VFD' and RFC'c ?

[Rbeckett]

Ok, now I am really confused. I was OK till Frank said the 3 phase motor is running and kicking out 3 phase juice. Could one of you really smart fellers please explain to me how this system works from beginning to end. The way I understood it the small motor turned the larger motor and it produced 3 phase electricity. The larger the three phase motor the more amps it would produce. Kind of like driving an auto alternator or a generator. Now with the 3 Ph motor runnig I am all confused. Please straightem me out if you can.
Bob

 

[Tony Wells]

Easy way to look at is that a 3 phase motor will run on single phase, just won't start on it. Once you get it spinning, and with one single phase fed into it, it runs, and magnetically it is actually generating the other legs (phases).

 

[Rbeckett]

So you end up tapping a live single phase feed and two phases manufactured on the motor to get all three? I think I got it now. Thanks for the help out, for some reason that just scrambled me completely.
Bob

 

[bcall2043]

So you end up tapping a live single phase feed and two phases manufactured on the motor to get all three? I think I got it now. Thanks for the help out, for some reason that just scrambled me completely.
Bob

Tony and your smarter guys jump on me if I am too far off base or totally wrong.

The way it was explained to me is that the 220 volts we normally have in our homes here in the states is two legs ("live or hot" wires) of the three phases (three hot wires) the power company manufactures and sends around the country. As Tony stated the three phase motor won't start without the third leg wire or true three phase. Once you get it turning by using a pony motor, pull rope or some other method, it will run and induce voltage into the other motor windings kind of like a transformer. Your not really manufacturing three phase but using the two you already have and generating (inducing) voltage into the third leg of the three phase motor wiring. There are higher voltages of the distributed three phase but in the home shop we usually only have to deal with the 220 voltage motors. I have ignored the neutral and ground wires in the explaination.


Have I totally confused the issue?

 

[7HC]

So you end up tapping a live single phase feed and two phases manufactured on the motor to get all three? I think I got it now. Thanks for the help out, for some reason that just scrambled me completely.
Bob

That explanation helped me too. Just to reiterate, the three phase converter motor is fed single phase power and is started rotating by drive belt from the small single phase motor. Once the converter motor is running from the single phase feed, the pony motor is disconnected and the converter sends two phases that it generates plus the 'house' single phase to the lathe, mill, or whatever.
Is that correct?

Regarding the disconnection of the pony motor, which I read could be problematic, a simple solution would be to use a pulley connected to an electromagnetic clutch, such as is used on a vehicle a/c compressor. As soon as power is cut to the clutch the pulley freewheels and the shaft remains stationary.

M

 

[Hawkeye]

Every spinning motor is a generator. Normally, when a motor is connected to the rated voltage, it generates Counter-EMF - a voltage opposite that fed into it. When the CEMF equals the supply, it stops accelerating. That's what sets the RPM for a given configuration running at a given supply voltage. A very basic explanation, but it may help the understanding.

Since any coil of wire moving through a magnetic field generates a voltage, the un-powered windings of the 3-phase motor will generate voltages at the phase angles dictated by the position of the windings. This makes the generated voltages at the right phase angles (120[SUP]o[/SUP] from each other). There's no such thing as a perpetual motion machine, so we can expect some losses. That's why we often hear about the motor in an RPC only able to generate about 2/3 of it's own horsepower.

 

[Tony Wells]

Yup, well rounded, yet simple enough that I think everybody got it.

 

[bcall2043]

Hawkeye,

You said it best.

Benny

 

[jumps4]

that was good mike
steve

 

[8ntsane]

This thread could get confusing to some. A phase converter needs 2 of the 3 legs powerd with single phase( not one) The 3rd leg , some times called the generated leg, sometimes called the wild leg will kick out its own power once the motor is running. This will give you 3 phases.

Just to make it clear for those that don,t understand, or know. You can get a 3-phase motor to run if you power 2- out of the 3 legs. You dont have to use a pony motor to spin it up and start it. You could , and many have wrapped a rope around the shaft, and give it a pull like you would with a old lawn mower.

Once the 3-phase motor is running, it generates power on the 3 rd leg on its own. Easy enough to check voltage on that leg while its runing. It wont be exactly the same as the other -2 but close enough to get the job done.

On general, the motor used for RPCs is twice the HP of the motor you intend to power off it. From what I have found, if I was to use say a 3hp RPC motor, and use it to power a 3hp lathe, the wild leg would have a very different voltage.

When the RPC motor is atleast twice the size, the voltage of the wild leg is much closer to the other -2 . So, in the case of the 3hp lathe, the close match for a motor would be a 7.5 hp motor for the RPC.

As said, general rule of thumb, is X-2 , and if you go to the next size up, as in the above example, thats fine too. The RPC can be started with a few methods. The pony start, is my perferred method. I wont bother doing it any other way on motors that are 7,5 hp and larger. I have no use for playing around with start caps either. Just adds to the mess of wiring, and as allready said, you can pull start it with a rip cord if you wish. That should tell you exactly what a pony motor is used for, starting the 3-phase motor only.

I,ll give you a for instance on how you can get it rolled over , and started. Now don,t laugh too hard now. A buddy of mine has been starting his with a automotive air gun on the shaft of his, and has for over 10 yrs. So a pony motor isnt the only way. Ive allso seen guys take a recoil off a old lawn mower, and that works too.

I hope that clears it up for some of the guys, if not, ask again. If I can help, I will.

I have assemble many RPCs, But they have all been pony start, to keep it simple. I never liked the massive amount of wiring with cap start RPCs.
As far as using run caps to balance the legs, I have tried before, and found its not worth the effort. On machines that change the load as a lathe does, trying to balance that, I see as pointless.

Just my 5 cents on the topic