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AdventureCoach

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Posted: 08/18/20 07:43am Link  |  Quote  |  Print  |  Notify Moderator

Just curious of why our Onan generators in our boat and RV are designed to operate differently. In the 2006 boat (11KW) it operates at a constant RPM regardless of load but in the 2018 RV (8KW) it runs at various RPMs dependent upon load.
I also like how the transfer switch operates in the RV. There is a slight delay when plugging in to shore power but a 2 minute or so delay when the generator is started. This gives the genset a bit of built in warm up time automatically.
Just curious why the designs are not consistent.

* This post was edited 08/18/20 08:25am by AdventureCoach *


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MountainAir05

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Posted: 08/18/20 07:59am Link  |  Quote  |  Print  |  Notify Moderator

Our runs at a constant speed if loaded or not. Does not change with a load. We have own 4 over our life time and they all have been the same.

IB853347201

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Posted: 08/18/20 08:21am Link  |  Quote  |  Print  |  Notify Moderator

MountainAir05 wrote:

Our runs at a constant speed if loaded or not. Does not change with a load.


Same here. We have had 2 MH's with Onan 5.5kw. Both have run at a constant rpm.


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ksg5000

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Posted: 08/18/20 08:42am Link  |  Quote  |  Print  |  Notify Moderator

Pretty sure std Onan generator output is directly related to RPM's ... if so then speed is suppose to be constant.


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oldave

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Posted: 08/18/20 09:08am Link  |  Quote  |  Print  |  Notify Moderator

It's all about frequency of the 60 hertz/cycle electricity and two methods
of maintaining it
Most RV and Boat generators maintain 60 cycles by either a constant speed
or electronically by an inverter .
My handy man generator is speed controlled , when I place a load it tries
to stay at the same speed as when it had no load ,all in an effort to maintain
60 cycles .
On the other hand my RV Onan Quite Diesel slows down or speeds up depending
on the load but still maintains 60 cycle a/c output .

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dougrainer

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Posted: 08/18/20 11:21am Link  |  Quote  |  Print  |  Notify Moderator

AdventureCoach wrote:

Just curious of why our Onan generators in our boat and RV are designed to operate differently. In the 2006 boat (11KW) it operates at a constant RPM regardless of load but in the 2018 RV (8KW) it runs at various RPMs dependent upon load.
I also like how the transfer switch operates in the RV. There is a slight delay when plugging in to shore power but a 2 minute or so delay when the generator is started. This gives the genset a bit of built in warm up time automatically.
Just curious why the designs are not consistent.


Most Transfer switches, the Transfer maker default is Shore Power with the time delay for Genset. But, most Transfer switches have a DIP switch that changes the priority. I believe the RPM change is on Onans( RV diesels) that have the "inverter" type internal controls. Doug

time2roll

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Posted: 08/18/20 11:50am Link  |  Quote  |  Print  |  Notify Moderator

Just a generator model thing. Not a marine vs RV issue.

You can use the delay transfer switch in your boat too. May even have a delay but was bypassed by the installer.


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valhalla360

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Posted: 08/18/20 12:28pm Link  |  Quote  |  Print  |  Notify Moderator

The difference between Marine and RV is largely about the cooling system.
- Dedicated boat generators will pull seawater to provide engine cooling and pump it overboard.
- RV generators will use air for cooling. Depending on the size and complexity, it could utilize liquid cooling with a radiator or it may be a simple air cooled engine (like the old VW bugs).

The RPM is unrelated to boat vs RV. AC power in N. America is standardized at 60hz frequency (in other parts of the world, they often use 50hz). There are two common ways to achieve this:
- The old school way and the way power companies do it, is to spin the generator at a speed that corresponds to 60hz and then adjust the throttle to keep it there. Often this is 1800rpm and the generator head will produce 2 cycles per rotation of the engine. 60hz is equal to 3600 cycles per minute and 1800rpm * 2 = 3600 cycles per min. The downside, particularly for small generators, is any sudden change in loading causes the generator to speed up or slow down until the throttle can adjust resulting in inconsistent power output. Another downside is even with a very light load, the generator has to keep feeding enough fuel to keep the RPM up at 1800rpm. Big power plant generators have so much momentum and the draw from thousands of houses averages out, it's not really an issue.
- The newer way for small generators is inverter based technology (newer being at least a couple decades in common use). The generator actually produces DC power which is immediately fed into an inverter, which converts it to 120v AC power. This has advantages for smaller generators. The electronics in the inverter can control the Hz very accurately. They can also eliminate the spikes and dips in voltage. This makes the power much nicer for sensitive electronics. Also, since the Hz is not dependent on RPM, when under light load, it can throttle way back to idle speed cutting fuel consumption.


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Ivylog

Blairsville, GA and WPB, FL.

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Posted: 08/18/20 12:48pm Link  |  Quote  |  Print  |  Notify Moderator

7800 and smaller Onan’s are DC generators into an inverter that only puts out 120V. 10K and larger are AC generators that put out 240V. They (your boat 11K) maintains 60 cycles by staying at the same rpm unlike the inverter in the smaller generators that maintains the 60 cycles and rpms vary as the load varies.


This post is my opinion (free advice). It is not intended to influence anyone's judgment nor do I advocate anyone do what I propose.

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dougrainer

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Posted: 08/18/20 03:20pm Link  |  Quote  |  Print  |  Notify Moderator

Ivylog wrote:

7800 and smaller Onan’s are DC generators into an inverter that only puts out 120V. 10K and larger are AC generators that put out 240V. They (your boat 11K) maintains 60 cycles by staying at the same rpm unlike the inverter in the smaller generators that maintains the 60 cycles and rpms vary as the load varies.


I believe that your statement should mean Onan DIESEL, not include Gas. Gas RV Onans are RPM based at either 1800 or 3600 constant RPM's with the Engine SPEED governor keeping the RPM's at 1800 or 3600 under load, and do not use Inverter technology. Doug

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