Relay to Relayless AC Generator AVR

I assume that this is for the 20kVA alternator whose exciter winding you just rewound. Typical small systems with brushless excitation systems usually rely on the residual magnetism in the main rotor to build up the voltage until the AVR can take over.

During the startup period your AVR should be firing the thyristors so that they conduct continually. The AVR simply runs full boost, taking the small residual voltage, rectifying it and putting it back into the stator of the pilot exciter which then adds additional current into the main field in a positive feedback loop until the terminal voltage reaches the setpoint of the AVR and automatic operation resumes. If this is not happening then your AVR is not operating properly.

In the case of an alternator that has not run for many years there is a good chance that there is not quite enough residual magnetism left in the rotor to start this process, that's where "field-flashing" is necessary. You simply take a battery of sufficient size, run your alternator up to operating speed, touch your battery leads to the appropriate terminals on the exciter field, and watch the voltage rise, then quickly remove the leads before you overvoltage the alternator. Usually you only have to do this once in order to build up sufficient residual magnetism for the next start.

If this fails and you have to flash the field for every start then it's time to do what was suggested to you in your previous thread, examine all the connections, rotating diodes, fuses, from the main output terminals all the way back through the main field. You may find corroded connections, blown diode(s), shorted turns, etc. somewhere in this loop.

hi mr RAMConsult,

thanks for your reply and no its not for the 20kva alternator, after i rewound it i used the avr which i have and its working fine so far.

i am just planning to do an avr which can be used for different types of brushless alternators. and i dont want to use any mechanical parts like moving contact.

as i understood from you, the volts will build up from the permanent connection of the scr at first. but how long it will take to build up? is it same as the relay contact build up time? i did not try it yet but i will try it with and without relay on a generator.

thanks for the help

I am designing avr for ac generator single and three phase.....

Which is it? Single or three phase?

A generator's AVR adjusts the field excitation to achieve correct alternator output voltage....is that your aim or are you designing an AVR to adjust the voltage post generation?

hi mr wal,

its for three phase self excited brushless ac generator.

thanks