Loss of Excitation

Loss of excitation is usually caused be getting older, falling into a familiar routine, doing the same old things all the time instead of changing up your life by taking her to different, fine restaurants, going out dancing and to movies and taking trips.

Cruises will also work well,

Trust me, a dozen red roses and a bottle of fine wine can go a long way toward restoring excitation in your and your mate's life.

Good luck!

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.Oh, GENERATOR!..................................................................... Never mind.

Maybe it stops....

http://control.com/thread/1348990642

Normally, the three phase current in the stator windings creates a rotating magnetic field. The rotor rotates synchronously with this field, and the magnetic field generated by the excitation couples the rotor to the rotating field and allows the rotor to deliver energy to it.

If excitation is lost, this coupling is lost, so the rotor, which is connected to some driving engine, can speed up faster than the rotating field. Since the rotor is now overtaking the rotating field, voltage is being generated in the unexcited rotor windings. The faster the rotor is turning over synchronous speed, the more voltage is generated.

https://en.wikipedia.org/wiki/Induction_generator

Dear sir

Thank you for your response

now after voltage is being generated in the unexcited rotor winding ,the current must pass through the rotor

winding to generated voltage in stator winding ,so how the current passes through rotor winding if the

rotor winding is open circuit after field circuit breaker open

best regards

If the rotor does not have amortisseur bars/windings and/or some form of path (field discharge resistor, flyback diode, etc.) then there is no current flowing in the field. Unlike a salient pole machine,a round rotor machine (without damper bars) has no poles, only residual magnetism, so the bulk of the magnetizing current tends to flow in the high reluctance/resistance iron of the machine resulting in very rapid overheating, usually observed as dark blue areas.

Machines still connected to the grid will rapidly overheat, turn blue, and rapidly self-destruct if allowed to operate for very long.

Your statement, "...loss of excitation it will work as induction generator..." is not always true, it is a function of three things, the physical arrangement of the rotor, the type of excitation system, and if brushed, whether there is a Field Discharge Resistor in the circuitry.

First is the type of rotor, round rotor or salient pole, and whether it has damper (armotisseur) windings.

Second is whether it is brushless or brushed, some brushless will conduct through the diode bridge, and certain designs of both types will have a "flyback" diode to control/absorb the sudden collapse of the field.

Third is the exact nature of the control circuitry surrounding the Field Breaker, some insert a Field Discharge Resistor to control/absorb the sudden collapse of the field.

So your doubt regarding how field current is going to flow, is well founded but incorrectly stated. It should say "...where does the magnetic field coupling the rotor to the stator come from?...". Either from the induced voltage in the field winding if shortcircuited, or thorough the armotisseur windings which act in the same manner as the rotor bars in a squirrel cage induction motor.

Well said.

Are you confusing excitation in the generator with the "exciter motor" that is coupled to the generator shaft and is used to run the generator up to speed? Once rotating at the correct frequency the steam or water valves that are providing the motive power for the generator are set to maintain that frequency and the required power input. The exciter motor speed is then adjusted fractionally to synchronize the phase output of the generator with the phases of the grid at which point the generator is connected to the grid. The exciter motor is then switched off by opening it's circuit breakers. Usually it is left to free wheel but I have seen instances where the exciter motor is de-clutched. Exciter motors can also be used as brake motors when a generator is stopping so that the generator passes quickly through any resonant frequencies it encounters as it slows down.

In the US the term "exciter motor" is used to refer to the motor that drives a DC generator that provides the field current to the alternator. The motor you are describing is usually referred to here as a "pony" motor, and is one of the ways that a large synchronous motor is accelerated to near synchronous speed prior to applying field current from an external source. Pony motors can also be used to accelerate very large induction motors prior to connection to a source to minimize the voltage sag as the machine accelerates.

Thanks for your input. The term pony motor is new to me. Another instance of two countries separated by a common language

Thanks for the feedback!! BTW- I've seen industrial gas turbines spun up by small motors turning the whole shaft, but I'm curious about spinning up a steam turbine to near synchronous speed and then admitting steam to it, seems like a formula for disaster with regard to differential expansion. Every startup I've participated in involved using steam to roll the turbine in stages while allowing everything to come up to a uniform temperature "soak" and expansion at each stage.