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#1
May 29th 04, 06:53 PM
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Alternator teststand
I want to build a teststand where I can test an alternator.
I plan on fabricating a test stand for the alternator. I have a variable voltage source capable of driving enough current to 'drive' the field. I have a dummy load to sink the output current. What I need is a variable speed electric motor to drive the alternator. But I am not sure how much horsepower is required to drive a 60Amp alternator @ the equivalent of 2500 RPM at the prop. Which brings up another question. What is the typical ratio of prop RPM to alternator RPM? I am thinking it is 2:1. Alternatively rather than wasting a bunch of my time - is this something I could just buy from an alternator repair shop? Thanks, Nathan 
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#2
May 29th 04, 08:53 PM
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60 amps X 28 volts = 1680 watts
1680 watts/746 watts/hp = 2.25 horse power Since the alternator is about 50% efficient you will need at least 5 HP drive for full output. The alternator speed is the ratio of the pulley sizes x engine RPM. On Sat, 29 May 2004 17:53:02 GMT, Nathan Young wrote: I want to build a teststand where I can test an alternator. I plan on fabricating a test stand for the alternator. I have a variable voltage source capable of driving enough current to 'drive' the field. I have a dummy load to sink the output current. What I need is a variable speed electric motor to drive the alternator. But I am not sure how much horsepower is required to drive a 60Amp alternator @ the equivalent of 2500 RPM at the prop. Which brings up another question. What is the typical ratio of prop RPM to alternator RPM? I am thinking it is 2:1. Alternatively rather than wasting a bunch of my time - is this something I could just buy from an alternator repair shop? Thanks, Nathan
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#3
May 29th 04, 10:26 PM
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'Course if you only needed 12-14 volts, then you would only need half as
much drive motor---Also, if you "full field" the rotor and don't spin it fast enough you can get stuff inside too hot--thas baddd. John wrote: 60 amps X 28 volts = 1680 watts 1680 watts/746 watts/hp = 2.25 horse power Since the alternator is about 50% efficient you will need at least 5 HP drive for full output. The alternator speed is the ratio of the pulley sizes x engine RPM. On Sat, 29 May 2004 17:53:02 GMT, Nathan Young wrote: I want to build a teststand where I can test an alternator. I plan on fabricating a test stand for the alternator. I have a variable voltage source capable of driving enough current to 'drive' the field. I have a dummy load to sink the output current. What I need is a variable speed electric motor to drive the alternator. But I am not sure how much horsepower is required to drive a 60Amp alternator @ the equivalent of 2500 RPM at the prop. Which brings up another question. What is the typical ratio of prop RPM to alternator RPM? I am thinking it is 2:1. Alternatively rather than wasting a bunch of my time - is this something I could just buy from an alternator repair shop? Thanks, Nathan
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#4
May 30th 04, 01:01 PM
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When I did my time in the army as a sparky, they had an alternator test
stand that was an off the shelf design. It had a vice type clamp to hold almost any model of alternator, generator or starter motor I ever tried to mount on it. Four big truck 6 volt batteries sat on the bottom shelf and the back panel had voltage and current meters. It's drive motor was mains powered and had variable position brushes that could be turned to either direction and would cause the motor to develop more power the further you pushed them away from neutral and in either direction of rotation. It gave out enough to drive the big 100 amp 24 volt Landrover radio vehicle alternators to full load so it had plenty of grunt. Although I have never seen a similar unit in my travels, most alternator repair places would have an idea about where these beasts might be supplied from. Hope this helps, Peter "Nathan Young" wrote in message ... I want to build a teststand where I can test an alternator. I plan on fabricating a test stand for the alternator. I have a variable voltage source capable of driving enough current to 'drive' the field. I have a dummy load to sink the output current. What I need is a variable speed electric motor to drive the alternator. But I am not sure how much horsepower is required to drive a 60Amp alternator @ the equivalent of 2500 RPM at the prop. Which brings up another question. What is the typical ratio of prop RPM to alternator RPM? I am thinking it is 2:1. Alternatively rather than wasting a bunch of my time - is this something I could just buy from an alternator repair shop? Thanks, Nathan
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#5
May 30th 04, 03:34 PM
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"Bushy" wrote in message ... When I did my time in the army as a sparky, they had an alternator test stand that was an off the shelf design. It had a vice type clamp to hold almost any model of alternator, generator or starter motor I ever tried to mount on it. I took an alternator I had bought at a flea market to test at the local auto parts store. It was a 100-amp 14-volt unit. The bearings seemed to be a little rough if you spun the shaft by hand. They hooked it up and clamped it down. The arrangement for the room's alternator test equipment was that once they got it spinning they would flip a switch to excite the magnetic field and then could read whether it was making sufficient voltage. As soon as the technician flipped the switch, and as I watched through a window from inside the store, the alternator tore itself away from its moorings and flew into a wall. The wall, which was sheetrock, did not stop the missile. It went on through, making a loud crash, into the parts supply section. As soon as a load had been put on it the bearings had seized. If that loose alternator had hit someone ...
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#6
May 31st 04, 02:32 AM
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#7
May 31st 04, 03:13 AM
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On Sun, 30 May 2004 10:34:57 -0400, " jls"
wrote: "Bushy" wrote in message ... When I did my time in the army as a sparky, they had an alternator test stand that was an off the shelf design. It had a vice type clamp to hold almost any model of alternator, generator or starter motor I ever tried to mount on it. I took an alternator I had bought at a flea market to test at the local auto parts store. It was a 100-amp 14-volt unit. The bearings seemed to be a little rough if you spun the shaft by hand. They hooked it up and clamped it down. The arrangement for the room's alternator test equipment was that once they got it spinning they would flip a switch to excite the magnetic field and then could read whether it was making sufficient voltage. As soon as the technician flipped the switch, and as I watched through a window from inside the store, the alternator tore itself away from its moorings and flew into a wall. The wall, which was sheetrock, did not stop the missile. It went on through, making a loud crash, into the parts supply section. As soon as a load had been put on it the bearings had seized. If that loose alternator had hit someone ... .... standing in the parts supply. Somebody got luckier than you think. Of course, you could have tried it on a running engine. I'll bet it would make an interesting sound under the hood. :-) -- dillon When I was a kid, I thought the angel's name was Hark and the horse's name was Bob.
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#8
May 31st 04, 01:08 PM
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Dan Thomas wrote:
: The alternator should be a bit more than 50% efficient, I think. It depends on where it's operating, but for a claw-pole automotive alternator (read: practically *all* alternators), efficiency sucks. It's designed to produce somwhere close to its rated output at somewhere slightly above idle. At 2-3:1 for a car, that'd be about 1500-2500 rpm. A Lycoming gearing would probably put it at 4-5:1, or roughly the same 2000-3000 rpm. Over its range of speed and loading, efficiency ranges from 50-80% is typical. -Cory -- ************************************************** *********************** * The prime directive of Linux: * * - learn what you don't know, * * - teach what you do. * * (Just my 20 USm$) * ************************************************** ***********************
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#9
May 31st 04, 04:15 PM
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