Wiring in a generator

[Snotter]

I plan to start the process of wiring in an older onan 7.5 kw into the boat's electrical system for the first time. I will be using two high current triple pole/double throw relays as a transfer switch. These relays are designed to handle more than the 63 amps the generator is capable of producing. This forum has been a great source for educating me concerning the need to adhere to the safety guidelines concerning the hot-neutral-ground connections and proper polarity. My question concerns how I identify the hot and neutral wires coming out of my generator's stator field.
My generator was manufactures sometime in the late 70s. I've seen no mention of a hot and neutral lead in any of the many documents and manuals that I have. I know that, when generator electricity on the boat, that the neutral and ground wires should be tied together at the source; however, when connecting to shorepower, the neutral and ground tie is done on shore.
If I had to guess, I would suspect that the choice of field leeds are arbitrary. Do I choose a lead to be hot and the other neutral? I would hate to guess wrong and tie a live wire to the generator case, which is electrically tied to the perkins primary propulsion engine. I'd hate to be climbing over machinery while the generator is producing electricity and getting zapped. Advice is always appreciated from this group.
BTW--I have two 30 amp shorepower cables coming in to the boat. The green wires from both cables go through a single galvanic isolator.

 

[flynfol]

I've tinkered with the RV version of the Onan and they all had a junction block under the cover. White wire was neutral and black and/or red were hot. The neutral was also grounded to the chassis with a green wire at the same junction block. You could also identify which is neutral with a voltmeter. Check for potential at the junction block between the chassis and either wire while the gen is running. One hand in your pocket, other for the probe....;-) I used a similar relay with mechanical lockout for the standby generator connection to my house. All lines(neutral and hot) went through the relay.....except ground. That was continuous. I truthfully don't know what CODE is for a boat.

 

[Snotter]

I've tinkered with the RV version of the Onan and they all had a junction block under the cover. White wire was neutral and black and/or red were hot. The neutral was also grounded to the chassis with a green wire at the same junction block. You could also identify which is neutral with a voltmeter. Check for potential at the junction block between the chassis and either wire while the gen is running. One hand in your pocket, other for the probe....;-) I used a similar relay with mechanical lockout for the standby generator connection to my house. All lines(neutral and hot) went through the relay.....except ground. That was continuous. I truthfully don't know what CODE is for a boat.

The only thing in the electrical box are the four wires coming out of the stator field windings. The four wires are joined to form a two wire 120VAC output. The wires are devoid of markings though, I believe, they are referred to T1 through T4 in the onan literature. The onan literature indicates that two inductive field windings are wired in parallel to provide 120VAC single phase in domestic sets. The onan schematic indicates neither a hot nor neutral, nor any color, nor any connection to ground. Also, a digital meter was used to measure resistance on the plant between the field output leads and ground. The leads are isolated from ground, at least in a DC sense.

I've had the generator apart on 3 separate occasions during the year and a half it has taken for me to refurb it and to get it to generate. I bought the plant off of ebay. The unit did not generate electricity when I bought it. I got the thing to generate for the first time yesterday after replacing the stator exciter field windings.

 

[flynfol]

You might have a reconfigurable head. Is it a 1R or 3 CR head? Depending on how you wire it at the junction block, you can have two 120 legs in parallel or 180 degrees out of phase for 120/240. If you have no need for 240.... I would wire it as a single 120 output. This keeps the gen balanced and allows starting heavier loads.

http://www.smokstak.com/forum/search.php?searchid=3748473

Check out this site for Onan info......these guys are awesome.

You had no continuity from any of the leads to ground/ chassis? I seem to remember a ground at the brush block assy. Maybe on a marine gen head they isolated the rotor......in which case pick one....;-)

My experience has been with magnaciter rotors. The 4 pole 1800 variety. Brushes are used for AC and DC(starting). The rotor output wires can be changed depending on genhead.

 

[flynfol]

Just ran out to look at my Onan. Wires coming off the rotor were all the same color. On the Onan I have, there is no regulator. The magnaciter is designed to output DC voltage to the stator windings and is rpm sensitive for AC voltage output. As long as none of your outputs are grounded( my neutral was grounded at the terminal block to the chassis) I would guess ...take your pick since it is wired for only 120.

 

[Snotter]

You might have a reconfigurable head. Is it a 1R or 3 CR head? Depending on how you wire it at the junction block, you can have two 120 legs in parallel or 180 degrees out of phase for 120/240. If you have no need for 240.... I would wire it as a single 120 output. This keeps the gen balanced and allows starting heavier loads.

http://www.smokstak.com/forum/search.php?searchid=3748473

Check out this site for Onan info......these guys are awesome.

You had no continuity from any of the leads to ground/ chassis? I seem to remember a ground at the brush block assy. Maybe on a marine gen head they isolated the rotor......in which case pick one....;-)

My experience has been with magnaciter rotors. The 4 pole 1800 variety. Brushes are used for AC and DC(starting). The rotor output wires can be changed depending on genhead.

I have the MDJE-3CR, which doesn't have brushes. The exciter is brushless.

Concerning smokstak--they are awesome. I used them to find the electrical manual for onan generators. The manual allowed me to figure out that someone installed two negative diodes on the rotor of my generator instead of one positive and one negative. I also used the manual to figure out my exciter field coil (stator) was open-circuited. Luckily, I had a spare generator I bought for parts and was able to swap the field coil.

 

[Snotter]

Just ran out to look at my Onan. Wires coming off the rotor were all the same color. On the Onan I have, there is no regulator. The magnaciter is designed to output DC voltage to the stator windings and is rpm sensitive for AC voltage output. As long as none of your outputs are grounded( my neutral was grounded at the terminal block to the chassis) I would guess ...take your pick since it is wired for only 120.

My MDJE-3CR has a solid state voltage regulator. (I bought a new circuit board off of ebay which was, imho, reasonably priced.) I've had no problems finding new parts for this generator.