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What Have You Done to Your Onan Generator Today?

len k

Subscriber
Last Subscription Date
12/12/2018
Zephyr7 ...... I'll do a test to get you those numbers you asked for.
I'll have to put together some test equipment.
Might be easier for me to just put 3-4 car bats in series , then use power resistors to limit field current to get gen output voltage of ~ 120VAC, then measure field voltage and current

Gen (7NHM) DEFINITELY will need 12V flashing EVERY time gen starts.
On this gen , residual field magnetism is about nonexistent.
Without field excitation there is NO AC output,
When using Simpson analog VOM on 120V scale, needle does NOT move.
I'll have to check again on a more sensitive lower voltage range, I seem to remember some gen putting out 2 - 4 volts.
This happens even if gen made 120V output just 10 seconds ago, then engine was stopped.
In fact gen can be making 120 V , then if disconnect field excitation with engine still running, output drops to ~ 0V
 
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Zephyr7

Registered
Bill, you might need to add into the circuit an auto field flash circuit, to flash a field from the 12 volt Supply every time the generator starts up, depending on the regulator's design of the residual startup voltage requirement.
It wouldn’t be difficult to add something like that. It would need a delay, with a bit of logic. Something like “the engine has been running a few seconds but there is no voltage output so apply 12v DC to the field to get things to start generating power”. This would need to be powered from the 12v system, so it would need to get a signal from the onan controls indicating the set is running, which could probably also power the circuit, with a delay so that it the 12v is present for more than a few seconds without seeing 120v AC output a relay would click over to flash the field with 12v DC from the battery for a second or so before dropping out and running normally.

This could be designed as an add-on to the regulator so that you don’t have to build both. Keep’n it simple and all :)

Bill
 

Zephyr7

Registered
Gen (7NHM) DEFINITELY will need 12V flashing EVERY time gen starts.
On this gen , residual field magnetism is about nonexistent.
Without field excitation there is NO AC output,
When using Simpson analog VOM on 120V scale, needle does NOT move.
Then also tell me what the AC output voltage is with 12v applied to the field, since I need to make it all start working with whatever voltage that is.

Would it hurt anything to make the genset flash the field EVERY time the set is started? This would simplify the circuitry since there wouldn’t have to be a “check for output before flashing” circuit. I could design the regulator to ALWAYS apply 12 v to the field UNTIL the output voltage came up to some setpoint, then have the 12v drop out and power the field normally. This would be very simple to implement.

Bill
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
Since 12 volts isn’t enough to develop rated voltage even at no load, you could simply use a diode between the engine ignition circuit power and the field.

The generator field gets battery voltage anytime the ignition does, and as soon as the regulator has enough supply voltage to work with, it supplies all of the exciting current, and the diode of course prevents any current flow back into the 12 volt system.

This assumes that your regulator is isolated between its input and output, since the field negative will need to be grounded.

Alternatively this circuit could be powered from the starter solenoid, so it is not active once the engine starts.

Keith
 

len k

Subscriber
Last Subscription Date
12/12/2018
Then also tell me what the AC output voltage is with 12v applied to the field, since I need to make it all start working with whatever voltage that is.
I posted those numbers on previous page, toward bottom, post # 5957,

Also Note: in that post the WIDE variation of rotor ohms with temperature ( copper resistance temperature coefficient)
Rotor temps of: 0 , 77, 125 deg F make rotor ohms of 18.3, 22, 24.3 respectively

=======================================

I believe same reg is used on BGE/NHE/BGD/NHD/BGM/NHM gens

Doesn't matter on 7NHM/5.5BGM gens , ( they just run).
But on NHE/BGE gens (and likely NHD/BGD ones) controls SHUT OFF engine when release start button IF gen is not making AC , specifically bat charging AC out stator B-windings.

So I would design reg to flash gen every time gen is started, that is what Onan did.

If gen doesn't make 120V type output then it won't make bat charging voltage output either.
 
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Zephyr7

Registered
That would actually be preferable, IMHO. Make it operable on 12 to 28 volts and it might work for the MEP-002A, etc.
Probably not too difficult, but a little tricker. 28 volts is getting close to max ratings for many cheap integrated components which usually top out around 30-35 volts or so. I could use a switching regulator such as those from CUI, but that means a “fancy” part that isn’t as commonly available which I want to avoid.

Vanman, neat idea with the diode but it would require some additional parts (at least one diode) to prevent reverse flow into the regulator output. I’ll have to see what’s easier to design, but typically I would build a regulator circuit with negative ground.

Len, resistance change with temperature doesn’t matter since the regulator will just swing the field voltage around to whatever it needs to be to get 120v AC out of the genset. Current and voltage in the field will be automatically adjusted since it’s inside the control loop, so the resistance change of the field windings gets handled automatically too :)

Bill
 

zuhnc

Subscriber
Last Subscription Date
12/08/2019
This is a fun discussion on the regulator. I understand all the concepts, etc, but I am woefully lacking in the expertise to design such a thing, being more mechanically inclined. Would it be possible to start another thread on this discussion, with a synopsis of what has gone before here? Make it easier for search and to keep track of progress. Everyone needs an inexpensive backup regulator for the electronically-regulated sets! zuhnc
 

Zephyr7

Registered
This is a fun discussion on the regulator. I understand all the concepts, etc, but I am woefully lacking in the expertise to design such a thing, being more mechanically inclined. Would it be possible to start another thread on this discussion, with a synopsis of what has gone before here? Make it easier for search and to keep track of progress. Everyone needs an inexpensive backup regulator for the electronically-regulated sets! zuhnc
I agree and I was thinking the same thing. Installing the regulator would probably fit in the “what have you done today...” thread, but designing it really doesn’t. I’ll do the design work for everyone, and explain how it all works so others can learn. I’m an electrical guy (an EE actually), but have learned much here on the mechanical side. I’m happy to contribute to the knowledge pool on the electrical side whenever I can :)

Len and all — I will start a new thread for this. Len, please post your measurements (all of them, including the output from the genset with 12v on the field) in the new thread so that the new thread is a complete record.

Bill
 
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