I have no clear cut favorites when choosing how to use 3 phase motors when
only single phase is available.

I prefer VFDs. They allow soft start, reasonably fast reverse and variable
speed. The only downside is cost. Some people here have figured out ways
to use one VFD for multilple machines.

I have a rockwell combination horizontal/vertical mill. I use one VFD
to run both motors. I put twist lock plugs on the motors so I can easily
change from one to the other. I need to change one parameter (full
load current) in the VFD when changing motors because they are very
different size (.5 vs 1.5HP).

In article , dnichols@d-
and-d.com says...
In article ,
Chuck Sherwood wrote:
Would it be correct that all these phase converters that are not based
on motor/generators, are unable to run more than one 3 phase device at
a time.

That is not correct. Rotary phase converters are often used to
run multiple machines. One at a time or even more that one at a time.

Agreed -- but he said "*not* based on motor/generators", which I
think he intended to also exclude idlers used as rotary phase
converters. The idler motor is serving both as a motor and a generator
at the same time.

"Static" phase converters ("Phase-o-Matic" is an example) are

phase-A-matic ( -a- rather than -o- )

not only limited to running one thing at a time,

They're limited to *starting* one load at a time. Once you
have one motor running it (and every subsequent motor) acts
like a rotary converter allowing you to start another
motor, up to the capacity of the branch circuit.

but are also designed
for a fairly narrow range of load motor horsepower. If you need to
start a 1HP motor, and a 2-1/2 HP motor, the odds are that you can't
even switch the converter from one load to the other to run one at a
time, because the starting capacitor value will be wrong for at least
one of them.

I think most phase-A-matics have a 2:1 range, 3/4 to 1-1/2
HP, for example. You can get around this by creative use of
the motors in other machines as RPCs as mentioned above.
Not convenient, but will work in a pinch to get something
going that the static converter won't otherwise start.

One very important thing to note is that you should never
connect anything with sensitive electronics to the leg
being generated by a static converter. There are likely to
be spikes present on that leg that may damage solid state
controls, etc.

Ned Simmons

"Ignoramus19508" wrote in message
.. .
Thanks to all. I have another question. For a phase converter motor,
how important is the RPM? I am a little puzzled that 3 phase motors
have different RPMs, 1740, 1750, 1800, 1760 etc. I cannot understand
how it can be with 60 HZ incoming power.

In any case, with a motor that is not spinning at exactly 1800 RPM,
how would the third leg agree with the incoming 60 Hz frequency? I am
quite confused.

Do I need a motor that is rated for precisely 1800 rpm, or can I get
away with a 1760 RPM motor?

i



I

I suspect that you arent actually "confused". Perhaps you dont yet have
enough information to show you what the induction motors do and how they
work.
If you want to get involved with studying the theory of electric motors,
there are alot of good books and web sites that have all the information on
these 3 phase motors.
If you want to build a RPC so you can have decent 3 phase available in
your shop, I'd suggest that you get Bob Swinney to send you a copy of his
writings.

Note:
The 3 phase motors commonly being referred to here in RCM are induction
motors. They necessarily run at an RPM slightly lower than the synchronous
RPM which is determined by the number of poles in the motor's construction.
By far, the most common 3 phase motor used to power machine tools is the
"1750". Actually the name plate RPM indicates the RPM to which the "1800
RPM" falls when the motor is fully loaded to its name plate HP. An
unloaded 1750 3 phase motor will spin at close to 1798 RPM, depending on its
static load from the fan and bearing load. The motor's RPM will gradually
drop to the name plate RPM when it is loaded to its name plate HP.

Jerry

Ignoramus19508 wrote:
Thanks to all. I have another question. For a phase converter motor,
how important is the RPM? I am a little puzzled that 3 phase motors
have different RPMs, 1740, 1750, 1800, 1760 etc. I cannot understand
how it can be with 60 HZ incoming power.

In any case, with a motor that is not spinning at exactly 1800 RPM,
how would the third leg agree with the incoming 60 Hz frequency? I am
quite confused.

This is a little difficult to explain without diagrams, but I'll have a
go. In an induction motor each phase winding encircles (i.e., is wrapped
around) a number of pole pieces. The pole pieces are formed from
laminated iron, and point towards each other on opposite sides of the
rotor. The minimum number of pole pieces that a winding can encircle is
two (one pair). A motor in which each phase winding encircles two pole
pieces is known as a two pole motor. A three phase, two pole motor will
have six poles in total (two for each phase). A single phase, two pole
motor will have just two poles.

Two pole motors will turn at rotational frequency (i.e., rotations per
second) slightly less that the frequency of the supply. This is true if
they are single phase or three phase. Four pole motors will turn at
slightly less than half the supply frequency, and six pole motors at
slightly less than a third. The motor must turn at a lower frequency
than the supply, or there would be no "flux cutting" taking place, and
therefore the motor couldn't develop any torque, but this is getting
into the physics of it. If you want to figure out how the windings
produce a rotating field, try to find a good book on electrical
engineering. The diagrams are a big help in understanding it. If you
can't find one, I have a book buried on my bookshelf somewhere and could
scan the relevant section for you. Let me know if you need it. You'll
also find that the poles are somewhat hard to identify if you look
inside a real motor because the windings are carefully formed into a
cylindrical shape around the rotor.

Do I need a motor that is rated for precisely 1800 rpm, or can I get
away with a 1760 RPM motor?

You won't find an 1800 rpm motor, because there must always be some lag
between the rotating field and the rotor. A 1760 rpm motor should be fine.

Hope this helps,

Chris

In article , Ignoramus19508 says...

Thanks to all. I have another question. For a phase converter motor,
how important is the RPM? I am a little puzzled that 3 phase motors
have different RPMs, 1740, 1750, 1800, 1760 etc. I cannot understand
how it can be with 60 HZ incoming power.

Get the Audel;s motor book. It explains about the number of
"poles" in an induction motor, and how induction motors work.

In any case, with a motor that is not spinning at exactly 1800 RPM,
how would the third leg agree with the incoming 60 Hz frequency? I am
quite confused.

The rotor in an induction motor sets up a rotating B field inside
the stator windings that is locked to the line frequency. Because
the windings are installed in the stator with correct spacing to
give the poles the correct phase relationships to each other, the
three wires automatically give you correct 'factory' three phase
at the output.

It's not magic, it's just a natural result of how polyphase induction
motors work.

Jim


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Iggy sez:

" ..... Is that the article on metalwebnews? ...."

I suspect you are referring to Jim Hanrahan's excellent article, most
generously contributed to the Internet several years ago. While the article
is "classic" in every sense of the word, I would not recommend it as a
"first read" on RPCs unless you already have some background in the subject.
Hanrahan's article describes 'self-starting' rotary phase converters,
although it fails to make the distinction between "self-starting" and the
balanced type more commonly used in the home shop. The "self-starter" has
no auxiliary, "start caps", relying instead on gross amounts of
single-legged run capacitance.

The self-starting RPC is inherently an "unbalanced" device. "Self-starting"
works against the object of achieving any sort of balance among voltages of
the 3 phases. Self-starting is achieved through the use of excessive
amounts of "run capacitance"; enough that if some means of switching out
isn't employed, the RPC will be drastically out of voltage balance and
incapable of delivering nearly perfect 3-phase power.

RCM's own Fitch Williams ran many posts a few years ago on voltage balanced
RPCs. IMO, Fitch gets all the accolades, credit, and all kudos for waking
up the home shop crowd to the advantages of the auxiliary-started, voltage
balanced RPC. If I had a drink at the moment, it would be raised to Fitch.

Thanks, to Jerry Martes for recommending my writings, but I've just about
given all of them out short of enlisting attention of the copyright police!

Best regards to all,

Bob Swinney

Someone wants $100 for a [supposedly] nice condition used 10 horse
motor... It is not exactly very cheap, but I can have it right now...

Its a time/money/hassel trade off.
Lost creek machine in Ottawa, IL sells used 3ph motors for
$10 per HP. Same price so its not out of site, but not cheap either.

Something to think about: You will need start and run caps. The
bigger your idler motor, the more caps you will need. If you are
buying the caps new, you might incure more cost for the caps than
your motor. I recommend that you figure out what you need before
you buy parts because you will spend more to build a 10HP converter
than a 5HP converter.

Ignoramus19508 wrote:
Someone wants $100 for a [supposedly] nice condition used 10 horse
motor... It is not exactly very cheap, but I can have it right now...

For a good quality motor (I always prefer the cast iron frame motors as
opposed to the aluminium ones) in good condition, guaranteed working,
$100 is probably a fair price. A 10 hp three phase motor might be $1000
new. If you want one now, take a trip to see the motor if you can. If
you can wait, you might be lucky and get something free.

Chris

Iggy sez: "Thank you. I am reading an article at
http://home.att.net/~waterfront-woods/, if you have any other
suggestions, I will appreciate. I found some posts by Fitch to
rec.crafts.metalworking, as well, and will soon look at them."

A good article. If you read it in detail, moreover, understand it in detail
you will be on your way to making great RPCs.

Bob Swinney

Would that be a better deal than one of Jerry's hand picked free motors, if
you paid shipping on each

Bob Swinney
"Ignoramus19508" wrote in message
...
Someone wants $100 for a [supposedly] nice condition used 10 horse
motor... It is not exactly very cheap, but I can have it right now...

i

Advise you take a close look at the starting capacitor arrangement used in
the wood working article you cited. A momentary push button to start and
some free caps is a lot better than a klunky pony moror.

Bob Swinney
"Ignoramus19508" wrote in message
.. .
On 21 Jul 2005 20:14:16 GMT, Chuck Sherwood
wrote:
Someone wants $100 for a [supposedly] nice condition used 10 horse
motor... It is not exactly very cheap, but I can have it right now...

Its a time/money/hassel trade off.
Lost creek machine in Ottawa, IL sells used 3ph motors for
$10 per HP. Same price so its not out of site, but not cheap either.

That's about what I figured, as well.

Something to think about: You will need start and run caps. The
bigger your idler motor, the more caps you will need. If you are
buying the caps new, you might incure more cost for the caps than
your motor. I recommend that you figure out what you need before
you buy parts because you will spend more to build a 10HP converter
than a 5HP converter.

I would like to do starting with a pony motor, actually. I have a
couple of crap 1/4 HP motors (actually broken vacuum pumps), which I
can possibly use, I have to open the pumps to see if the motors have
shafts etc, but I think that they do. I also have a 1/2 HP Dayton
gearmotor with a removable gear part, I could use that if 1/4 HP is
not enough or if the pumps are directly coupled to the rotor without a
bearing in between.

So start capacitors are not necessary.

As for run capacitors, I looked at ebay and saw some caps such as this
one:

http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7530235319

and I can have a 100 mF and a 60mF on every leg, which would make my
cost about $60 for caps. That would mean that I will have a 10 HP
phase converter for $160 plus the cost of electrics (which usually is
rather substantial).

i
--

I think of a rotary phase converter consisting of a three phase motor
as a motor generator. Of course I think of a single phase motor as a
motor generator too. Works for me, but apparently not for a lot of
people.
If you did not have the back EMF generated in a motor, the motor would
draw a huge amount of current.

Dan



jim rozen wrote:


As others have suggested, motor-generator setups are not typically
found in this application. The rotary phase converer which consists
of a large three-phase motor, acting as a sort of rotary transformer,
has rotating elements and will provide nearly factory-fresh 3~
power, and can run many machines at once.
Jim


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You might want to consider that a induction motor that is driven acts
as a induction generator. Jim Pentagrid has posted some very accurate
information about induction generators. To summarize, they are very
load sensitive if they are used as a stand alone generator. But if
they are connected to the power grid, they work very well.

So you can make a motor generator using a single phase motor driving a
three phase motor which has one phase connected to the same single
phase that drives the single phase motor. Nota Bene In order to
actually be driving the three phase motor, you have to drive it faster
than the rpm that it would run at when running on single phase as a
idler. The way to do this is with a belt drive. One pulley should be
a variable pulley. So one can measure the current drawn by the single
phase motor and adjust the pulley so the single phase motor draws its
nameplate current or somewhat less.

This is the way to make a phase converter for driving something as a
surface grinder that is sensitive to variations in torque.

I made one like this some twenty years ago to run a centerless sander.
Worked slick. The single phase motor worked as a pony motor to start
the three phase motor and then drove the three phase motor to produce a
better three phase than most rotary three phase converters.

Dan

Robert Swinney wrote:
What Grant said . . . a single phase motor driving a generator would, in
fact, be a motor/generator -- but -- the 3 phase generator portion of the
combination would have to be an alternator; a machine with a stationery set
of coils (stator) and a revolving magnetic field set of coils, or vice
versa. The field would either have to be excited by direct current (DC) or
be made up of permanent magnets. Large steam driven turbo-alternators are
an example. Those are synchronous machines, whereby the speed of rotation
determines the output frequency; their magnetic fields are generally excited
by DC generators of 110 volts, sometimes 220, and their may be other DC
voltages for excitation.

Contrast this to a rotary phase converter. The type RPC we are familiar
with is basically 2 three-phase motors running on single-phase current with
their 3rd legs tied together. The motors are not operated in parallel.
They may run with or without capacitor augmentation which aids in supplying
phased current to the 3rd leg of each motor. In a manner of speaking each
motor is a sort of rotating transformer which is responsible for energizing
the 3rd leg or "manufactured phase". The idler motor, viewed as a source
( not a strictly accurate analogy) has to be larger than the load motor in
order to keep the 3rd leg "generated" voltage from sagging down.
Capacitance from the single-phase line to the 3rd leg(s) lowers impedance in
those paths and may be thought of as forming a very broadly tuned series
resonance circuit. Current flow in a RPC is quite complex and does not
readily yield itself to mathematical modeling..

Bob Swinney

wrote:
You might want to consider that a induction motor that is driven acts
as a induction generator. Jim Pentagrid has posted some very accurate
information about induction generators. To summarize, they are very
load sensitive if they are used as a stand alone generator. But if
they are connected to the power grid, they work very well.

So you can make a motor generator using a single phase motor driving a
three phase motor which has one phase connected to the same single
phase that drives the single phase motor. Nota Bene In order to
actually be driving the three phase motor, you have to drive it faster
than the rpm that it would run at when running on single phase as a
idler. The way to do this is with a belt drive. One pulley should be
a variable pulley. So one can measure the current drawn by the single
phase motor and adjust the pulley so the single phase motor draws its
nameplate current or somewhat less.

This is the way to make a phase converter for driving something as a
surface grinder that is sensitive to variations in torque.

I made one like this some twenty years ago to run a centerless sander.
Worked slick. The single phase motor worked as a pony motor to start
the three phase motor and then drove the three phase motor to produce a
better three phase than most rotary three phase converters.

Dan,

This is a really interesting suggestion. What size motors did you use? I
can get a 2.5 hp three phase motor free, and a 3/4 hp single phase motor
for little money. How do you think this would work? From what I've read
phase convertors are pretty forgiving about the size of motors used.

Many thanks,

Chris

In article , Robert Swinney says...

A good article.

I've seen better....

g

Jim


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wrote:
You might want to consider that a induction motor that is driven acts
as a induction generator. Jim Pentagrid has posted some very accurate
information about induction generators. To summarize, they are very
load sensitive if they are used as a stand alone generator. But if
they are connected to the power grid, they work very well.

So you can make a motor generator using a single phase motor driving a
three phase motor which has one phase connected to the same single
phase that drives the single phase motor. Nota Bene In order to
actually be driving the three phase motor, you have to drive it faster
than the rpm that it would run at when running on single phase as a
idler. The way to do this is with a belt drive. One pulley should be
a variable pulley. So one can measure the current drawn by the single
phase motor and adjust the pulley so the single phase motor draws its
nameplate current or somewhat less.

This is the way to make a phase converter for driving something as a
surface grinder that is sensitive to variations in torque.

I made one like this some twenty years ago to run a centerless sander.
Worked slick. The single phase motor worked as a pony motor to start
the three phase motor and then drove the three phase motor to produce a
better three phase than most rotary three phase converters.

Dan

Dan,

That's a really interesting suggestion. What size motors did you use? I
can get a 2.5 hp three phase motor free, and a 3/4 hp single phase motor
for little money. How do you think these would perform in a phase
convertor? From what I've read phase convertors seem quite forgiving
when it comes to the size of motors used.

Many thanks,

Chris

The motors I used were a 7.5 hp three phase motor and a 2 hp single
phase motor. My motor picks were dictated by what I had available,
but seemed to work well. The centerless sander had a 5 hp main drive
motor and a couple of smaller three phase motors.

I think a 3/4 hp single phase and a 2.5 hp three phase motor should
also work well.

If you do a' little googling on induction generators, you will find
they are not uncommon. But mostly used on things as street cars when
braking.

Dan

wrote:
The motors I used were a 7.5 hp three phase motor and a 2 hp single
phase motor. My motor picks were dictated by what I had available,
but seemed to work well. The centerless sander had a 5 hp main drive
motor and a couple of smaller three phase motors.

I think a 3/4 hp single phase and a 2.5 hp three phase motor should
also work well.

If you do a' little googling on induction generators, you will find
they are not uncommon. But mostly used on things as street cars when
braking.

Thanks very much for the opinion. I'll probably make a trip soon to get
the 2.5 hp motor and start experimenting. Will let everyone know how it
goes.

Many thanks,

Chris

In article , Robert Swinney says...

Advise you take a close look at the starting capacitor arrangement used in
the wood working article you cited. A momentary push button to start and
some free caps is a lot better than a klunky pony moror.

I resemble that remark!!

Jim


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