waross
Electrical
- Jan 7, 2006
- 27,202
In a recent thread we had some misunderstanding and disagreement concerning the rating of a generator when converted between three phase and single phase or between single phase and three phase.
I have recently encountered a generator that may clear up the misunderstandings.
KVA: Historically, the maximum KVA rating of a transformer winding or a generator winding has been determined by multiplying the maximum safe current by the rated voltage.
That is: The KVA was limited by the maximum safe current with respect to heating.
In sizes above about 20 KVA the major gen-set manufacturers typically did not manufacture single phase sets.
If a customer needed a single phase set, an existing three phase model was re-connected and re-rated.
Two windings would be working at 60 degree phase angles, so the KVA rating for single phase would be 2/3 of the KVA rating of the three phase rating.
Typically the prime mover would remain unchanged so that there would be an excess of power available and the rated PF would jump from 80% to 100%.
I have recently encountered some sets that were built as primarily single phase sets rather than rerated three phase sets.
This aproach allowed a smaller and more economical prime mover to be used.
A 50 KVA/40 kW single phase set would have a prime mover capable of 40 kW rather than the 60 kW that a rerated three phase set would have.
These sets are capable of being re-connected for three phase use.
Three phase use does some interesting things to the ratings.
For single phase use, the prime mover will supply 40/2 or 20 kW to each winding.
For three phase use, the prime mover will supply 40/3 or 13.3 kW to each winding.
Conventionally generator sets are KVA rated at 80% PF
The KVA rating per phase now becomes 13.3 kW / .8 = 16.7 KVA.
The KVA in these ratings is based on 125% of available kW and the three phase current per phase is based on the KVA rating which is based on the available power, not on the maximum heating of the winding.
Any single winding may be safely loaded to 20 KVA but the published KVA rating is based on 125% of 1/3 of the available prime mover kW output, not on the maximum safe current.
I believe that this is the root cause of our previous misunderstandings.
Bill
--------------------
"Why not the best?"
Jimmy Carter
I have recently encountered a generator that may clear up the misunderstandings.
KVA: Historically, the maximum KVA rating of a transformer winding or a generator winding has been determined by multiplying the maximum safe current by the rated voltage.
That is: The KVA was limited by the maximum safe current with respect to heating.
In sizes above about 20 KVA the major gen-set manufacturers typically did not manufacture single phase sets.
If a customer needed a single phase set, an existing three phase model was re-connected and re-rated.
Two windings would be working at 60 degree phase angles, so the KVA rating for single phase would be 2/3 of the KVA rating of the three phase rating.
Typically the prime mover would remain unchanged so that there would be an excess of power available and the rated PF would jump from 80% to 100%.
I have recently encountered some sets that were built as primarily single phase sets rather than rerated three phase sets.
This aproach allowed a smaller and more economical prime mover to be used.
A 50 KVA/40 kW single phase set would have a prime mover capable of 40 kW rather than the 60 kW that a rerated three phase set would have.
These sets are capable of being re-connected for three phase use.
Three phase use does some interesting things to the ratings.
For single phase use, the prime mover will supply 40/2 or 20 kW to each winding.
For three phase use, the prime mover will supply 40/3 or 13.3 kW to each winding.
Conventionally generator sets are KVA rated at 80% PF
The KVA rating per phase now becomes 13.3 kW / .8 = 16.7 KVA.
The KVA in these ratings is based on 125% of available kW and the three phase current per phase is based on the KVA rating which is based on the available power, not on the maximum heating of the winding.
Any single winding may be safely loaded to 20 KVA but the published KVA rating is based on 125% of 1/3 of the available prime mover kW output, not on the maximum safe current.
I believe that this is the root cause of our previous misunderstandings.
Bill
--------------------
"Why not the best?"
Jimmy Carter