Posted by LJD on October 31, 2011 at 05:52:37 from (67.142.130.19):
This isn't a trick or rhetorical question that I already have an answer for.
I've been turning wrenchs and doing electrical work for over 50 years. That being said, I just got a surprise.
This past year I've been working with something that's new to me. Also seems to be relatively new to many household electrical inspectors. 12 volt DC wiring in residential households. Especially with high-amp service panels, circuit breakers and switchs rated for DC, etc.
I am building a well pump based on an old piston-pump and converted to a 12 volt DC motor drive. Not long ago these old piston pumps were considered "junk." Now, they are considered "high end" and cost a small fortune. Often $2000 for new one.
Here is my "surprise." My old Sears pump has a 1/4 horse, 1720 RPM, 115 VAC motor on it. These motors are common and a "dime a dozen."
Specs: Will make 1/4 horse continuous and draw 5.1 amps at 115 VAC. That comes to 586 watts.
The 1/3 horse version will draw 6.6 amps at 115 VAC. That comes to 760 watts.
Now, I just replaced with a 1/3 horse DC motor. It draws 26.4 amps at 12 volts at 1800 RPM. That comes to 317 watts.
I am amazed at this. Why is the DC motor so much more efficient then any of the AC motors?
To the converse, I know that an old DC generator on a farm tractor is inefficient because it makes AC power - and wastes much of it by custom selecting only the DC via brush placement on the commutator. Thus the reason why modern alternators using diodes make much more power per size.
I'm wondering if AC motors also waste near half the current to get rotation?
I'm also wondering if the part of the reason the DC motors are so much more efficient is the permanent magnet fields that don't use electricity?
By the way, if you EVER need to install a DC service panel in a house, I discovered a little known "secret." The only major brand that is "dual rated" for AC and DC is Square D type QO. In stock at Home Depot although they are cluelss about the obsure DC rating. Now . . . finding DC rated wall switches is another story.
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