On 08/23/2010 09:11 AM, Existential Angst 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....

1 phase: 0 degrees, with return.

2 phase: 0 degrees, 180 degrees. Same as one phase, only we've renamed
the return.

3 phase: 0 degrees, 120 degrees, -120 degrees. Because the three phases
always sum to 0, a neutral wire is only needed to carry current for
single-phase wiring on one or more leg, and for broken machines.

4 phase: 0 degrees, 90 degrees, 180 degrees, -90 degrees. With just a
bit of thought you can see that this is really only two independent
phases, with returns. You could go to one return, but then it'd have to
carry current so couldn't really be a 'neutral'. Transformers would be
weird. Motors would be weird. The 3 phase motor hook-up dance "try it
out and swap two wires if it goes backwards" wouldn't work -- get phases
wrong and all hell would break loose.

5 phase: five wires instead of three, with the electrical phases 72
degrees off. There's no significant smoothness advantage, and now if
you get the wrong two phases swapped all hell breaks loose instead of
just having a motor run backwards.

6-phase: just like three phase, only with individual returns. Why?

etc.

--

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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