In article ,
John Manders wrote:
"SRF" wrote in message
m...
I'm setting up a hobby machine shop and am getting into the issue of 3
phase
motors. Most of the motors are going to be in the 1 to 3 horsepower
range.
For motors of this size, why would I want to have make three phase power
to
run these machines instead of just swapping out the motors for single
phase?
Thanks.
Steve.
There are a number of answers.
As others have said, industrial 3 phase motors are much better made than the
cheap single phase equivalents.
Amen!
Used 3 phase machinery is generally cheaper than single phase stuff.
Again amen.
You only need to make one converter instead of buying a number of separate
motors so the total cost is less.
Correct -- or you can set up a VFD (Variable Frequency Drive) to
generate three phase at the machine, which offers you the advantage of
being able to fine-tune the motor speed (while it is running) to
eliminate chatter.
I would suggest that for machines which have only steps in their
speed selection (e.g. gears or belts and step pulleys), the VFD is
preferred, while for those with variable speed pulleys, and for running
multiple machines at a minimum initial investment, a home-built rotary
converter (built around a larger three-phase motor, some capacitors, and
some voltage-sensitive relays) would be the better choice.
For long-term operating cost, the VFD will put more of your
energy budget directly into the motor, but the initial investment is
greater.
*My* choice is VFDs. I have two machines which currently have
three-phase motors, and a third three-phase motor ready to go into my
lathe to replace the existing single-phase motor.
A number of machines use non standard motors so replacing them is difficult.
The answer quoting 120 times per second that the power is zero made me think
a bit. The first reaction was to realise that you are on 60 Hz mains and we
(UK) are on 50 Hz. The other thought was that a single phase motor doesn't
actually rely on single phase as such but it makes another phase by use of
an inbuilt capacitor. This second phase is frequently of a lower power than
the primary phase though hence 3 phase is generally a better motor.
Well ... that depends on the motor. There are some single-phase
motors (capacitor-run motors, and hysteresis-synchronous motors (which
would be found on audio turntables and tape recorder decks, not on
machine tools)), but most found on machine tools are only capacitor
*start* motors -- the manufactured second phase is present only when the
motor is at a standstill, or very low speeds, just to get it started in
a desired direction, and more quickly. If you are starting in under
load, it is possible to spin a motor by hand, then switch on the power,
and do entirely without the starting capacitor. And once the motor is
up to speed, the starting capacitor and winding are disconnected
automatically, so you are back to the once per 120th of a second (or
once per 100th of a second in the UK) zero power point.
For surface grinders in particular, the three phase motors
produce a much better finish than the single-phase ones -- thanks to the
vibration introduced by the zero-power points.
Enjoy,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. |
http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---