Disable generator output with excitation circuit?

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Todd0x1

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Not sure if this is the right place to post this. If I have a 'standard' 3 phase diesel genset with a 12 wire alternator and its accompanying AVR, is it possible to disable the alternator's output simply by interrupting the excitation circuit?

I have a couple rental units where the output is always live if the breaker is on, this has been a problem when a customer has run it out of fuel, or shut it down under load, as the voltage and frequency drop along with the engine speed. Looking into how I can retrofit these units to be like the other ones that have the load shut off (without using a contactor) by the controller. The controller on the units in question supports what I am looking to do so I don't need help there, just with the how to kill the alternator output portion.
 
If it is a rental unit, I wonder if the rental company is going to go along with your plan to modify their units.

Most exciters that I have worked with can be disabled in some way. There is probably a manual somewhere that would tell you how.
 
If it has that capability, maybe an rpm sensor? A phase loss/under voltage relay would not work because you would need a time delay to allow the exciter circuit to start voltage output, and that same delay would be useless because by the time it trips, the generator would already be shut down. Unless…..you have a manual push button bypass to allow excitation until it is up to voltage.
 
Todd0x1 said:
I have a couple rental units where the output is always live if the breaker is on, this has been a problem when a customer has run it out of fuel, or shut it down under load, as the voltage and frequency drop along with the engine speed.
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Can you elaborate what you mean by this?
 
hillbilly1 said:
If it has that capability, maybe an rpm sensor? A phase loss/under voltage relay would not work because you would need a time delay to allow the exciter circuit to start voltage output, and that same delay would be useless because by the time it trips, the generator would already be shut down. Unless…..you have a manual push button bypass to allow excitation until it is up to voltage.
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The deepsea controllers on the generators do all this, I just need to figure out how to switch the output. I know there are some units that accomplish this without use of a contactor on the generator output I just need to figure out how theyre doing it.

I'll try to post a schematic of the alternator + AVR
 
Todd0x1 said:
The deepsea controllers on the generators do all this, I just need to figure out how to switch the output. I know there are some units that accomplish this without use of a contactor on the generator output I just need to figure out how theyre doing it.

I'll try to post a schematic of the alternator + AVR
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This would be an added independent relay, not part of the genset controller.
 
hillbilly1 said:
This would be an added independent relay, not part of the genset controller.
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So the genset controller has an output to connect/disconnect the load, the plan would be to use that to drive a relay which disconnects the two exciter coil wires that go from the AVR to the Alternator. I don't want to burn anything up by trying it so I guess I'll call the alternator manufacturer on monday.
 
It wouldn't surprise me if the exciting coils have significant inductance, which would mean you would need some way to dissipate the stored energy when interrupting the circuit.

This might leave you with slowly decaying excitation and exactly the sort of problems you are trying to prevent .

Just a hunch here, I've no direct experience.

Jon
 
I think I have my answer. I found a schematic for a 168kw diesel generator that has the feature I am looking for, and they are interrupting the power supply (aux winding output from alternator) to the AVR with a relay. The excitation or field coil remains connected to the AVR
 
I think it comes down to a combination of what controller you will add, what features it offers, maybe how your AVR is powered, and what you are trying to protect. What you are willing to add/spend is always a factor.

If it was my rental unit I'm protecting my generator first. So I'd want a shutdown on low oil pressure, low fuel, high temp, abnormal voltage/hertz, etc. And I'd want a shut trip on the output breaker and a lockout relay.

In some cases powering the load is more important than saving the generator.

Best to give some thought as to all the things you want to accomplish.

All my experience is with larger systems with generator paralleling switchboards and ATS's so I can't make any specific suggestions.
 
Russs57 said:
I think it comes down to a combination of what controller you will add, what features it offers, maybe how your AVR is powered, and what you are trying to protect. What you are willing to add/spend is always a factor.

If it was my rental unit I'm protecting my generator first. So I'd want a shutdown on low oil pressure, low fuel, high temp, abnormal voltage/hertz, etc. And I'd want a shut trip on the output breaker and a lockout relay.

In some cases powering the load is more important than saving the generator.

Best to give some thought as to all the things you want to accomplish.

All my experience is with larger systems with generator paralleling switchboards and ATS's so I can't make any specific suggestions.
Click to expand...

It's got a Deep Sea controller which has all the protections you mention. Thanks.
 
There are others here with more experience and brains. Personally I would want a motorized output breaker, or at least a contactor. I wouldn't want my generator to ever be started or stopped when connected to a load. I'd also want a cool down period after load is disconnected depending on fault.
 
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