On 08/23/2010 11:32 AM, Cross-Slide wrote:
On Aug 23, 11:11 am, "Existential wrote:
"Pete wrote in message
ter.com...
Bob La Londe wrote:
Why are so many big milling machines 3 phase?
Its not about horsepower. I have a compressor with a 5 horse motor that
runs on 120V. It draw close to 20 amps on startup, but...
Straight single phase 220V is more than adequate for any power for most
shop
machines. There are some big machines out there, but for most shop
machines
its fine.
I can think of two possible answers.
1. Price of electricity. It was explained to me that power companies
often charged based on the peak demand of the leg drawing the most
current
at any given time. By balancing the load you can reduce your power bill.
2. Smoother operation. With the vagaries of the power supply from
many
electric companies by going 3 phase you can get a smoother operating
motor
with less variance of speed.
(I have worked in facilities with 440 and even 680 motors, but that is
a-whole-nuther conversation.)
Your "5 horse" motor isn't, 5HP is 30A+ at 120V. Pretty much all real
5HP motors are 240V or higher.
Three phase motors a
- Simpler, no start or run capacitors or centrifugal switches.
- More efficient.
- Have better low RPM torque and starting characteristics.
- Require smaller wiring due to lower currents on three wires for a
given HP than on two wires for single phase.
- Higher HP for a physical size.
- Less expensive due to their simplicity.
- More reliable.
- Can be accurately speed controlled.
- Smoother operation, less torque ripple.
Mebbe 4 phases would be even better? 
Seriously, tho....
--
EA
Actually, Two phase, three phase, any polyphase circuit can be
converted into another polyphase circuit.
You can take three phase and get two phase or visa-versa. Or Five or
12 or whatever you want.
I used to work where we took three phase power and converted it into
twelve phase power with transformers.
Look up Schott-T transformers for more magic.
The original generators at Niagara Falls were two phase, (IIRC) and
transformers converted the power into three phase for power
distribution.
I have been told, but did not confirm that three phase wiring is the
most economical for copper vs power delivered.
Two phase power is two phases 90 degrees part. NOT 180 apart as
someone suggested.
I was being lazy. Yes, two phase, as marketed, is 90 degrees apart, not
just single phase with a return.
Two phase (by your correct terminology) needs at least three conductors,
with legs at 0 degrees, 90 degrees, and a return whose current is -135
degrees away from the current in the other two (or with separate returns
for each phase). So it requires the same number of conductors as three
phase, but the return wire has to carry 41% more current than either of
the other two conductors.
Three phase still needs three conductors, but the requirements on each
conductor is the same as any other. It has a whole bunch of handy
symmetries that make it easier to use than two phase (which was really
my point in my other post, where I got too fast and loose with terminology).
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
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