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
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
What he is planning to use, then, is the equivalent of a
capacitor temporarily connected to to the "not connected" leg of the
motor. This gets it spinning in the right direction and then
disconnects.
The sure indicator here is the range of horsepowers (instead of
"up to X horsepower). This is because the capacitor needs to be sized to
the horsepower of the motor which it is being used with. Too large a
motor or too small a motor will not start properly. (Of course, the
word "Static" in the name is another clue.
You can perhaps get nearly full horsepower from the motor with
this, but at the cost of running one winding at a much higher current
than it was designed for. It would be more likely to burn out if run at
this level for quite a while, though you can get away with it for short
spurts.
And this is not what *I* would call an "electronic converter".
If you want to do it right, the better way to go (other than
making a rotary converter) is to get a VFD (Variable Frequency Drive), a
truly electronic device which converts the incoming power (single phase
or three phase) to a high DC voltage, and then converts that to true
three phase -- with the added boon of generating the three phase at a
frequency of your choice, so you can run the motor significantly slower
or significantly faster.
When running significantly slower (say about 25% of full speed
or so) you do have the problem that the internal fan in the motor won't
keep it cool enough, but if you wish to run it at such slow speeds for
long periods, you can add an external fan to keep it cool enough. Just
mount it to blow into one end (in the same direction that the internal
fan blows) and you are fine.
I forget what kind of machine tool this was to be used with, but
quite a few benefit from the ability to tune the speed to the task at
hand. For example metal lathes can produce variations in finish quality
as you face from outside towards center or vice versa. The reason for
this is that at certain surface speeds you get buildup of metal on the
cutting tool which will eventually break clear and give good finish for
a short while before building up again. If you can turn the spindle
speed up as you face in towards the center, or down as you face out from
the center to the outside, you can tune away from the speeds which give
the problem. And this is something which you can't do with a step
pulley, since stopping to change belt steps would change the finish at
the point of the stop. (Though if you have a variable speed pulley, you
can do quite well with this.)
I personally would not use a "Static" phase converter, and would
choose either a rotary converter (home shop made) or a VFD (commercially
made). Since I have three VFDs and no Static phase converters, you can
see that I live by that.
Enjoy,
DoN.
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Because the phase shift from the capacitor is a function of the
motor horsepower (and related inductance) the capacitance, and the
frequency. Since the frequency is fixed at either 60 Hz (here) or 50 Hz
(UK) we have the motor inductance/horsepower and the capacitance. Too
small a capacitor will not produce enough phase shift to get the motor
started quickly enough to avoid blowing the capacitor, and too large a
capacitor will generate too much phase shift, again resulting in very
slow motor starting and likely failure.
Note that the static converter was spec'd for a range of
horsepower (in the part of the article now trimmed), not "up to
such-and-so horsepower".
This is a common limitation for a static converter -- at least of
the capacitor and relay style. So -- if the machine tool has three
motors -- one 4 HP, one 2 HP and one 1/2 HP (say for perhaps spindle, feed,
and coolant pump), then while it may start the 4HP motor nicely, it will
fail to start the 1/2 HP coolant pump if that is needed first.
Obviously (in spite of what Nick thinks) once you have the spindle motor
running, it can start the others, but if you need the smallest to start
first (say it also pumps lubricant around to the bearings before you
start moving things) you may have problems. For this, really, an
external rotary converter would be a better bet. Let the static
converter start the rotary, and it can start all of the load motors with
no problems.
Obviously -- if it is a homebuilt static converter, or if the
owner is competent to open it up and replace the capacitor with the
appropriate size for the other motor, then this can be overcome.
Also -- if he is not careful to wire it so the real 240 VAC
comes in to the terminals which feed the control circuitry, he may not
be able to get the spindle motor to start with the static converter
alone.
Enjoy,
DoN.
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Hi Don
I have fallen short on my study in this 3 phase stuff. When I was doing
the testing with my litle dyno on 3 phase motors, I recognized that the
bigger the capacitor I used to spin up even the fractional HP 3 phase
motors, the quicker the motors spun up. I suspect I never approached the
"too much" capacity.
At one time I thought the static converter was a partially useful method
of running 3 phase machines at home, on single phase. But, the more I
learn about them, the more I recognize that they are an excellant device for
most home users for running 3 phase machines.
The only literature I have read related to static converters ghas been
sales brochures. I am a little sceptical of what is written in those sales
papers.
I accumulated some data while dyno testing the effects of 3 phase motors
and rotary convertyers. I saw that a very much smaller idler could spin up
a larger tool motor when the tool motor is spun up while very lightly
loaded. But, as you would expect, the little idler helps little or nothing
to the tool motor's ability to deliver power.
Where can a guy find some good tech info on static converter design?
Thanks for the "heads up" on excess capacity for spinning the 3 phase
motor.
Jerry
You may not have even reached the level of "right value". :-)
You were perhaps using oil-filled AC capacitors, instead of the
electrolytic motor starting capacitors which are more common in motor
applications -- because they are smaller and they cost a lot less.
Oil-filled AC capacitors are a lot more tolerant of long start
times.
That depends. If you need to "plug reverse" a motor -- say do
it to a lathe spindle motor when you are threading up to a shoulder to
avoid a crash -- a static converter is useless.
The static converter (one version) is something like this (view
with a fixed pitch font such as Courier to avoid distortion of the
drawing):
(1)--------+ +----------------------------------(A)
| | +--> |
+----------+ | |
| I R | | |
+----------+ | |
o o
| |
(2)-------------------+-----------------------------(B)
|
| C1
+---|(------------------(C)
The block marked "I R" is a current sensing relay. When the motor is
drawing the high current when it is switched onto power with no ability
to start, it is sufficient current to close the contacts. One contact
connects to either (1) or (2) of the 240 VAC input lines (neutral is
ignored, so I did not draw it here). The capacitor (C1) applies
phase-shifted current to the motor terminal (C).
Once the motor spins up to perhaps half of its normal speed, the
relay is no longer seeing enough current, and thus the contacts open,
leaving terminal (C) on the motor totally disconnected.
That's OK for most things, because the single-phase power
applied between (A) and (B) is sufficient to keep the motor spinning at
normal speed -- though if it is loaded to full horsepower rating, the
current through the single winding will be higher than it was designed
for, and the winding will overheat and eventually burn out if you are
running it near its load rating for a long time.
However -- if you want to reverse the motor quickly, you are out
of luck. Normally, a three-phase motor is reversed by interchanging any
two of the three power leads. At that point, the three-phase power will
be trying to turn it the other way and it will reverse *quickly* (and
with a lot of current for a very short period). But, remember that the
(C) feed to the motor goes to nothing as long as the motor is not
drawing a lot of power. And once the motor is spinning, it doesn't care
which way it is going when fed by single phase.
So -- it won't reverse until the relay contacts close, and that
will require a lot of current, such as you get from a stalled motor.
But since it is spinning at normal speed, you won't get that current and
the relay will keep the connection to the (C) terminal open, so the
motor will keep running in the original direction, and your lathe tool
will crash into the shoulder.
A VFD does not want switches between the VFD and the motor, so
you have to ask it to reverse the motor, and that will require it to
slow the motor down and then speed it up in the other direction. Not
quite a fast as just reversing two leads on a real three phase, but
usually quick enough.
A rotary converter (which can be thought of as a static
converter starting a motor (called the idler motor) which then acts to
generate the third phase) can handle plug reversing -- as long as the
idler motor is enough larger than the load motor. I have heard of
attempts to plug reverse with too small an idler motor resulting in
reversing the idler motor instead of the load motor. Plan on at least
1.5 time the load motor rating for the idler motor -- or two times is
even better.
Good!
Not from the makers -- they don't want you to see how simple
what they are selling you is, and how little it would cost for you to
make your own.
BTW That current relay is another reason for the minimum horsepower.
If the motor is two small, even with it stalled it won't draw
enough current to draw in the relay and engage the start
capacitor.
Some may be built using voltage sensing relays, waiting for
enough voltage to come back from terminal C instead of current sensing
relays -- or perhaps even a timer which just assumes that the motor must
have started by now and thus disengages the capacitor. All of this is
doing just what the centrifugal switch in a motor built for single phase
does.
If you want to study circuits which do what a static converter
does -- look at the home-brew plans for rotary converters -- they often
use the "static converter" design to start the idler.
And good rotary converter designs add oil-filled AC capacitors
to tune the output of the rotary converter for better balanced three
phase. You need to adjust the total capacitance according to the idler
motor, and ideally with the load motor connected too, so you are tuning
for best balance when under load.
Now -- I have seen mention of static converters which use a
tapped inductor to accomplish the phase shift instead of a capacitor.
That might avoid the need to switch out the third phase as soon as the
motor is spun up -- but it certainly increases the cost of
construction. Basically -- if you have difficulty lifing a static
converter, it is probably an inductor-based one, and may be better.
But personally, I like the VFD for most home shop conversion,
since it lets you tune the speed. The only place where I would consider
the rotary converter better is when you have a lot of three phase motors
on a single machine (which I believe is what started this). And of
course real three phase from the power company is even better -- if you
can get it.
You're welcome.
Good Luck,
DoN.
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Hi Don
You have alot of information on Converters. I began a study a few years
ago by building a dyno and using Excel to make charts of HP while running 3
phase motors from a single phase line. Don Foreman taught me lots and lots
about all the stuff I built and tested. he is world class instructor. I
have disassembled the dyno but keep the set of four 200 mic electrolytics in
parallel that I can apply to the unfed motor lead thru a solid state switch.
I use the capacitors to spin up 3 phase motors here at home. I wont be
surprised if i sometime fail to spin up a small 3 phase motor due to the
excess capacity.
I have several VFDs in systems I work on. So, I do like them and have
used alot for stuff I've built. I even repaired a 120 KW solid state
converter last week. It is fixed frequency at 60 Hz in and 400 Hz out.
The problem is 'I dont know what I did to make it work'. Thats not too
satisfying, but the unit did get shipped after I disassembled part of it and
then put it back together.
But, the more I learn about "Static Converters", the more I respect their
value to home use where space and budget is a factor.
Jerry
Hmm ... that is enough to handle rather large horsepower motors.
An interesting device -- and something which I would like to
have -- except much smaller -- to run an old gyrocompass and an
artificial horizon which want 115 VAC 400 Hz. Probably on the order of
20 W each.
I'm not too sure about the budget matter. The price from
vendors seems to be not too different from that for a VFD from eBay or
one of the good dealers -- at least until you get into seriously high
horsepower ratings.
Enjoy,
DoN.
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I agree, Jerry; static converters get an undeserved bad rap. I have
three machines (mill, lathe, surface grinder) that benefit from VFDs,
but my vertical band saw, belt sander, and one bench grinder run
happily on a static converter. (And, yes, turning one of those
machines on will help start a motor that the converter would otherwise
not start.)
I do have a large rotary converter, and can easily switch between the
static and rotary, but rarely find it necessary to run the (noisy)
rotary.
And yet it works. I ran a Bridgeport for many years on a static
converter and regularly plug reversed when power tapping. A friend who
has a commercial shop without access to 3-phase runs 4 BPs on static
converters, and has for as long as I can remember. Plug reversing
isn't a problem for him, either.
Here's what Phase-A-Matic has to say:
**********************************************
Next, you must determine whether to use a Regular or Heavy Duty
Converter. Keep in mind you can always use a
Heavy Duty Converter in place of a Regular Duty model. However, there
are some applications for which you
should always recommend Heavy Duty Converters. They are:
1. Frequent starting or instant reversing (more than once a minute).
2. Unattended equipment, such as air compressors.
3. Long, heavy starting cycles, such as lathes without a clutch,
flywheel driven equipment, etc.
4. If jogging is required.
5. If there is a chance of the motor being stalled during use
(woodworking equipment, etc.)
********************************************
Capacitors cant store enuf energy to be used to convert single phase
to 3 phase , you must use rotary machines to store the energy .
Its real simple , a motor is powered to store energy , which it
delivers
to a phase wire on a 3 phase motor , you simply control the energy
applied to the single phase so it can deliver this energy , but
delayed
to the 3 phase motor .
The more energy needed , the larger the rotating mass of the single
phase
motor .
motors and transformers can be modeled as a magnetizing inductance in
parallel
with a leakage induct'+resistive load .
At no load , all current flows thru magnetizing induct' .... bad
P.F. ,
so you pay more to the electric company .
But as the current flows to the load , which is series to the leakage
induct'
the P.F. improves to 85% ( cause the load is resistive , not induct'
as the L-induct is ).
The stored energy is in the Leakage induct' , and it is effecient .
But it needs a large heavy rotor to deliver that energy to a 2nd
motor .
It also needs a small air gap , to reduce the reluctance of the path
from
stators to the rotor .
we need to move above 400 hz , to get higher power , but that means
diameter
of motor also increases .
Hi Werty
I find no fault in what you write here. But, there is a situation where
3 phase motors can be run nicely when connected to single phase. And, it
requires only that a capacitor be temporily connected from one of the single
phase lines to the "not connected" third wire from the 3 phase motor. That
temporary connection of the capacitor gets the 3 phase motor spinning so it
can then deliver appreciable power.
The capacitor doesnt need to store power. It is quickly "bumped across"
the two terminals of the motor wires. The major benefit from having a VFD
or a Rotary Converter would be to get the tool motor to delive close to its
name plate rated power to a load for extended times.
Jerry
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