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Onan Cummins 35kw conversion to 480V from 208V


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  #61  
Old 03-14-2019, 04:43:02 PM
w-cummins w-cummins is offline
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Default Re: Onan Cummins 35kw conversion to 480V from 208V

Stupid question...

What powers these transfer switches? Power off, gen off, caps in the power sentry? starting battery??

William....
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  #62  
Old 03-14-2019, 05:03:41 PM
Zephyr7 Zephyr7 is offline
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Default Re: Onan Cummins 35kw conversion to 480V from 208V

Quote:
Originally Posted by w-cummins View Post
Stupid question...

What powers these transfer switches? Power off, gen off, caps in the power sentry? starting battery??

William....
The generator powers them. The usual sequence for a “normal” ATS is like this:

1 - normal source (utility) powers everything
2 - normal source fails, ATS controls lose power
- time delay starts (May be a mechanical timer or a small battery powered timer)
- time delay ends, start relay contacts close
3 - generator starts. Generator starter, controller, and the control circuit through the start contacts in the ATS are all powered by the generators starting battery
4 - generator power re-energizes ATS controls
5 - ATS closes on emergency source using emergency source (generator) power to run the actuator

When the normal source is reenergized, the cycle goes in reverse but with a “qualifying” timer that makes sure the normal source is really ok before switching back to it. A cooldown timer keeps the generator running at least a few minutes before opening the generator start/run control circuit to shut the generator down.

The ATS control logic is designed such that it fails sending a start signal to the generator (start/run relay contacts closed). This way if normal fails and the generator doesn’t start, as soon as the generator is fixed everything still works and the ATS will switch to emergency regardless of how long it didn’t have power.

Bill

Last edited by Zephyr7; 03-14-2019 at 06:55:09 PM.
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  #63  
Old 03-14-2019, 05:50:59 PM
w-cummins w-cummins is offline
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Default Re: Onan Cummins 35kw conversion to 480V from 208V

Quote:
Originally Posted by Zephyr7 View Post
The generator powers them. The usually sequence for a “normal” ATS is like this:

1 - normal source (utility) powers everything
2 - normal source fails, ATS controls lose power


- time delay starts (May be a mechanical timer or a small battery powered timer)
- time delay ends, start relay contacts close
So the contactor/switch remains connected to the utility power at this point? In the case of the Cummins switch there appears not to be a battery (at least I didn't see one in the parts list) so... what happens if the gen doesn't start? It just powers out and sits there??

So the ATS can see power on either side and power its self from either side and by passes the large switching contactor? Is there a mechanical interlock that prevents cross connecting the sources?



Quote:
The ATS control logic is designed such that it fails sending a start signal to the generator (start/run relay contacts closed). This way if normal fails and the generator doesn’t start, as soon as the generator is fixed everything still works and the ATS will switch to emergency regardless of how long it didn’t have power.

Normally closed relay?
I thought that if it had an over crank it locked out the starting?

William..
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  #64  
Old 03-14-2019, 05:52:55 PM
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Billy J Shafer Billy J Shafer is offline
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Default Re: Onan Cummins 35kw conversion to 480V from 208V

Power sentry from an OT no. Power sentry from the OT III. You would have to swap the whole control system. Probably be cheaper to find a complete switch.
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  #65  
Old 03-14-2019, 07:03:10 PM
Zephyr7 Zephyr7 is offline
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Default Re: Onan Cummins 35kw conversion to 480V from 208V

Quote:
Originally Posted by w-cummins View Post
So the contactor/switch remains connected to the utility power at this point? In the case of the Cummins switch there appears not to be a battery (at least I didn't see one in the parts list) so... what happens if the gen doesn't start? It just powers out and sits there??
If the generator fails to start, the ATS just sits there with the start contacts closed. There is no power to make the ATS transfer, but there’s no other source to transfer to anyway without the generator running.

Quote:
Originally Posted by w-cummins View Post
So the ATS can see power on either side and power its self from either side and by passes the large switching contactor? Is there a mechanical interlock that prevents cross connecting the sources?
Yes and yes. The ATS can be powered from either source, and often is interlocked so that the power to transfer to a source comes from the source that will be transferred to.

Usually an ATS is only mechanically capable of connecting to one source at a time (one metal piece that switches to contacts from one source or the other). Some of the older switches had different switch mechanisms for each source, but a mechanically interlocked actuator system so they couldn’t both be connected at the same time.

Quote:
Originally Posted by w-cummins View Post
Normally closed relay?
I thought that if it had an over crank it locked out the starting?
Yes, usually the generator start signal comes from a normally closed relay in the ATS.

Overcrank protection and lockout is handled by the generator controller, not the ATS. A typical ATS is pretty dumb and doesn’t talk to the generator except for sending a “start/run” signal. The ATS signals the generator to start by opening that start/run contact. That is the usual “two wire start” system.

There was a three wire start too, but it’s much less common than the two wire system.

Bill
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  #66  
Old 03-14-2019, 08:50:42 PM
zuhnc zuhnc is offline
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Default Re: Onan Cummins 35kw conversion to 480V from 208V

Generator start is through a normally OPEN set of contacts that CLOSE to signal the generator to start. You can put that N.O. switch anywhere you want, and parallel a bunch of them to enable starting from multiple locations, if desired, even without a transfer switch. From the OTC manual - Although the logic is more involved, the two-wire starting circuit can be thought of as a single pole, single throw switch. A closed switch signals the generator set to start. An open switch signals the electric generator set to stop. zuhnc
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  #67  
Old 03-14-2019, 10:52:27 PM
Zephyr7 Zephyr7 is offline
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Default Re: Onan Cummins 35kw conversion to 480V from 208V

Correct, start signal is a CLOSED set of contacts. But the relay itself is NOT normally open. “Normally open” refers to the state the contacts would be in while the relay is NOT enegized. Since the relay in an ATS keeps the contacts open while its energized (because it needs to close to signal a start, and it needs to do that when the power fails), the relay itself has to have normally closed contacts.

The relay in the ATS is energized while the normal source is present, and closes when the normal source fails. This means the relay itself has a set of normally closed contacts that are held open while it’s energized. Normally closed relay, but a normally open control circuit (the open condition is the generator NOT running).

It’s all a matter of perspective, and how the terms are defined.

Bill
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  #68  
Old 03-15-2019, 09:00:12 AM
zuhnc zuhnc is offline
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Default Re: Onan Cummins 35kw conversion to 480V from 208V

Correct; we are on the same page! Onan always shows their components on the schematics in the "de-energized" state, which is confusing to some. I find it easier to trace out operating paths on such a schematic. zuhnc
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