- Power Rating Definition: The power rating of an alternator is defined as the maximum power it can safely and efficiently deliver under specific conditions.
- Losses and Heating: Copper losses (I2R) depend on armature current, and core losses depend on voltage, both contributing to the alternator’s heating.
- Independence from Power Factor: Alternator ratings are in VA, KVA, or MVA because these losses are not influenced by the power factor.
- Output Calculation: The electrical output is the product of power factor and VA, expressed in KW.
- Additional Ratings: Alternators also have ratings for voltage, current, frequency, speed, phases, poles, field ampere, excitation voltage, and maximum temperature rise.
The rating of an alternator states the output it can supply under defined voltage, frequency, cooling, ambient and duty conditions without exceeding its electrical, mechanical or thermal limits. Higher load increases losses and temperature. The conductor current and the thermal class of each insulator help determine the permitted temperature rise. The nameplate rating applies only under the stated operating and site conditions.
Stator copper loss (I2R loss) varies mainly with armature current, while core loss depends mainly on voltage and frequency. These limits make apparent power a useful stator rating, so AC generators are commonly rated in VA, kVA or MVA. The operating power factor still matters.
Power factor determines real-power output for a given apparent power and affects the excitation needed to hold terminal voltage. A capability curve may therefore impose separate stator-current, field-current, core-heating and stability limits.
Real electrical output in watts equals apparent power in volt-amperes multiplied by power factor.
If an alternator is specified by real power in kW, its rated power factor or corresponding kVA rating must also be known.
A complete rating also states items such as voltage, electric current, frequency, speed, phase count, pole count, field current, excitation voltage, cooling method, duty and permitted temperature rise.
| Illustrative alternator rating plate | |
| Make | …………… |
| Kilowatt Rating | 500,000 |
| Power Factor | 0.85 lag |
| kVA Rating | 588,000 |
| Stator Volts | 21,000 |
| Stator Amperes | 16,200 |
| Rotor Volts | 340 |
| Rotor Amperes | 4040 |
| Speed | 3000 rpm |
| Frequency | 50 Hz |
| Phases | 3 |
| Armature Connection | Double Star |
| Coolant | Forced water and hydrogen |
| Gas Pressure | 3.5 bar |
| Insulation Thermal Class | F |
| Type | …………… |
| Specification | IS 5422 and IEC 34 |
| Product Serial Number | …………… |
| Manufacturing Year | …………… |
