Hi Keith;
I agree, you won't find the ratings on the nameplate.
You have to look at the manufacturers technical info sheet for the set model.
When you order the set, it will be given nameplate with the single set of ratings that you have selected.
The ultimate KVA that a set will produce is determined by two basic factors;
The maximum current that the set may handle safely, and the maximum voltage, limited by insulation and saturation.
The maximum KW is limited by the capability of the prime mover.
The power factor is the ratio of KW/KVA. The prime mover is normally selected so that the power factor will be 80%.
There is normally a lower rating when a set is designated for use at 50HZ.
This is not because of the speed drop as with a motor, but because the normal 60HZ. rating will be at 480 volts, and the 50HZ. rating will be at 400 volts.
Coincidentally the voltage ratio, (480/400=1.2) is the same as the frequency ratio (60/50=1.2), and is the reason for the derating.
Voltage issues. An electric motor is excited by line voltage. An alternator of the size that I am discussing is excited by the exciter, controlled by the voltage regulator. These components have a wide range of adjustability.
These questions may be considered three ways.
1> The ultimate rating of an alternator.
Maximum allowable voltage times maximum allowable current equals KVA.
2> The rating of the alternator when applied to a standard system.
System voltage times maximum allowable current equals KVA.
The allowable rating will change in the ratio of the original or ultimate voltage to the application voltage.
3> The rating of a complete set with a prime mover.
The power of the prime mover determines the maximum KW. that the set may produce. If the torque curve is fairly straight over the range that the speed is changed (3600RPM/3000RPM, 1800RPM/1500RPM) the reduction in speed will result in a reduction in power (KW). This fits nicely with the normal drop in system voltages from the NEMA world to the IEC world.
I didn't mean to write this much, but as I am this far, please let me share with you an experience with generator ratings.
A customer (a movie theatre) had a fixed load and ordered a diesel generator for standby service.
His system was 120/208 volts. As I remember the load was 32 KVA. The supplier sold him a set rated at 32 KVA at 120/240 volts 4 wire delta. The rated current was 77amps. At the utilization voltage of 120/208, this resulted in a KVA rating of 27.7 KVA.
The set supplier was uncooperative and we had to reduce the load.
To make things more confusing, a diesel set manufacturer will rate the same alternator differently for standby service as compared to prime power service.
To compare ratings, look at the following sites;
Note, F G Wilson used to manufacture the Olympian Brand for Caterpilar. Several years ago, Caterpilar bought F G Wilson.
Caterpilar markets both Olympian (F G Wilson) and Marathon branded gensets.
On the Cat site, go to Latin America for dual 50/60 hz. ratings.
Cummins Power
and click on:
On-Site Power Generation Systems - Full Line PDF icon (1.8MB)
When the set is ordered, the governor, dead-rack adjustment, voltage, Under Frequency Roll Off, motor overspeed protection etc. will be set for the frequency and class of service, (Prime, Standby, or Continous) and the Nameplate will be supplied showing only one frequency.
I haven't mentioned widely variable temp. rise ratings and "Fudge" factors. Both common in set ratings.
Respectfully
