Automation or not ?

[Eric Nelson]

Wait, I found more...

If the motor is small enough, I think Vetteboy's suggestion of a VFD is the best way to go. In any case, I found yet another method in the same book. This sounds like it would take up less room. Again, from "The Electrician's Book of Trade Secrets" (which I guess aren't so 'secret' anymore!)...

• RUNNING A THREE PHASE MOTOR WITH SINGLE PHASE POWER
  
  A three phase motor can be run on single phase power by the use of capacitors to create a type of rotating magnetic field in all three windings of the motor.
  
  If you refer to Fig. 1, you will see that one leg of the single phase power is connected to one motor lead, and the other single phase leg is connected to another of the three phase motor's leads. The third motor lead is run first to a capacitor, and then back to one of the single phase leads.
  
  As you notice, there are two capacitors shown in the drawing; one is a "run" capacitor, meaning that it is in the circuit at all times. The other is a "start" capacitor, which is used only for a few seconds at start-up; this is necessary to provide the extra torque that is needed to start the motor. The run capacitor should be an oil type capacitor, as it will be carrying a significant amount of current, and needs to be hefty enough to take a fair amount of heat. This run capacitor should be sized at 25 to 30 mfd per horsepower. The starting capacitor can be of the electrolytic type (which is cheaper), as it will only be in the circuit for a few seconds. This start capacitor should be sized at about 60 mfd per horsepower.
  
  If you have any trouble finding capacitors of the right size, remember that you can "stack" your capacitors; that is, you can hook them up in parallel. Capacitors connected in parallel are additive (a 30 mfd capacitor and a 40 mfd capacitor hooked up in parallel equal 70 mfd); so this should eliminate any such problems.
  
  To operate your motor manually, you will need a regular start switch (single pole, normally open, momentary contact), and wire it as shown in Fig. 1. You must push your start switch, then turn on your single phase power, and then let off of your start switch after the motor is up to speed. In Fig. 2 we are showing a three phase motor being run on single phase power in this same way, but with an automatic (Magnetic) starter.
  
  Because one leg of this three phase power is being made by the use of capacitors, you can get some very unbalanced currents. Because of this, a motor run on this set-up should not be run at more than 75% of its full load for an extended period of time.
  
  In Fig. 3 we are showing a variation of this method which also incorporates an autotransformer to balance the currents better.
  
  You will need to experiment with the autotransformer to find out which of its taps would be the best to use, but using the autotransformer will give you a much more efficient operation. Using this set-up you can run motors up to 100 horsepower or more if necessary.
  
  If properly adjusted, this set-up will operate extremely well at full load, with current in all three legs balanced. At no load however, its operation is not too good.
  
  One note about autotransformers; Autotransformers can be kind of tricky, so handle them with care. Make sure that the autotransformer that you use is rated for the amounts of current and voltage that it will be required to carry.
  
  Also remember that the voltage rating of your three phase motor must match the voltage of your single phase supply."

[attachment]
Pardon the crappy drawings, I was just trying to duplicate the original author's hand-drawn diagrams as closely as possible...

beerchug

-Eric

P.S. Vetteboy, I agree with you about the '2 phase' jibberish, but I have seen (with my own eyes) motors labeled "TWO PHASE". Go figure!...

 

[LadderLogic]

Just been wondering...

Where that "two-phase" thing might be coming from? I know for a fact, that in some languages what we call "hot" and "neutral" wires are traditionally called "phase" and "null" ones - or something like that.
Therefore, depending on the type of AC system used, a single-phase device may be connected to a pair of "phase" leads; hence "two-phase".

This observation is rather about linguistics than engineering...

 

[Variblade]

Hey everyone,

I finally feel useful here. Ha!

Single phase is when 1 or two hot wires are used. Three phase is when 3 hot wires are used. You can use 1 line or two lines (2 phases) of a 3 phase system to run a single phase component.

An example would be:
A 115 volt motor uses one hot wire and a neutral.
A 220 volt single phase motor usually uses 2 hot wires 115v each.
a 220v three phase motor uses 3 hot wires.

The 220v single phase motor will run on a single hot wire of 220v of a higher voltage system but normally this is not the case.

 

[rsdoran]

Just my 2 cents

3 Phase from Single Phase

Everybody knows that 3 phase power is expensive to get, and most homes only have single phase (110 or 220) power. But 3 phase motors run smoother, because power is constant, so they would be nice to have. You can even get ahold of them used for free/next-to-nothing because almost nobody has 3 Phase Power! You can buy some very expensive electronic converters, but why bother?

You can run a 3 phase motor from standard 220 2 phase power. Really. It is an old technique, known mostly only to old tinkerers and the like. It works like this:

First, you get the 3 phase motor turning it (manually, or better, with a small 110 v motor), and THEN turn on the 220 (connected to two legs) it will run. It will not run at rated power, or smoothly, but it will run (at speed). This is OK for some machines. Now for a magic trick.

Take a second (free, same size or smaller) 3 phase motor, and connect it to the first (three leg switch, and zing!, the second motor turns on instantly, and both motors run smoothly! And with more HP than with just 220. The first motor is acting like a "generator" to provide the third leg. This is not "full three phase power", but it works quite nicely. The 220 supplies power to both motors, with one more wire to connect them. You can can connect additional motors also.

This works best if the first motor is a larger HP (2 vs 1), or higher speed rating (3400 vs 1750) than the slave. Older, "beefy" motors are preferable, due to their mass. A 3 HP, 3400 RPM motor works nicly. The reason for this is that if you try to start a bigger load than your master motor, you may reverse the rotation direction of the master (3 phase motors can be wired to run either direction). A large/fast motor will have enough momentum to resist changing directions. The more motors you get running, the more stable the system becomes. The limit may be the amount of power drawn through the third leg of any given motor, the wiring, or your switch, etc.

A real world, functioning system:

master motor: 3 HP, 3 Ph., 3400 RPM
Starter "pony" motor- 110, 1/2 HP, single phase washing machine motor or eq.
Slave Motor- 3 Ph, 1750 RPM punch press or lathe. (Insert your machine here)
The Washing machine motor and the master are mounted to a board/bench, and their shafts are connected with a flexible coupling. Power to them is switched with a Double Throw, Double Position, CENTER OFF switch. The 3 legs of the master also go to a 3 pole switch, to the slave machine.

  (DP/DT/C off Switch, shown                          (Three phase Switch,  
   in 110 V motor on position)                         Shown off)
          SWITCH 1                                SWITCH 2  /
220 leg1     o--------------------------------+-----------/   o---->
>--------o\   o                               |
            \o                                |                      Three
                                              |             /        Phase
220 leg2     o---------------------------+----------------/   o----> Power
>--------o\   o                          |    |                      To
            \o--------|                  |    |                      Machine
                      |                  |    |             /        Motor "Slave"
Ground                |                  |    |   |--------/  o---->
>--------o------|     |                  |    |   |
                |     |              |--------------|
             |------------|          | 3 HP, 3 Ph.  |
             |(110V Motor) |==[]====|    Motor      |
             |------------|          |  MASTER      |
                                     |--------------|

Operation:

Start with Switch 1 OFF.
Make sure that Switch 2 is OFF.
Turn on the 110 V "start up" motor with switch 1. (as drawn) This will drive the master motor.
Once running, flip Switch 1 so that power now goes to the Master Motor. It will now run at it's speed, and drive the 110 motor. (note that power is NEVER connected to both at once)
You may now turn on Switch 2, which will start the machine motor.
Turn off BOTH switches when done.
Additional motors must have their own switch (of course), wired to the master, and should only be started one at a time.

CAUTION:
Use this information at your own discresion, and your own risk. No warrenties implied. Always observe proper safety proceedures when working with electricity! If you don't know what you're doing, DON'T mess with it.

This is posted at http://www.patchn.com/3phase1phase.html

I see now that this is basically the same thing Terry posted. I have used this setup in my shop and will probably do so again because its an easy/inexpensive way to obtain 3 phase.

 

[Vetteboy]

Just been wondering...

Where that "two-phase" thing might be coming from? I know for a fact, that in some languages what we call "hot" and "neutral" wires are traditionally called "phase" and "null" ones - or something like that.
Therefore, depending on the type of AC system used, a single-phase device may be connected to a pair of "phase" leads; hence "two-phase".

This observation is rather about linguistics than engineering...

Hi LadderLogic, I know people get confused and you have just explained why. If I want 480 volts at work (let's say a control transformer for 120v control power) I can pick any two leads from my 3 phase system and get 480v. In this instance we use 2 phases to get our high voltage, and one phase to N for our lower voltage. We have all discussed before how 3 phase works so I won't go into great detail. What I'm saying is that even though the utility is supplying 3 phase, we don't use it the same way every time.

Same goes with single phase. The 3 wire edison circuit is a single phase supply with a dual voltage. When you use both "Hots" you use a bigger 240v phasor, which is the result of two 120v phasors in the same plane added together, but still 1 phase. The only real 2 phase I know of is not supplied by a utility, but created by a capacitor in a single phase motor to get rotation going. Hope this makes sense, I'm certainly no instructor.

 

[seppoalanen]

As Vetteboy told, VFD is good solution for small motors:

 

[drewcrew6]

When I was in school (Vo-tech) they talked briefly about 2 phase.
From what I recall there was such a system and at that time still was.
It was actually 2 seperate phases 90 degrees out from each other and used 4 wires. Essentially 2 sets of 240v residential 90 degrees out.
I was told that in Philadelphia PA area there was a few select companys that still used this. Due to the large expense of converting the motors and wiring over to 3 phase the system was intact.


Maybe someone in Philly can confirm????


Drewcrew6

 

[Jnelson]

drewcrew6,

I have worked with some "old timers" that call that type of system an "edison". I have never seen one but they did try to explain to me how they had to wire a three way switch in a house, now that you would not believe. The way you have described it is pretty much how they drew it out on paper.

You know how those "old timers" are though (sorry Terry), I cannot confirm as I have never seen an edison?

 

[David_Emmerich]

Hi Group,
Mr. Tesla designed a 2 phase system very early in his efforts to establish a workable distribution system for the Niagra Falls power plant that he collaberated with Westinghouse on ....when I get home tonight I will look up the prints and post them for y'all. Most of the prints that you folks have posted were origanily drawn by Tesla as part of his Patent papers...from the 1890's.
More later, got to run....
David