One of our clients has an old (approximately 25 years) standby generator, It is an Onan 3-phase that runs on natural gas. The problem is, that approximately 15-45 seconds after start-up the 20a breaker on the cover trips and the unit stops putting out power. I believe it may be shutting down due to over-voltage because the voltage meters get over 250 volts prior to shut down. However, I'm not positive that's it and wouldn't know where to begin to correct an over voltage.
I have no experience with generators and am looking for someone to point me in the right direction or perhaps recommend a local guy that knows the old generators. The customer is aware that the unit will have to be replaced before too long, but they have a lot of hurdles to get through with the city due to diesel storage etc that will be required when the replace it.
Thanks
Most cities will hardly allow deisel permits anymore due to noise if not fuel storage +/or emmisions - GOOD LUCK.....
I think the 20 amp CB is for the excitation voltage.
Determine if the CB is defective.
Check voltage regulator.
Check windings
Check governor.
I can ask our gen techs on Tuesday.
All good advice. I wouldn't touch it without a schematic though - sfav8r, I would start here.... While I would not assume there is a defective breaker, or even that the voltage output is bad just yet. And definately not windings...
It just makes me ask more questions from the comfy chair:
- What type of breaker?
- Output or control/VR - GFI theromag? Contactor?
- Load or no load?
- Any voltage on the line side?
- Where is this 250 being read? Control panel meters are averaging, and often higher than real output.
- Engine stays running - or just drops the load?
Me - I would get the schematic out and go through it's operation step by step. Generators often have a number of safeguards, and depending on the model may operate differently, and some may be 'dumber' than others in not having the luxury of idiot lights to tell you it dropped the load due to over speed, or over-voltage, etc. Which can be confusing since over the large phone diagnosis - the action would say one thing, reality another. Way off in my past - when I started this trade all I did was troubleshoot generators being refurbed or stored while I was in the Marines....
Most generators will start, get the engine to speed, THEN give a voltage from the BATTERY to excite current the stator via a separate circuit in the voltage regulator (Often higher than normal), then once at speed, and a voltage has been sensed at the output, it then becomes the input voltage for the regulator - it sounds as this is when your output voltage falls off. The swap from battery power to AC output power to the voltage regulator sounds like an issue due to the timing at start-up. Voltage down = amps up - tripping that breaker (if it is one, and not a contactor) and only if the set is under load when it transfers. But that is from the comfy chair....
If this is an output breaker - rule out any load - or transfer equipment voltage back feeding. Disconnect it completely.
