Can somone post a link to a description of how 3 phaze power works. My
brother is looking at buying a Felder combination woodworking machine
with 3 4kw motors in 3 phaze and while we both worked for an
electrician as kids we never worked with 3 phaze. If he buys it he's
planing on buying a converter as he can't get 3 phaze power at his
place. He worked in a shop with converter while in college but has
never had to set one up.
Thanks
Karl
Hi Karl
Try searching for Three Phase Power in Wikipedia.
Bob Swinney (of this group) can tell you how Rotary Converters work if
you want to make your own 3 phase from a single phase source.
Jerry
Hi Karl
I support your brother's decission to buy an "electronic one". I suspect
it will be a device that starts the 3 phase motor spinning then allows the
motor to run on single phase. A 3 phase motor will supply nearly full power
when running on single phase, but it will stall much more easily than when
it has 3 phase power.
If all 3 of your 4 KW motors are spinning at the same time from the same
power connection, you will have made your own 3 phase power within the
unit's wiring anyway.
Jerry
Hi Nick
I recognize that you are a smart guy, so there is a misunderstanding.
I say that a 3 phase motor can deliver nearly its full name plate rated
power to the load when it is running from a single phase power source. I
suspect that you agree.
I say that a 3 phase motor will stall more easily when running from a
single phase power source than when running from a 3 phase power source.
You certainly agree with that.
I considered that when one of the three motors is spinning, it becomes a
"rotary converter" for the other motors. My thought was (is) when the
system of 3 motors are included in one machine, not all will be loaded
simultaneously. That assumption could easily be an error.
I am open to learn. Where is my thinking wrong?
Jerry
Jerry
the unloaded RPC motor acts as an LC network and creates the needed
phase shift.
the RPC motor is not being used as it was originally intended as a
LOAD its being used to pass power through it and apply a phase shift
rather than to transform the power into mechanical energy.
The rpc will not generate the third leg properly until it is spinning
at speed. once it is spinning at speed it will take a tiny amount of
power to keep it spinning at speed but that will be insignificant
compared to the load
the 3 motors in the machine are running as loads and are taking power
out of the system rather than generating extra power on the third leg.
and should be treated as loads that remove power
3 Phase power is very desirable. Motors do not suffer velocity modulation
like single single phase motors do and they are more efficient. I am
American, but live in Europe where most homes are fed with 3 phase. It is
wonderful. When I return to the US, I will do my damdest to live where 3
phase is available, but having said that, I do appreciate the single phase
dilemma in the US. My solution would be a Motor Generator set where I would
run a large single phase motor and drive a 3 phase alternator with a very
large flywheel in between. These are very efficient and infinitely more
reliable than an electronic phase converter, as they are very prone to back
EMF caused switch failures with the large inductive loads. I know the
puritans in the group may flame my solution because by using an inductive
motor as a rotory source wwould create a secondary frequency 10% slow due to
inductive slip, but my experience using 60 Hz motors on 50 Hz power has
proven that it works flawlessly and it is definitly the least expensive
solution. Remember that all converters imitate the third phase, but do not
drive the motor in a balanced condition, as this manufactured phase is only
a helper.
Steve
Most of us would kill for 3ph in the home. It would also make breaker boxes
for industial settings a hell of a lot cheaper.
I don't know about that slip thing but a motor / generator with a fly wheel
sure would keep the lights from dimming when you switched on your industrial
sized machine out in the back yard polebarn.
Of course that presents the problem where you start up the gen set. Having
worked with a number of mechanical die cut presses, spinning up the flywheel
is the MOAL (mother of all loads). So now you need a soft start VFD as an
input to the motor/genset. Most HSM'ers would just skip the genset and use
the VFD.
Wes
Heh, I remember an 8'x1" capacity shear with a 20 HP motor and the MOAL
flywheel. Had a manual starter: a 2' long lever that you flipped one way
to energize the start windings, wait about 15 seconds then flip it the
other way to energize the run windings. If you needed a break, just make
sure to not wait quite long enough, it would blow the fuses on the run
windings. Of course that meant having to put up with the plant
electrician giving you an earful about your origin, habits, morals, and
destination but a break is a break. :)
Hi Brent
It isnt clear that the OP is considering a Rotary phase converter. A
converter is often a capacitor that is temporarily connected to the 'not
connected leg' of the 3 phase motor. Once the 3 phase motor is spinning, it
will run well with single phase input power while the third leg of the motor
disconnected.
3 phase motors run well when fed single phase, once they have "spun up" to
near their synchronous speed. A 3 phase motor running from single phase
will delliver very nearly its name plate reted power to a load.
Once one 3 phase motor is spinning, any additional motors connected in
parallel with them will see the "spun up" motor as its being a Rotary phase
converter with single phase power applied to their input.
Bob Swinney wrote a good article describing most of the theoritical
aspects of Rotary Converters.
Jerry
If you have to ask, you probably really don't want to be mucking with
the making of a rotary phase converter anyways.
The vast majority of what you need to know, is that there are three
wires running into the motor, and if you disconnect two of them and hook
those two up in reverse to where they started, it will cause the motor
to run in the opposite direction to the one it was.
A VFD (Variable Frequency Drive) is a pretty wise choice, if you are
not already fairly familiar with, and well stocked with, three phase
motors. Cost is pretty close to or sometimes less than having to buy a
motor and capacitors to use, as well as the requisite switchgear. If you
can read a simple diagram, and follow it up with connecting the wiring
as per, you should have no troubles. If the VFD is used to simply supply
fixed frequency three phase power off the single phase input, you need
not give it a lot of effort in locating it. With three motors to feed
power to, I would suspect this is the way planned.
If you wish to use the "variable" part of the VFD name, consider where
it will be located, and how the inputs are to be fed to the terminals as
required. Tou could, possibly, incorporate the contlols into a pendant
on the end of a cable that would allow the use of the variable speed
feature as desired, when it is appropriate, and use just fixed speed
when it is a better choice.
If you search Ebay for the phrase "VFD" many of the vendors link to
the makers sites where the manuals can be downloaded from. Worth looking
at a few. They will give you an idea of how simply or complicated it can
be made to be.
Cheers
Trevor Jones
I use a rotary 3-phase and it works nicely. I drive transformers with it
to step-up transformers.
There is so much 2 phase around and in backyards I'm surprised he can't
get the power company to set up a set of transformers for him. I suppose
it is money.
Martin
Martin H. Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
TSRA, Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal.
NRA Second Amendment Task Force Charter Founder
IHMSA and NRA Metallic Silhouette maker & member.
No. It only draws 1/3 of the current (only one leg connected). Albeit, the
speed is the same.
I a agree. And it stalls easier, because it only has 1/3 the power. You
can't say it has the same power and at the same time say that it stalls
easier.
Draw a diagram how the legs are connected. You'll see that all
the "generator legs" of the same phase are interconnected. If you think
that one leg of one motor is generating power for an other motor, you
invented a perpetuum mobile.
Nick
Despite you saying you live in Europe, presumably Germany from your
email address I'm not sure where you get the idea that most homes in
Europe are fed with three phase. Some commercial and most industrial
premises are, almost all single occupancy homes aren't, the only real
exceptions are homes that are heated by electricity. Given the cost
of generation with fossil fuels this tends to be those with a
significant proportion of hydro such as Norway.
Hi Nick
You've made some sort of mistake here somewhere. Thats not your usual
style. Ou, maybe I misunderstand what you are stating when you write "No.
It only draws 1/3 of the current (only one leg connected). Albeit, the
When a 3 phase motor is running with 2 of it's 3 wires connected to single
phase power, it can be loaded to deliver nearly its FULL name plate rated
power. That means that alot more than 1/3 of the currentfrom the single
phase power.
Nick, I cant draw a diagram of that machine with three, 3 phase motors.
He certainly hasnt posted a diagram. I considered that when one motor is
running after being easily "spun up", it becomes a Rotary Converter for the
other two motors. A 3 phase motor can deliver more than it's name plate
rated power when connected to a Rotary Converter with single phase input
power.
I wonder why you write statements like you do that seem to mock my
statements. Do you think I am stupid? Do you actually think any of
the information I have posted is wrong? Sure, I often make errors while
trying to explain things, but I dont yet recognize any mistakes in any of my
posts to this thread. You might take some time to think about this Rotary
Converter stuff before writing about *my* errors.
Jerry
Jerry
You forgot, that the motor's shafts aren't connected. This is a saw (or
such) with three separate motors.
You have to draw a diagram how the three legs of the three motor are
connected. You don't want?
I'll describe it with words:
L1, L2, and L3 are the phases. M1, M2 and M3 are the motors.
L1 is the only phase available, so you connect it to M1L1. M2L1 and M3L3 are
open, because we don't want to let them run. OK so far?
Now the voltage generated in M1L2 and M1L3 goes where? Nowhere, because they
are not connected on M2 and M3 (because we don't want to let them run).
Now we let run all three motors at the same time.
Connections:
M1L1 to M2L1 and M3L1
M1L2 to M2L2 and M3L2
and
M1L3 to M2L3 and M3L3
Now the voltage generated in M1L2 is feed into M2L2 and M3L2 (same for L3).
But at the same time M2 and M3 generate voltage at L2. Now do you really
think that one motor can drive L2 and L3 of the two other motors and at the
same time doesn't lose any energy?
Note that I disregarded losses and was talking about an ideal machine.
Absolutely. :-)
This ain't a rotary converter!
OP cite:
| ... is looking at buying a Felder combination woodworking
| machine with 3 4kw motors in 3 phaze ...
Nick
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