mamiller@rodan.acs.syr.edu (Mark A. Miller) (03/29/91)
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Hello! A machinist friend of mine is buying some equipement that runs
on 3 phase power and requested me to build a converter for him. My
question to the net is : What exactly IS 3 phase? Is it simply one
ground line and three 'live' lines, each 120 degrees out of phase
with the others, or is it something else? Also, could you point me
in the direction of some good references for building a converter
(preferably non-mechanical; i.e. no motors/generators). Also, could
someone tell me WHY it would be preferable to run a machine on
3 phase power? I can't really think of a good reason, but there
MUST be one, right? Thanks alot for any responses! (I'm a practicing
THEORETICAL physicist, so please keep the jargon to a tolerable
level ;-) ).
Mark Miller
mamiller@rodan.acs.syr.edumcovingt@athena.cs.uga.edu (Michael A. Covington) (03/29/91)
3-phase is indeed 3 sine waves, 120 degrees out of phase with each other. Sometimes it is delivered on 4 wires and sometimes on three. Its advantages have to do with motors and generators that have 3 windings (120 degrees out of phase with each other of course!) instead of just one. Also, with full-wave rectifiers, if you have 3-phase input you can use 6 diodes and get much less ripple than with 1-phase input. That's all I know. Single-to-3-phase conversion sounds very difficult. -- ------------------------------------------------------- Michael A. Covington | Artificial Intelligence Programs The University of Georgia | Athens, GA 30602 U.S.A. -------------------------------------------------------
fval_ltd@uhura.cc.rochester.edu (Trey Valenta) (03/29/91)
In <1991Mar28.205341.11182@athena.cs.uga.edu> mcovingt@athena.cs.uga.edu (Michael A. Covington) writes: >That's all I know. Single-to-3-phase conversion sounds very difficult. I would highly imagine. We use it here on campus anytime a large power drop is required by various events such as concerts with a lot of equipment. The lines come that way from the power company. I would think that, if this is a business venture, you friend would want to have three phase added, although this would cost a bunch, I'd assume. trey valenta -- Trey Valenta | It's 3am; do you know where your Mac is? fval_ltd@uhura.cc.rochester.edu | CPU Box 272253 (716-274-4233) | Corollary: Rochester, NY 14627 | It's 3am; do you know where you are?
jgd@Dixie.Com (John G. DeArmond) (03/29/91)
mamiller@rodan.acs.syr.edu (Mark A. Miller) writes: >Hello! A machinist friend of mine is buying some equipement that runs >on 3 phase power and requested me to build a converter for him. >in the direction of some good references for building a converter >(preferably non-mechanical; i.e. no motors/generators). You don't want to spend your friend's money trying to build a 3 phase sine wave inverter with sufficient reserve to supply starting inrush to the motors. Not only would the components be expensive, the development would be bloody, with lots of blue smoke leaking out. This is the voice of experience speaking. What your friend wants to do is a) find out about a 3 phase drop from the power company. The way to go if the minimum is not too high and is available. b) get a rotary converter or c) get a motor-generator. The rotary converter is a motor-like device with a special rotor that runs on 1 phase power and outputs 3 phase power at pretty much whatever voltage he needs. They have slightly less efficiency and a poorer power factor than motor-generator (M-G) sets but are a lot less expensive. An M-G set is better than a rotary converter, having a better power factor, a better efficiency, and with an appropriate flywheel, the ability to supply locked rotor current requirements typical of starting motors. ON the downside, it requires more maintenance. Check your local motor supply or motor repair shop for prices for either device. >Also, could >someone tell me WHY it would be preferable to run a machine on >3 phase power? I can't really think of a good reason, but there >MUST be one, right? Thanks alot for any responses! (I'm a practicing >THEORETICAL physicist, so please keep the jargon to a tolerable >level ;-) ). Sure. 3 phase motors don't require any external phase shifting components such as capacitors to generate the rotating magnetic field that drives the rotor. Neither does a small ( up to a few hundred HP) motor require any special starting equipment. And since the magnetic field rotates faster than single phase motors and for other reasons, more power per volume and weight is possible. Finally, the efficiency and power factor is generally better. John -- John De Armond, WD4OQC | "Purveyors of speed to the Trade" (tm) Rapid Deployment System, Inc. | Home of the Nidgets (tm) Marietta, Ga | {emory,uunet}!rsiatl!jgd |"Politically InCorrect.. And damn proud of it
lrk@k5qwb.UUCP (Lyn R. Kennedy) (03/30/91)
mamiller@rodan.acs.syr.edu (Mark A. Miller) writes: > Hello! A machinist friend of mine is buying some equipement that runs > on 3 phase power and requested me to build a converter for him. My > > Mark Miller > mamiller@rodan.acs.syr.edu Yes it really is three phases 120 degrees apart with or without the ground/neutral. The motors work better using three phase. A solid state convertor is possible but you won't be happy with the price. First call the electric company. They have neat ways to do 240 VAC three phase without a proper neutral (you still get 110/220). Otherwise look for similar single phase motors as replacements. --------- lrk@k5qwb.UUCP lrk%k5qwb@kf5iw.UUCP 73, utacfd.utarl.edu!letni!rwsys!kf5iw!k5qwb!lrk Lyn Kennedy K5QWB @ N5LDD.#NTX.TX.US P.O. Box 5133, Ovilla, TX, USA 75154 -------- "We have met the enemy and they are us." Pogo -----------------
jb@lydia.mae.cwru.edu (Jim Berilla) (03/30/91)
In article <1991Mar28.191005.1653@rodan.acs.syr.edu> mamiller@rodan.acs.syr.edu (Mark A. Miller) writes:
[ needs a single to 3 phase converter ]
Many companies make variable speed drives for 3 phase motors. The smaller
ones (5 horsepower or less) can use single phase for input. They also have
a number of useful features. They can vary the speed of the motor (by
changing the output frequency) from about 30% to 150%. They can provide a
soft-start by ramping the motor speed up over a period of time. Overload
protection is usually built in.
They're not cheap, though. About $800.00 for a 5 hp controller. They're
small (9x12x6 inches for 5 hp), quiet, and easy to use. Given time, you
could build your own, but I haven't seen any cookbook plans for building
them. I've done some work with high power circuits like this, and the
first few weeks were spent watching power transistors turn into plasma.
Subtle problems can cause not-too-subtle results. (If anybody's inter-
ested, the TL494 under some cases can clock the same output twice in a
row, rather than alternating the two outputs. Mail me for the gory details.)
A freind of mine is using a 5 hp unit to run a 2 hp mill in his basement.
Works great. The variable speed control is an added bonus.
Another person tried to run a 1 hp machine with a 1 hp controller. I don't
know the details, but he said it didn't work.
One nice thing about 3 phase motors is that you can flip them into reverse
without waiting for it to stop. I was tapping a bunch of holes on the mill
once, and that ability sure saved me a lot of time. No overload indication
when going from full speed forward to full speed reverse.
--
Jim Berilla / jb@falstaff.cwru.edu / 216-368-6776
"My opinions are my own, except on Wednesday mornings at 9 AM,
when my opinions are those of my boss."mzenier@polari.UUCP (Mark Zenier) (03/31/91)
In article <1991Mar28.191005.1653@rodan.acs.syr.edu> mamiller@rodan.acs.syr.edu (Mark A. Miller) writes: >Also, could >someone tell me WHY it would be preferable to run a machine on >3 phase power? I can't really think of a good reason, but there >MUST be one, right? The three phases add up so power is constant over a cycle, while with single phase, the power pulses at 2 * frequency. Mark Zenier markz@ssc.uucp mzenier@polari.uucp
jgk@osc.COM (Joe Keane) (04/06/91)
Almost all `serious' power is three-phase, because that's the easiest to work with. In a power system the generators and transmission or sub-transmission lines are almost always three-phase. Also all motors of any substantial size are three-phase. It's only when you get down to a few houses that you use a single phase. I think Nikola Tesla deserves a lot of credit for coming up with the three-phase alternating-current system we use today. The neat part is that although the voltages are constantly changing, the total amount of power transmitted is constant. Now, to the original subject, converting single-phase to three-phase power. The executive summary is that it's a pain in the ass, and you shouldn't do it unless you have a good reason. It's a lot easier to just get the power company to run a couple more wires. There are basically two ways to do it, and neither one is easy. One way is to use some sort of rotating machinery. The simplest is just a motor-generator set. The real way people do it is more complicated to explain. The general idea is that you wire up a rotating machine so that it presents a low impendance to negative-sequence and zero-sequence power, while not affecting positive-sequence power. If you understand what this means then you probably already know what i'm talking about. Then you can feed power into one phase and have it come out balanced. This is like a phase balancer in reverse. The other way is to convert it to DC and back. You rectify and filter the incoming phase, and then invert back to three phases. Although this has the advantage of being solid-state, it's still pretty expensive. Why would you want to do such a thing? One example is electric trains. Clearly you can only get a single phase from a catenary or power rail. But you may still want to use three-phase motors, just because they're so nice. So this is a good application for phase conversion. -- Joe Keane, not a real EE jgk@osc.com (...!uunet!stratus!osc!jgk)
grege@gold.gvg.tek.com (Greg Ebert) (04/09/91)
You can build a trivially simple 3 phase converter *IF* (and that's a very big IF) the building was improperly wired. I used to live in a condo, and the '220' plug was actually two 120 VRMS lines which were 120 degrees out-of-phase (which is why the votage was 208 VRMS and not 230-240 VRMS). This is easily determined with a crummy voltmeter. Nope, you don't even need a scope. The third phase can be reconstructed by using two 1:1 isolation transformers. Basically you 'add' the two phases which you do have, then invert this sum. Now you have 3 phase AC. I got 2 transformers at a surplus store which were rated at 120V 20A / 240V 10A [now way dude, they aint for sale]. I gave my solid-state inverter a few years rest with the above setup, except when I wanted to run 60 Hz motors at 330 Hz [boy do they scream !]
larry@tapa.uucp (Larry Pajakowski) (04/09/91)
Some of the back issues of the magazie "Live Steam" about 2 years ago had an article with a couple of easy ways to do this. One comment is that all this stuff is at line potential. The first is with an idler 3 phase motor. Your feed your single phase into 3 of the 3 connections and use 2 capacitors to the third. One capacitor is a motor starting (short duty) and the second is a motor run cap. Second is to use a capaictor as a phase shifter into the 3rd lead. Again another larger capacitor is needed to get things started. I've only seen this used on smaller motors like grinders. Refrigeration supply houses are a good source of motor run caps. This isn't as far fetched as it sounds. I know several people who run milling machines and the like with these circuits. Molyphase is a commercial brand of the idler motor type. I've seen them advertised in some of the machinery catalogs. Larry
chuckh@apex.com (Chuck Huffington) (04/09/91)
In article <4725@osc.COM> jgk@osc.COM (Joe Keane) writes: |Why would you want to do such a thing? One example is electric trains. |Clearly you can only get a single phase from a catenary or power rail. But |you may still want to use three-phase motors, just because they're so nice. |So this is a good application for phase conversion. |Joe Keane, not a real EE |jgk@osc.com (...!uunet!stratus!osc!jgk) Another really good reason is that you can't always get 3 phase power when you need it. A lot of machine tools have 3 phase motors. People who have home machine shops often find it nearly impossible to get three phase power. If the area you live in was not wired 3 phase it will cost a fortune to get it. A lot of residential areas are NOT supplied with three phase and you will pay by the foot to the nearest three phase power. I've seen it to be miles away. Rotary converters are not that hard to get. They aren't free but they aren't that expensive either. I've seen some pretty cheap solid state stuff, but have never tried one. I have third hand reports that they are not very reliable. The rotary ones are. The cheap solid state ones also only work on lightly loaded motors (<75% load). As I recall a rotary unit to run a 7hp motor will weigh about 100 lbs and cost under $1000.
josh@happym.WA.COM (Joshua_Putnam) (04/11/91)
In <1991Apr9.004732.8605@apex.com> chuckh@apex.com (Chuck Huffington) writes: >Rotary converters are not that hard to get. They aren't free but they >aren't that expensive either. I've seen some pretty cheap solid state >stuff, but have never tried one. I have third hand reports that they >are not very reliable. The rotary ones are. The cheap solid state ones >also only work on lightly loaded motors (<75% load). >As I recall a rotary unit to run a 7hp motor will weigh about 100 lbs and >cost under $1000. Since I just happen to have my J&L catalog handy: A solid-state phase converter for 7.5 hp, good for 75-85% loading with "limited" duty cycle costs $279 and weighs 17 lbs. A rotary phase converter capable of running one 7.5 hp motor and a total of 20hp load costs $1103 and weighs 200 lbs. This is the manufacturer's recommended loading, and may be conservative so that you could get by with a smaller unit. I don't know, since I don't use one of these myself. I have seen used rotary converters in this size range sell for as little as $100, but I don't know what condition they were in. -- Josh Putnam josh@happym.wa.com 206/463-9399 ext.102 Happy Man Corp. 4410 SW Pt. Robinson Road Vashon Island, WA 98070-7399 SOLID VALUE, the investment letter for Benj. Graham's intelligent investors
wreck@fmsrl7.UUCP (Ron Carter) (04/11/91)
One quick and dirty method of producing 3 phase from 1-phase power is to connect a 3-phase induction motor as a delta winding and then run one side off the line power. It can be started using a big starting capacitor along one of the other two sides. Once it is running, the motor's inertia provides the necessary energy storage and the magnetic field of the armature induces currents in the non-driven windings. The result is a pretty close approximation to "real" 3-phase power available across the 3 motor terminals. Of course, for this you pay in hysteresis losses, resistive losses in the windings and armature, and windage losses in the bearings and cooling airflow. Electronics are definitely superior for everything except tolerance to electrical abuse. The nice thing about the motor method is that motors are much more likely to be lying around in usable condition. Yes, I'm a real EE.
sherwood@cbnewse.att.com (charles.a.sherwood) (04/12/91)
In article <39787@fmsrl7.UUCP> wreck@fmsrl7.UUCP (Ron Carter) writes: >One quick and dirty method of producing 3 phase from 1-phase >power is to connect a 3-phase induction motor as a delta winding >and then run one side off the line power. It can be started >using a big starting capacitor along one of the other two >sides. ^^^ How big is big? Is the size motor dependent? 2HP=?? 5HP=?? Do you need a start cap and a run cap or just a start cap??? chuck sherwood att!iexist!cas