AVR

Sorry to appear stupid, but what is an AVR?

As to your other questions about the output voltage of the alternator it depends on the regulating current in the field coils... but at overload the regulator should protect the alternator by shutting off the field current...

Obviously if you have a short on the output the output voltage is zero.

John.

THe ackrontm AVR pertains to an Automatic Voltage Regulator.

Very close to a good question, if you provided the brand, model & year of generator, you may get an answer with the additional info.

AVR just adjusts the voltage gain or drop due to load changes , any thing excess than the rated range of AVR will trip it or reset it , higher current drawn will drop voltage AVR will tend to regulate output to keep voltage constant , AVR is just medium to purify the supply with its own rating if you tend to extend the ratings it will trip or reset

I thought the AVR is used to self-adjust the exciting current on the magetic coils side of an AC generator. By this way the output voltage will be kept stable.

The AVR is a hardware module from what I have seen. It could be a microcontroller based design or it could be a voltage comparator-based pure hardware close-loop design.

I could be wrong.

If there is a short on the output, I can't see any way that the voltage could possibly be kept stable??

The output voltage with a direct short will be zero volts!!

John.

Some of the new AVR with Microcontroller should be able to detect the fault and sound alarm for service when severe problem happens.

Assuming a quick-response solid state regulator design and insignificant line length between the generator and the load:

(a) In most cases, the Voltage Regulator will not have a chance to respond before the output circuit breaker trips. If the breaker fails to open (or the short is between the generator and the breaker), the AVR will ramp up to the setpoint of the field current limiter.

(b) Again, the AVR will ramp to the current limit setpoint if it has a reactive compensator. For an induction motor, as the starting current drops off, the voltage will restore, and the AVR will ramp back down.

In case (a), the design of the sensing circuit compared to the type of fault is critical. For a 3-phase bolted fault, the regulator will behave as noted above, regardless of design. However, if the regulator has single phase (or 2 phase) voltage sensing, a single phase fault on the phase without voltage sensing will warp the sensed waveform and provide unpredictable results.

In both cases, a lot will depend on the stiffness of the prime mover response. If the engine or turbine slows significantly (due to the countertorque of the excessive load), the voltage drop will be compounded by frequency reduction.