What is automatic voltage regulator (AVR)
An automatic voltage regulator (AVR) is a electronic device for automatically maintaining generator output terminal voltage at a set value under varying load and operating temperature. It controls output by sensing the voltage Vout at a power-generating coil and comparing it to a stable reference. The error signal is then used to adjust an average value of the field current.
Most cheap portable generators have fixed excitation. In such machines, when an alternator is loaded, its terminal voltage drops due to its internal impedance. This impedance is formed of leakage reactance, armature reactance and armature resistance. The Vout also depends on the power factor of the load. That's why to maintain Vout within tighter limits, more expensive models use an AVR. Note that all AVRs help regulating the output in a steady state operation, but are generally slow to respond to fast transient loads. Some high-end models, such as Honda EB10000 use more accurate digital DAVR.