Author Topic: Older 7.5 Kw Onan generator rewire  (Read 5070 times)

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Frank S

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Older 7.5 Kw Onan generator rewire
« on: November 10, 2015, 01:43:18 PM »
 I recently bought an Onan 7.5 Kw diesel generator about a 30 year old machine with only 49 hours on it, that was wired for 3 ph this was fine as I have some 3ph needs but for single phase 220. naturally I could only get around 5 Kw or power
 so I pulled the plate and noticed it was a 12 wire generator meaning that all terminals are pulled out for connections.
 Looking at the charts it showed that I needed to wire it for double delta (3.5)(9.11.)  (6.1. L1) (10.8. L2) (4.2.7.10.L0). After doing this it would pull my Miller 210 mig welder with .045 E71T1 duel shield wire even on the max setting with no problems for a while. but after a few hours of welding it felt like I was only getting around 5 KW again. If I flashed the (3.5) and the (9.11) together for a short second it would go back and produce power just fine for a while again.
 the generator now has 90 hours on it this problem only started yesterday
 I was wondering if I could wire it up as a zig zag and maybe prevent having to flash the connections. I believe the zig zag is (2.8.6.12)
 (1.7. L1) (3.9.L2) (4.10.5.11 L0)
 the coils are 1-4,2-5, 3-6 ,7-10,8-11,9-12.
 What are the advantages / disadvantages of double delta over zig zag or can this be done on this machine?
 
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Frank S

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Re: Older 7.5 Kw Onan generator rewire
« Reply #1 on: November 10, 2015, 01:46:57 PM »
Also in future I would like to add a selector switch so I could switch from 3 ph to single phase without having to re wire each time I was thinking of a rotary knife switch like that used on Miller Bobcat welding machines to switch their amperage's or preferably something smaller if I could find something
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bart

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Re: Older 7.5 Kw Onan generator rewire
« Reply #2 on: November 10, 2015, 06:02:48 PM »
   I suggest you try the smokstak forum for your genny questions.
http://www.smokstak.com/

joestue

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Re: Older 7.5 Kw Onan generator rewire
« Reply #3 on: November 10, 2015, 07:25:09 PM »
the zig zag winding is usually  used for providing a ground connection to a floating delta system, that won't saturate the core when dc flows through it.

what might be happening is the SCR control of your welder is not zeroed perfectly and you are drawing some dc from the ac source. this is certainly a problem for my single phase AC welder when certain rods draw asymmetrical current.

but a three phase welder rectified to dc should draw very little or zero dc current, and a zig zag winding, i'll have to check the books again but i don't think it will do you any good.

what you might try doing is getting three, 100uF run caps connected across your generator (if wired for 240), this will suck up 30 amps of reactive current, pushing the voltage higher and canceling out the inductive load of the welder.


also you shouldn't have to switch between single phase and three phase, just wire it for center tapped delta. 120/240, and 240 delta. no wires to change, just a different outlet to plug into.
« Last Edit: November 10, 2015, 07:33:17 PM by joestue »
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Frank S

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Re: Older 7.5 Kw Onan generator rewire
« Reply #4 on: November 10, 2015, 10:56:00 PM »
Thanks Joestue;
 I'll try wiring it (T12T1-L1) (T4T7-L0) (T10T2-L2) (T8T5) (T11T3-L3) (T5T9) and see what it does
 I have  some 3 ph welders but the smallest Mig machine draws over 12k and the DC stick machine can draw up to 20 KW
 This little Miller is the only machine I have that is small enough to run on my generator.
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Frank S

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Re: Older 7.5 Kw Onan generator rewire
« Reply #5 on: November 11, 2015, 10:41:44 PM »
this is the way I wired it today  \
 (T12T1-L1) (T4T7-L0) (T10T2-L2) (T8T5) (T11T3-L3) (T5T9) with L0 bonded to ground. The 3ph worked as expected
 I wired a 50 amp single ph plug to L1 L2 and ground the welder initially welded as hot as it normally should but after a few minutes the amperage seemed to fall off like it was only receiving about half the input current.
 Tomorrow I think I will drag out one of the larger 3ph welders to use as a load for all 3 phases this should tell me if the problem is in my generator or in my mig welder
'
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Frank S

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Re: Older 7.5 Kw Onan generator rewire
« Reply #6 on: November 12, 2015, 11:13:26 AM »
OK IO connected my 300 AMP Airco DC Bumblebee to the generator then with a 5/32" 7018 in the stinger I shorted it to the ground and left it there until the rod turned red then took a few more rods and welded several stringers on a scrap piece of 1/2" plate after 30 minutes the arc strength had not deteriorated this tells me that my issue is not in the generator but in the Miller welder itself. Which is highly probable since it is about 12 years old and has always been used to weld with .045 duel shield wire. Pretty much having the little machine maxed out all the time.
 One more issue I found with the generator was the 120v duplex was originally wired to the L3 and the generator when I got it was wired WYE
 now that it is Delta the L3 was putting high voltage to the duplex High enough that it instantly blew out all of my CFL lights in the machine shop trailer. moved the connection to L1 this fixed that problem
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Frank S

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Re: Older 7.5 Kw Onan generator rewire
« Reply #7 on: November 12, 2015, 12:59:29 PM »
Problem solved with the Miller welder.
 I got to thinking as old as that machine is the contractor may be arced out badly, so I opened it up to find that all contacts were just about completely eroded away
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joestue

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Re: Older 7.5 Kw Onan generator rewire
« Reply #8 on: November 13, 2015, 01:35:05 AM »
Standard disclaimer: i don't have the standard motor connections memorized and a lot of time i find out that some things aren't standard..
Sorry to hear you blew up your CFL's with 208 or more volts, $#|+ happens.

I suspect you toss some motor run caps across those larger welders, run them on low-er-ish amps and you won't notice the difference between a 7.5KW genny and the utility grid.
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Frank S

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Re: Older 7.5 Kw Onan generator rewire
« Reply #9 on: November 13, 2015, 11:01:28 AM »
Joestue; when I re wired the genney  I failed to notice the breaker for the 120v duplex had been previously wired to the L3 connection of the main breaker on the hot side  other wise I would have moved the wire before using the 120v
 I am thinking that since whoever wired it to parallel  star originally they tapped the L1 and L2 for the 220 plug and the L3 for the duplex  the 3ph plug did not have the L3 connected to it either.
 Looking at the wiring chart in the  panel I could wire it to series star and get 415 to 480v depending on the connection code in the AVR

  I have a 7.5 Hp Cleereman Index  drill press with the old tape reader controller on it. in the past I've always used it with a 15KVA step up transformer to run it because it was 460V. Some day I might see if the genny will pull that but I am pretty sure that for that I will have to add those caps you mention for voltage stability. either that or re wire it to lower voltage
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Frank S

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Re: Older 7.5 Kw Onan generator rewire
« Reply #10 on: November 29, 2015, 12:17:29 PM »
I've now put more hours on the Onan than it had when I bought it.(100 vs 49 hrs) It is getting easier to start, probably due in part to being used regularly and the addition of fuel and injector system cleaner additives. I haven't filled it with the rape seed oil Bio juice that I get form a friend of mine yet and I haven't set up my used oil re-processor to make diesel to run in my Listeroid and I don't plan on running any of that in this engine either. 
I live so far outside of the box, when I die they will stretch my carcass over the coffin