Null Meter On Avr Panel

I certainly know what a NULL is but if you want accurate help you should clearly identify any acronym. Knowing when an Atmel AVR microcontroller returns to memory x00 seems useless to me. We also do not do Aortic Valve Repair. This is an engineering blog. We like precision in all communication.

Eeerr, Fred are you sure your in the right thread?

Where did Atmel AVR microcontroller and Aortic Valve Repair come from?

Well if the acronym AVR is not defined, I'm allowed to choose any of the possible things that this could be. Come to think of it, the Allegheny Valley Railroad would have a null grade as long as the train track follows the middle of the valley. If the Aboriginal Voices Radio looses its carrier, would that be considered a null or only when the carrier is not modulated?

Two things: 1. Null isn't an acronym. 2. Null should not be capitalized. 3. Null is nothing.

_{I know.................That's three things.}

True. The appearance of a NULL on the null meter ensures that the transfer of excitation from Auto Mode to Manual Mode OR from a Manual Mode to Auto Mode OR From Auto Mode Channel 1 to Automode Channel 2 (in a dual channel AVR) is BUMPLESS. There is real time automatic tracking within the available channels to ensure a BUMPLESS transfer.

If AVR is automatic voltage regulator, then we are probably talking about a generator.

Then I would guess its a 3 phase generator.

The null meter, unless I am badly mistaken, is used when paralleling two generators together as switching when "not" null could result in severe damage and maybe one generator leaving its bed and taking a trip through the air!!!!

Its possible that the meter is always there, but not needed in a single generator system....

My apologies if I am off base, but the OP gave so little info, its a bit like guessing on a game show!!!

I worked at cargo ships for six years as onboard electrical engineering officer.

I am responding regarding a generator parallelling panel's NULL meter. This voltmeter is designed in nonlinear scale. At very low voltages to show very precisely and at maximum applied voltage (sum of grid and generator voltages) to deflect to maximum. In old technology it was accomplished by connecting in series a voltmeter with a iron wire resistor (PTC) or an incandescent bulb. At low voltages the cold PTC has low resistance, thus the major voltage drop occurs at voltmeter and and at high voltages PTC heats up , resistance rises and major voltage drop occurs at PTC, thus voltmeter has maximum deflection and safe voltage drop without bending the needle.

The old method of sensing the "Null" (going from an over 40 year old memory) was 3 small lamps, set in a pyramid format in a cabinet. The top lamp is connected between the same phase on two different generators that are to be parallelled.

I will use the NATO naming for each phase of Red, Yellow & Blue for simplicity.

So the top lamp is connected say between the "R" phases of both generators.

The other two lamps are connected between "Y" - "B" and "B" - "Y", that means the top lamp is in phase and the other two 120° out of phase.

Probably each lamp has a resistor to drop the voltage so that regular standard bulbs are usable.

Now when the phases match (Synchronise), the top lamp will be out as the voltage difference across it will be Zero - Both red phases are in phase.

The other two lamps will be at full/same brilliance as they have the voltage of two phases across them. Both lamps are connected now across 2 phases, at 120° out of phase......full voltage across each lamp

The connection "switch" should be made just as the top lamp is out and the bottom two are at full brilliance. That way they are "in phase" with each other.

The "oncoming" generator should be set up to be slightly faster than the running generator so that it takes some load when connected, otherwise the reverse power relay may drop the "it" off immediately.....