| Author | Message | ||
| Dave Silva (Cypress)
Rating: N/A |
see my other post regarding 28 volt HUMVEE alternators. No one in my area is able to test them so I'm on my own. Using the limited resources of "what's lying around" I am attempting to test them and get some idea which ones might be good. using a 1.5 HP motor I've gotten two results on two of the alternators; One spins freely with 24V applied to the field voltage. There is no voltage at the power side. When I attach a drained (but not dead) 24V battery to the power post the brakes come on hard and the motor stops spinning. The second one begins to brake when the field voltage is applied even when there is no load. If I get it spinning good without the field voltage applied, put my meter on the power post I get about 1.5 VDC. I leave the meter in place, apply a field voltage of ony 12VDC and I get a spike at the power post of 28-29 V for a second before the motor chokes and can't spin any more. Am I making sense? I am puzzled that the field voltage creates such resistance even when there is no load. I've read everything I can on alternators but I can't quite interpret my results. Ulitamately I need a more substantial poweplant to keep the alt spinning under load. I'd be grateful for any insight offered. Dave | ||
| herman Rating: N/A |
Well, the first one sounds normal; if you want to play around more, try voltages intermediate to zero and 24V. The second one sounds like it might have an internal short in the armature windings or the diodes. If you have an infrared thermometer, you might be able to pinpoint a hotspot which in turn might be the point of failure, e.g. a particular diode. | ||
| JW Smythe (Jwsmythe)
Rating: N/A |
... and you need a bigger test motor.  I don't suppose you know if either one of them was good, so you can easily identify the proper behavior. I agree with Herman, the first one sounds right. This really demonstrates what kind of load an alternator puts on the engine. I know people with race cars that don't use alternators, so they can keep that little bit of extra horsepower. They usually start doing worse as the race goes on though, because their electronic ignitions need the full 12V to operate properly. I've seen kits for putting a car alternator on a lawnmower engine. I believe they use a 5hp lawnmower engine for that. I can't say it would keep up with the alternators you're trying. | ||
| Gary Stadler (Boogiethecat)
Rating: N/A |
Herman is probably correct, the first one sounds ok and the second one likely has an internal short- windings or diodes is about all there is to short. When you apply full field volage to the second one, since it's probably shorted, it acts as if it's loaded down fully. Don't know what the amp value of your alternators is, but if they were, for example, 60 amp units, ohms law ExI=P so 24x60=1440 watts. Since 800 watts is aproximately a horsepower, it would take, inefficiencies included, 2-3 horsepower to keep it spinning under that load. So it makes sense that it stops your 1.5HP motor. If they are 100 amp alternators, it would take at least a 4hp motor to keep them spinning under full load. But you probably dont have to change your motor- A better way to test things would be use the motor you have and get a variable DC supply to power the field. That way you can vary the field to the point that your 1.5HP motor grunts, but it won't have to stop. Then you can compare alternators output currents vs field input voltages to see how they are all performing, within the limitations of your 1.5hp motor. Sounds like you're on the right track anyway. Get an amp gauge for the alternators outputs and the variable field supply, and you'll be testing like nobodys business in no time! | ||
| Phil Dumpster2 Rating: N/A |
Take an ohmmeter and measure the field resistance. If it's less than 2 ohms and is in good shape then trying applying only 2 ot 4 volts to the field. | ||
| Dallas Farnworth (Dal300)
Rating: N/A |
Dave, Just a thought, but, go down to your local National Gaurd Armoury or Reserve Post and ask them where they get maintenance done on their vehicles. When I lived North of Louisville, Ky, I got one of the Mechanics to work on a Deuce and a half I bought. Again, it's just a thought. Dallas | ||
| Dave Silva (Cypress)
Rating: N/A |
Ok, we are getting closer but I'm still not entirely clear. They are 60 amp alternators but I would not think they would need 3hp without a load? And I would not think the field voltage itself would create a load? But the first one described above, if it was working, should have 24 volt at the power terminal? The second does put out 24 volts at the power post but seems to have a full load with just the field voltage present. What else can I learn from just an ohmeter? I can easily seperate the diode pack from the rest of the alt. It's a great design; the solid state voltage regulator is encased in apoxy in the rear cover of the unit, connected with press in lugs. If I knew what meter reading to expect I could seperate the "might work" from the "don't work" Here's the wiring diagram: http://www.prestolite.com/images_alts/wiredia/pdf_web/AMA-5104UT_wiring.pdf Thanks | ||
| Gary Stadler (Boogiethecat)
Rating: N/A |
Dave, think of an alternator as if it were a multiplier. RPM times field voltage = output. If your output is loaded down as in a shorted diode or field coil, or a discharged battery, then the equation is more like RPM times field voltage = load on your driving motor. So since your motor is a constant RPM, that only leaves the field as the thing you can vary to mess with the equation. Or the load... So, given constant RPM, if you're hitting the field with 24 volts, something has to give, and the only thing left to give is the RPM, meaning it's going to stop your drive motor as you've seen. So you are correct, without a load, they will need very little horsepower. With a full 60 amp load they will need about 3HP to keep it spinning. And your field by itself will not create a load unless the alternator's output is hooked to something or shorted. If your diodes or field coils are shorted, that constitutes a full load as far as the driving motor is concerned...even more than a full load actually. If the first alternator was working perfectly, it should have 24 volts at the output terminal but likely not unless a fully charged battery is hooked to that terminal. Alternators often won't output anything unless hooked to a battery. If it's not doing that I'd suspect the regulator first. The second alternator, if it's putting out 24 volts AND loading the motor down, I'd still suspect a shorted diode. There are 6 diodes in there and if only one or two are shorted, the alternator could still grunt and put out 24 volts while loading down your motor considerably. You can probably figure out where the problem is, as Herman said, with an IR temperature meter, or your finger although you may get burned doing it the finger way... Kinda like asking if you can instantly do calculus because you now have a nice new pencil and a piece of paper, An ohm meter could probably tell you all you need to know, if you know electronics and how to use the meter, and what all the things you could read with it mean. But to troubleshoot with an ohmmeter kinda requires years of experience doing it, and it's not something that can easily be taught over the internet, because there are simply too many things you have to know to use it right. That said, I think you are on the right track. Keep at it and keep asking questions, and you'll get there.... Personally, I repeat, I think the first thing you need to do is get a variable DC power supply that you can hook to the field and then you can start doing meaningful troubleshooting. Without that as your first and most important tool, you're kinda shooting in the dark. Just my opinion, and boy do I have a lot of those these days!! | ||
| Dave Silva (Cypress)
Rating: N/A |
Great info. Thanks. I kinda like the challenge of figuring this out with only the tools I have. Based on what you've said I think I can do that; Assuming the first alt is good, if i put a fully charged battery on the power terminal, (in addition to having field voltage), and if the charge in the battery is equal to what the alt wants to put out then it should spin freely on my small motor. I can also adjust down the output voltage (there's and easily accessable pot) So, with field voltage applied and a fully charged battery the working alt is spinning away, now if I put a load on it, like a dead battery or a 24 V inverter and it bogs down the motor, then I can be reasonably confident the alternator works, Right? thanks Dave | ||
| Gary Stadler (Boogiethecat)
Rating: N/A |
That sounds right Dave. When you speak of "Field Voltage" as in the above post, I'm assuming at that point that you mean field voltage that is coming from the alternator's voltage regulator, right? (ie not straight 24v) If that's the case, you probably have a good one. | ||
| Stan Rating: N/A |
There is a big difference between an alternator working and an alternator working right! To do a valid test you hve to have enough power to drive the alternator to full output, a variable voltage supply for the field and both voltmeter and ammeter to measure the field supply and the output. If you apply full battery voltage to the field it will go to maximum output voltage (over 100 volts) and high current into your load battery which will stall you electric motor. Assuming that you got these alternator on a surplus sale there is likely something wrong with each one. Automotive alternators are three phase and without proper testing, you can't tell if one, two or all three phases are working. After going back and reading your other posts and looking at the wiring daigram that you linked to, I don't think you have access to the field winding. It is connected internally to the regulator and this creates another complete scenario. When you apply voltage to the regulator, you don't know if it is working and so you don't know if it is putting out 0 volts, 28 volts or something in between, which is what is needed. You have to access the field winding directly and apply a variable DC voltage to test the alternator. Testing the regulator is another whole story. Putting a good regulator on a bad alternator can ruin the good regulator. |