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18 KW generator low-voltage question

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jdemaris

12-23-2005 06:27:58




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My question is in regard to general "theory of operation" on an AC generator. I'm working on a 18KW generator powered by an F162 Continental gas engine. The unit is a Fairbanks-Morse built in 1964. Information is no longer available from the company. This generator has hardly ever been used since new - it was a self-starting standby unit in a local school. But, it then sat outside in a farmer's field for 10 years. So, I've been trying to figure it out, trace the wiring, etc. with no schematic. It has many relays, an "overcrank button", an "overcrank circutit breaker", a three position main switch with "Manual", "Off", and "Automatic" modes. Also has a control knob on a variable resistor marked "Exciter" that can be turned from zero to one-hundred (ohms maybe?). Inside the control panel - not normally accessible, is a Colt voltage regulator that also has a little adustable pot on it labeled "lineator." So, now my question. I've got the engine running nicely. The generator is rated 18KW, 120/240 volts at 1800 RPM. When I first ran it - the AC voltmeter on the panel read 120 volts - so I thought all was working great. But, I found that it taps both hot-legs. So, it should be reading 220-240 volts, not 120. I then checked voltage between each hot leg to neutral - and both checked out at 60 volts. Then, I put a hand-held tach on the generator and it was reading 1500 RPM which is slow. I turned the governor up to 1800 RPM and the voltage came up accordingly. It is now at 160 volts across two hot-legs, or 80 volts on either hot leg to neutral. So - now I am wondering if I have an RPM problem, a voltage regulator problem, or something else? If I turn up the RPMs, the voltage will rise more - but I don't really want to do that. I know that with DC generators, you can get proper voltage at almost any RPM - it is the current available that is most RPM dependent. But - how does it work with an AC head? I am assuming, but don't know for sure - that the reason Honda, and Yamaha, et. al. have started using DC heads on their AC "inverter" series generators, is to offer the capability of running at low RPMs at low AC loads. I am not an expert - and neither is anyone at my local electric-motor shop (I called and asked, they are clueless on the matter). I'm trying to figure out what the next logical step is in order to get the proper voltage.

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Richard H.

12-24-2005 06:40:25




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 Re: 18 KW generator low-voltage question in reply to jdemaris, 12-23-2005 06:27:58  
Just a thought, I have been around a few installations of gen-sets and they usually calibrated with either a load on it or a load bank device (giant toaster lookin thing). There are also a few "Y"s in the nomenclature which has me thinkin 3 Phase possibly. Just a few thoughts that hopefully might arouse some answers on this from these guys.



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NEsota

12-23-2005 11:13:37




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 Re: 18 KW generator low-voltage question in reply to jdemaris, 12-23-2005 06:27:58  
A synchronies electric kitchen clock compared to an accurate one minute timer will get you in the ball park on the proper RPM. You can get it exact by comparing it to your shop-hold 60 cycle line power. Check this at 110 volts by hooking two 110 volt bulbs in series and wiring to both the line and the generator set. Recommend, for safety reasons, three prong grounded plugs with this series connection. You may be driving a DC generator that supplies excitation voltage for the main power unit. If this is the case that DC voltage may vary with speed and exacerbate the main output voltage fluctuation.

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55 50

12-23-2005 10:25:22




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 Re: 18 KW generator low-voltage question in reply to jdemaris, 12-23-2005 06:27:58  
It may be possible to contact one or more companies that are buildings electric generators today and see if they would be willing to give you some clues. Companies like Generac, Coleman, Lincoln, etc. They might have some "white papers" on theory that they would share. Maybe this stuff is even available by "internet searches".
As an aside, I have a schematic and wiring diagram for a 10KW Army unit at 240 volts single phase and 60 hertz that was used for the standby power of an AM/FM radio station that I used to do the engineering work for. It was built in the 1940s by Okeefe and Merritt Company of Los Angeles, powered by a 4 cylinder Jeep engine 35 HP and ran at 1800 RPM. I"m not familiar with it"s technical details. My area of expertise is Radio Frequency stuff, not AC power generators. If this might be of some help to you, I could send you a copy if you give me an email address to communicate to. thanks

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jdemaris

12-24-2005 07:35:01




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 Re: 18 KW generator low-voltage question in reply to 55 50, 12-23-2005 10:25:22  
Thanks for the offer on the military stuff, but my son just sent me some info that is probably similar. He was stationed on a island military airfield that has similar older gensets as you describe. In regard to contacting companies - I've lost patience with that. In the recent past, I've contacted some of the big name companies with what I thought were simple issues - and got nowhere. For example - I was just recently trying to repair a new Coleman generator - 1000 watt - appears to be a inverter series that they quickly discontinued. So, they don't actively sell them anymore, yet there are many out there, brand new on dealer's shelves. I called Coleman for a schematic and their tech. department told me it is not available. They also told me no parts are available except for the complete control-board - at a price of $390. Well, that's more than the entire generator is selling for. So - with Coleman - just about no support. Then - I called several big-name companies trying to get data on the output their generators. By output, I mean how clean the AC voltage is. Grid current normally called 120 volts AC, usually spikes in a wave form to 170 volts. Many battery chargers are designed to operate in the upper part of that wave form. So, many battery chargers will not work properly in most portable generators. At the time, is was a major issue with what I needed to do. It is a major problem with people using solar-electric hybrid systems that use a battery bank, solar panels, a generator, and a large inverter/battery charger.
Not a single company could answer my question. Now, I see some companies are paying attention - that is why Honda, Yamaha, et. al. are making the "inverter" series now - that uses a DC head and a full-wave inverter. They claim to make power similar in wave-form to grid-current. Also, as a side-benefit, they can maintain voltage at low RPM if the load is light - and throttle up when more is needed.

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RND

12-23-2005 10:23:04




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 Re: 18 KW generator low-voltage question in reply to jdemaris, 12-23-2005 06:27:58  
I don't know anything about your specific unit, but I do know a fair bit about synchronous AC generators. Open circuit (no load) voltage is directly proportional to speed. Electrical frequency is also proportional to speed. You have a 4 pole generator which will produce 60 Hz electrical frequency at 1800 rpm.

Open circuit voltage is also directly proportional to field current (assuming all is working as designed). Do you have a way to measure field current? I would expect the generator name tag to list no load field current (perhaps labeled IFNL). Field current will increase (if the voltage regulator is working) with load to maintain constant terminal voltage (field current needs to compensate armature reaction). I also would expect the name tag to list full load field voltage (IFFL). If only one field current value is listed, I would expect it to be full load field current.

It is possible that you have shorted field turns. Actually voltage is proportional to field current (If) times number of field turns (Nf) times speed (rpm). If you have shorted turns it will take more (perhaps excessive) field current to produce rated terminal voltage at rated speed. The name tag might also list field resistance, which could be another way to check for shorted field turns.

email me if you want to discuss in more detail.

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jdemaris

12-23-2005 11:00:36




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 Re: 18 KW generator low-voltage question in reply to RND, 12-23-2005 10:23:04  
Tag on gen-head reads:

Model 5SJ4324P23Y4-F Series YX4109012

Type SJ Frame 324Y KVA 18.75 KW 15

Cycles 60 FL Amp 78 Encl DP

C Rise Arm by Therm 70 degrees

FLD By Resis 80 degrees

Exc Field Amps RVFL 1.3



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Roger in Iowa

12-23-2005 09:18:52




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 Re: 18 KW generator low-voltage question in reply to jdemaris, 12-23-2005 06:27:58  
You can check cycles by using a photo tach and a florescent light. If you can get the light to light. The photo tach will indicate 7200 when at 60 cycles per second.

Roger in Iowa



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Gary Schafer

12-23-2005 07:53:02




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 Re: 18 KW generator low-voltage question in reply to jdemaris, 12-23-2005 06:27:58  
Could it be that it is supposed to run at 3600 rpm and not 1800? It almost sounds like it from the voltage you are getting.

First though you need to measure the frequency output. Set the rpm's so you get as close to 60 hz as possible. The governor should hold the rpm's so the 60 hz is maintained with and without load.

Once you have the frequency set then adjust the voltage regulator until you get the proper voltage output.

Regards
Gary

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jdemaris

12-23-2005 08:32:39




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 Re: 18 KW generator low-voltage question in reply to Gary Schafer, 12-23-2005 07:53:02  
I know that it is intended for 1800 RPM. It is stamped all over the spec. plates on the generator head itself, and also the genset tag. And, it is obvious, upon inspecton of the governor, that this thing never ran 3600 RPM. I suspect the F162 Continental is not even capapble of sustaining that speed. Also, the 1800 RPM horsepower rating of the engine istself seems correct. Same basic engine was used in the Massey Harris 30 and tested at 33 horsepower at 1800 RPM. Figuring with a genset, one horsepower is good for 580 watts, that's about right since this thing is rated at 18.75 KW it would need 33 horsepower to run at full load. I don't have a good way of checking the Hertz cycle - unless I dig out my old oscilloscope. I'm still trying to find out how variable the voltage is with an AC head. At present, the voltage rises as I raise the RPM - just like my PTO generator does - but I think the PTO generator is supposed to be that way. It has a green-zone on the voltmeter with the correct voltage setting is - and you are supposed to rev up the tractor until you hit that green zone. Now - with this genset - since it has a voltage regulator - I assumed that the voltage (once the minimum is met) will be controlled - but how? Is it done by controlling the field input?. So, if this thing is not reaching the minimum - can it be due to lack of field current - caused by the voltage regulator? This - I have no knowledge of. I might just take the regulator off, and take it apart. I suspect once I see inside of it I will be able to understand how it is supposed to operate. Considering is was built in 1964 - most should be visible to the naked eye- unlike modern micro-processor controls.

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Gary Schafer

12-23-2005 08:50:35




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 Re: 18 KW generator low-voltage question in reply to jdemaris, 12-23-2005 08:32:39  
Yes the voltage will rise with rpm, that is normal. The voltage output is very dependent on rpm on a generator. But you need to set the rpm at 1800 and leave it there. You can find some of the cheap multimeters that have a little frequency counter in them. Not much of a counter but ok for this kind of thing. But setting the rpm at 1800 should get you close on that.

Once you have the proper rpm then you must adjust the regulator to get the proper voltage. Don't do it by changing rpm as that will throw the frequency off which you don't want.

Does the generator have any outlets marked 240 volts or only 120 volts? It may be that the 120 volt outlet, or wires, are center taped. You might see all sorts of strange things on some generators.

Regards
Gary

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jdemaris

12-23-2005 10:19:03




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 Re: 18 KW generator low-voltage question in reply to Gary Schafer, 12-23-2005 08:50:35  
No outlets on it - it had been hard-wired when it was the school backup system. I will be adding them. It has a output box with a large two-pole breaker and a busbar next to it with the neutral. The breaker has no writing on it - but it must be a double 80 amp or 100 amp. I must have the RPM pretty close now. My old Delco hand-held tach shows 1800 RPM and my old Allen tach and dwell meter shows 1950 RPM. Nice to have old equipment. New newer, digital photo tach crapped out and it won't work at all. I still don't know if AC output is directly effected by the exciter input. As I said - I'm not real knowledgable on this. The outside panel with all the easily accessed controls and gauges are - has a rotary knob - just like the volume on an old radio - that goes from 0 to 100 and is marked "Exciter Rheostat." I checked it with an ohm-meter and it seems fine. But turning it makes no difference on the AC voltmeter when running. I just took the voltage regulator off and apart. Every part in it is instantly recognizable except for one little plastic cube. Everything else includes - carbon resistors, wire-wound resistors, capacitors, one variable pot, one transformer, several small diodes, and two big diodes as you might see in an automotive alternator or a battery charger. No transistors that I can see - but I don't know what the little plastic cube is. It is marked MTC 15500862 and is soldered to the circuit board with four prongs. It's an old-fashioned circuit board that you probably couldn't break with a hammer. This regulator has a bus-bar with nine terminals - seven of which were being used. At this point - I wouldn't mind buying a generic modern replacement if available. But - I also have no idea if this regulator is the problem. You said the voltage varies with RPM - but should it level out at some point? If not, what exactly does this "Voltage Regulator " do if it doesn't regulate voltage? Also, anybody have an idea of what the variable pot in the regulator does? It is labelled "linearity adjust." By the way, the regulator is a Colt Industries, Fairbanks-Morse Power Systems Division, Magneto and Engine Accessories, Model Ho. 155010/0

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Stickler

12-26-2005 00:15:40




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 Re: 18 KW generator low-voltage question in reply to jdemaris, 12-23-2005 10:19:03  
I don't know exact specifics on your machine, but I operate hydro power stations for a living, in fact I'm at work right now. "Exciter rheostat" on a large generator is used to adjust the exciter field voltage, which in turn varies the output voltage. That would be like a coarse adjustment, then you have a voltage regulator, which makes constant small or fine adjustments to keep voltage where you set it as load changes. Without a regulator, the output voltage would change with every load change, not a desirable condition. It sounds to me like the small pot inside may be for adjusting regulator sensitivity.

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KEB

12-23-2005 13:51:34




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 Re: 18 KW generator low-voltage question in reply to jdemaris, 12-23-2005 10:19:03  
Have you tried adjusting the "exciter" pot? Frequency is set by the engine speed, so it should be set to 1800 RPM and left there. The frequency will be 60 Hz at 1800 RPM, period. Small variations in frequency (a few Hz) will have very little effect on any loads except those that use synchronous motors.

At a fixed speed, voltage is proportional to the current through the field windings. I'm not familiar with that particular generator, but it would seem that adjusting the output of the exciter would adjust the current through the field windings and hence the output voltage.

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jdemaris

12-23-2005 14:30:23




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 Re: 18 KW generator low-voltage question in reply to KEB, 12-23-2005 13:51:34  
Yeah - that's what I was expecting - but turning the pot makes no difference. I turned it all the way both directions while running and the output voltage stayed the same. I checked the pot itself and it is working fine (I think), but I only checked it with an ohm-meter and it read from 0 - 100 ohms as I dialed. A better test probably would have been a voltage test on those pot leads while it was running. I'm in the process of pulling the head all apart to take a close look at the diodes inside of it.

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buickanddeere

12-23-2005 18:50:44




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 Re: 18 KW generator low-voltage question in reply to jdemaris, 12-23-2005 14:30:23  
Before you get too keen to tear
it apart. Put an ammeter, good from 0 to 2 amp in series with the field excitation leads. Either one going to or from the brushes will do. Now tweek the pots and see if there are any changes in current and how much. Try measuring the voltage across the brushes too in DC and AC. If an AC voltage appears then a diode has failed. What is the resistance measured from the rotor's slip ring to slip ring without the brushes touching it or the wires connected to the brushes. Don't measure resistance through the brushes, they will wobble your results.

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jdemaris

12-23-2005 18:58:44




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 Re: 18 KW generator low-voltage question in reply to buickanddeere, 12-23-2005 18:50:44  
There aren't any brushes. It is a brushless design, it appears to have dual amatures and dual fields, one big set for output, and one small set for producing field. The small set, on the armature is a ring with three rectifiers - IN1185s.



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buickanddeere

12-24-2005 06:39:23




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 Re: 18 KW generator low-voltage question in reply to jdemaris, 12-23-2005 18:58:44  
That does change things a bit. Still check the input to the staionary excitor field windings for varying voltage and current when the pots are tweeked. If the stationary excitor parts check ok it's time to test the rotating parts. The diodes maybe be soldered in? Supplying a limited AC supply across the inputs of the diode assembly as if the power was coming from the rotating windings can work. Then measure for DC, AC or both coming out of the diodes and into the excitor field. Could be as simple as a smoked diodes? Anything look or smell smoked or bubbled? Last one I was into was cheapo Coleman. The Father-inlaw had rebuilt the engine but unknowingly snagged a rotor field wire when jamming it all back together. It didn't make power until the field was flashed. Ran fine for for the brief test run but quit later in a ball of spinning sparks. Had to drill and tap a slip ring, unwind a turn to rotor wire to get enough length and solder it to the connecter which was locktited into the slip ring. Outfit still works much to my suprise.

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