% for all 208 Volt loads.
Kilo Watts for single phase use.
Typically, no changes are made to the engine when the set is converted for single phase use.
For comparison, consider a 75 KVA set at 60 kW reconnected for single phase use. The new KVA rating is 50 KVA.
Although the KVA rating is 50 KVA the engine has enough power to deliver 60 kW. As result the single phase power factor rating is 100%
The set is somewhat over powered. A set in the 40 KVA to 75 KVa range may find much application as a residential standby set where multiple air conditioners are in use. The extra power is a help in starting large air conditioners.
The simple answer to your question:
Each phase may deliver 1/3 of the maximum kW rating of the set. However the total kW loading may not exceed the maximum rating of the set. This is limited by the power of the prime mover.
Downside: Unequal loading will cause rotor heating and extended unbalanced loading may shorten the rotor life expectancy.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

Another way of looking at it:
The engine (or Prime Mover) can't see any wiring configuration or anything else electrical related, it just supplies torque at a certain speed. Thus, the real power is limited by the engine (i.e. a 40kW set gives a maximum of 40kW).

Exactly how that gets delivered to the load is entirely due to the configuration of the alternator, whether single phase, 3 phase or some other exotic configuration.

The statements above are a generalisation, waross's details are a more in depth description.