expanation of 3 phaze power

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

Will do. Didn't think of that. I asked my brother if he was going to make a converter but he's looking at buying an electronic one. Thanks Karl

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

I see a slight contradiction in this statement.

And you only have to feed in 1/3 of the wattage. Quick, quick! To the patent-office! :-)

Nick

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

Mart> >>

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

My brother is planing to use one of these of suitable size.

everyone. Karl

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.

At least that holds true for Germany. Many don't know that they do have three phases, but a (closer) look at their fuse box shows it.

Nick

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

Fuse box? How last millennium!