This paper presents two basic results with proof. One result backs up the industry practice of using the first-order synchronous machine model for automatic voltage regulator (AVR) tuning. The other result gives a condition on when the loading of the generator results in an unstable system with the AVR tuned for the first-order model. This analysis also implies that when generator loading destabilises the system the standard IEEE AVR models are inadequate to guarantee stability for the loaded generator.