How it works
Voltage regulation VR = (E0 − V)/V × 100%, where E0 is the no-load terminal voltage and V is the rated terminal voltage. The synchronous impedance method (pessimistic): Zs = Voc/Isc from OCC and SCC; then E0 = √[(V cosφ + IaRa)² + (V sinφ + IaXs)²]. For a 10 MVA, 11 kV machine, rated current Ia = 10×10⁶/(√3 × 11000) = 524.9 A. The MMF method and Potier method are more accurate because they account for magnetic saturation by working directly on the magnetisation curve rather than assuming linear Zs.
Key points to remember
Three methods for finding voltage regulation must be known: synchronous impedance (EMF) method, MMF method, and Potier (ZPF) method. The EMF method overestimates (pessimistic), the MMF method underestimates (optimistic), and Potier lies between them. Parallel operation of two alternators requires identical terminal voltage, frequency, phase sequence, and phase angle — these are the four synchronising conditions checked by a synchroscope. If one alternator's speed is slightly higher, its phase advances and it takes more active power load from the bus.
Exam tip
Every Anna University alternator paper has a numerical on voltage regulation using the synchronous impedance method — memorise the phasor expression E0 = V + Ia(Ra + jXs) and its magnitude form to save time under exam pressure.