Alternator teststand

[Nathan Young]

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

[John]

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

[jerry Wass]

'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

[Bushy]

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
>

[jls]

"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 ...

[Dan Thomas]

(John) wrote in message >...
> 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.

Which on an O-320 is between 3 or 4 to one, so the alternator
will spin at about 8000 RPM.
The alternator should be a bit more than 50% efficient, I think.

Dan
>
>
>
> 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

[Dillon Pyron]

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.

[[email protected]]

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$) *
************************************************** ***********************

[Nathan Young]

On Mon, 31 May 2004 12:08:06 +0000 (UTC),
wrote:

>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.


Thanks for the input everyone!

-Nathan