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Mythical RPC efficiency....

viper

Titanium
Joined
May 18, 2007
Location
nowhereville
I talked with an RPC OEM after emailing them looking for efficiency charts. The guy indicated that an RPC is ((98%)) efficient from 0-60% load with efficiency dropping with load... I about shit myself and asked where he got that data from.. Of course his engineers just throw that out for him. I did ask him for giggles if that means there is virtually no work done when an RPC is idling with no load (0% load)?

I further explained my minimal load tests that indicate something quite different. Actually at partial load 40-60% efficient is a little more in the ball park. I cannot help but believe these manufacturers DO know about the efficiency but down play it because it is bad for business. All I kept hearing was "how well they are balanced".... Then I asked "at what load?", the said "any load"..... BS!!!!

Very grade school basic testing by comparing the input power to the output power will account for electrical losses as well as slip and heat losses in the idler motor. I have no idea how they account for their information but a bunch of bullshit if you ask me.

Just thought I would share.
 
Actually as a hand-waving argument, the manufacter's rep doesn't have a bad story.

Think about it: it really depends on what you define as the "efficiency."

If you call it work done by the load motor, divided by the real power input to the
converter, then the net efficiency defined that way really is ZERO until the
load motor is turned on!

All the input power to the idler is dissipated as heat.

It's only when you begin running the load motor to power a mechanical load,
that the number calculated their way starts to go up. I can extract about
hp from my load motor on a good day - that's 750 watts. The idler has
real losses that are about 250 watts. Total load then is 1KW and real work
is 3/4 of that. So maybe 75 percent effecient, right?
 
Yeah, I am sure you can bend the truth around several ways but if you were to tought certain numbers to the unknowing public, they would probably dismiss this as "highly efficient, whoooopieeee!". The reality is that what I think of in terms of efficiency is a "converter" AKA, "conversion unit" The actual "conversion" is NOT all that efficient. For seldom use, it might not make a difference but let that sucker run all day, every day or with minimal load, it can add up.

I guess all I was getting at is OEMs lying about specs. If you want to look at this from a primal physical calculation, work in/work out = eff. single phase in compared to the 3 phase coming out.
 
In my particular case, it doesn't make much sense for me to lose sleep over
an efficiency number. All I need to know is, whenever that converter is running
I'm burning a few 100 watt lamps. Because the lighting alone in my basement
shop is probably 5-600 watts I consider the converter to be down in the noise.

Obviously I don't run it unless I'm using the machines that need it. In the wintertime
I sort of like the waste heat as it does warm the shop a bit. Summertime? Not so
nice of course.

The machines running off VFDs are probably a lot less wasteful, they probably
only idle at ten watts or so. If I did not really enjoy the plug reversing on the
lathe, and the two speed motor on the hardinges, I suspect I'd convert them too.
 








 
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