Thanks. I generally don't like to see this forum spend time on talking about the bad guys out there, but sometimes it deserves mentioning.
In this case, I'd love to be told that this company has changed its ways, adding useful information and removing misleading and false statements, but I see that they haven't changed any of the false claims that have been in place on their site now for several years.
I didn't elaborate on that PMA motor spec and why it's misleading, but it provides a great discussion point.
Here it is again:
So, you look at that and the first head scratcher is the labeling on the axes. Vertically, at first it looks like 150RPM increments, but then you see that above 600RPM it appears to be 300RPM/line, then above 1200RPM it becomes 350RPM/line, and above 2600RPM it's 1300RPM/line, but then the scale drops to 1200RPM/line above 5200RPM, then finally takes a huge jump in scale above 7500RPM. Seemingly a similar story with the horizontal axis.
And so, say you want to use this as a generator and you want to be able to deliver current to a 12v battery bank and you can know your steam engine can only spin it a max of 1800RPM. What does the curve above promise?
Well, first, you see the 1800RPM vertical location and see that it intersects the curve at a point midway between what is labeled as 31 and 81amps---so you figure that this means you should be able to deliver 50amps or so into 12v.
But then you think, "wait a minute, how do I know those aren't 'short circuit' currents instead of delivered currents?" A: You don't.
And then you wonder "uh, what voltage is the generator delivering at 1800RPM?" A: it appears to indicate 16v, but who knows?
Then you ask: "What's the internal resistance of this generator?" A: They don't and won't tell you.
The curve is highly misleading and virtually useless.
With the amount of heat that this thing put out to deliver 20amps into 12v (240watts), we figured that this motor/generator was probably about 50% efficient and was losing 240watts to itself in heat. This was a great departure from their grandiose claims made on the website and the efficiency was even less than ordinary alternators at that speed. A reputable, honest vendor would have taken our observations and suggestions seriously and worked with us to come up with an amiable solution. This company would not.
Anyway, whenever you see questionable curves or specs---it is because they are either too lazy, incompetent or intentionally misleading you.
Motor and generator curves have been around for over 100 years---there are standard ways to represent their performance and from them you can derive Kt, Km, Rm and other factors. That cannot be done from the curve above.
-MT
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of GNHBus@aol.com
Sent: Friday, March 18, 2011 11:26 AM
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Re: Catalina 27
Awesome Myles, a great example of why these forums are so valuable,
unfortunately there are human beings that lack, and think nothing of a big lie to
intentionally rip someone off.
In a message dated 3/18/2011 2:17:27 P.M. Eastern Daylight Time,
matwete@comcast.net writes:
Regarding that MANTA motor....
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