Why 3-Phase Motors Instead Of 2-Phase?

The various encyclopaediae that offer explanations point out that 3-phase transmission transfers power smoothly 'twixt generator and load. 3-phases are the minimum needed to confer direction of rotation, and use the minimum of copper to do it.

A single or dual phase motor will work however requires a separate field coil to ensure the motor goes in the correct direction.

The most efficient is the three phase motor which does not need a separate coil to ensure proper direction.

Also, If you add all three phases, The average voltage is zero

the three-phase system needs the minimum amount of conductors for the maximum transmission of power for a balanced load. If you drive a lathe with a three-phase motor the torque is even over 360 degrees rotation and a better turned product will result, a two-phase motor (that's single plus capacitor or genuine two-phase) will have uneven (lumpy) torque resulting in a discernible difference in turned product, all other elements being equal. The permutations arising from three wires - ok and a neutral are incredibly useful, allowing two voltages and capable of supplying a combination of loads. You need already 3 (or 4) wires for two phases, so this is three phases for the price of two! If it was soap powder you would buy it!

OK. So riddle me this... A girlfriend of mine is into pottery. Recently, we purchased a wheel. Our decision was based on the fact that one particular wheel (controlled by a foot pedal) could turn very slowly without losing any torque! This was a single-phase 110V setup. So why no loss in torque at low revs?

I suspect modern electronics.....

Now Andy. Don't make me send you to the Principle again!

What else could be the reason, I am willing to learn!!

Do you remember last week's phonics lesson? OK, Andy... And the word "how" means what?

Is this a Christmas Quiz ? or don't you now either ?

G'on, show us how clever you are, pretty please?

Yes it is a Christmas quiz. How do you keep the torque going when the revs drop on an electric motor. If I knew how, I wouldn't have axed the question!

My apologies for thinking you were making a quiz.....

If it is a DC motor (of almost any type) or a Universal motor, then Pulse Width Modulation is a method favoured by many that allows good torque at low speed, but as to whether this has been done on this potter's Wheel, I could not say.....the AC mains would need to be rectified first......

With PWM, the motor is supplied with 100% full voltage pulses, but the width (timing) is variable from say 5% on to 95% off, to 100% on to 0% off......

I am of the opinion that there are further electronic methods for AC motors that I have never come across in detail.....maybe someone else can let us know more.....

Vermin, I don't subscribe to any threads, but check back occasionally, so I don't know if you will read this. There is the element of gearing. Your girlfriend's wheel even when flat out will still rotate slowly compared to the motor rpm, and when rotating slowly the motor will be rotating much more quickly. I had a go at pottery once, even with the bit at the start (throwing?) which I imagine is the highest load likely to be imposed on the motor, she would not notice the lack of torque. Compare this with putting >1mm cut on steel at 1800 rpm. Regarding lack of torque, once playing with a 1ph to 3ph inverter on the bench driving a fairly chunky motor (no size given but it was a heavy lift) it was really easy to stop the motor with my hand on the very lowest ranges (speed control selected via a pot); no torque to speak of. As another point of reference I once had a treadle metalworking lathe and used it on treadle power. To do big cuts I used the backgear which gave me about 10X the torque.

HUX,

Thanks for the answer. As far as I could tell the foot-pedal contained only a potentiometer. They were doing something much trickier with the torque. For example, if you grabbed the wheel and tried to stop it at low RPMs it would practically throw you across the room. It was quite impressive, because potters like to do things to their creation at very low revs.

Thanks all for the answers, please let me know if I am getting all my facts correctly:

- A real 2-phase motor starts by itself, and you change rotation direction just exchanging phases. A +90 degree phase diference would set one direction, and -90 the other one. Is that right?

- The number of cables to deliver 3 phase is the same needed to deliver 2 phases ,because 3 phases which are 120 degrees out of phase add to 0 volts, but that's not the case for two waves 90o out of phase.

- In a 3-phase motor, torque is even along the whole rotation. In a 2-phase one, torque changes as the adition of the voltages change.

I don't really think you understand it fully, maybe I am wrong, but here goes.

Two phase motors run on single phase, a better name is split phase. They usually use a capacitor to try and get the second field to be at a max at a different time to the first field which is attached directly between phase and neutral. The turning moment is not very good and they have problems with starting loaded.....Sometimes the so called starting field is taken out of circuit once the motor is running using a centrifugal switch. Sometimes, for more shaft power, the starting cap and field are left in circuit, but need to be rated for that.....

3 phase motors have the 3 ( or multiples of 3) fields connected one per phase, the voltage maximums are 120° to each other, so that basically each field is "on" at a different time, so that the strongest field point basically rotates. This drags the so called "Squirrel (Skwirrel Kris?) cage rotor around at almost the same speed as the field rotates. The difference being the so called "slip" which varies from motor type and with load.....

Basically 3 phase have a clearly defined rotating field and single phase do not have it quite so well defined and suffer from lower starting torque because of that....

Ya know, if someone was to draw a graph of the fields (sine waves and their phases?), it might make it clearer.

Efficiency is the main reason I would think. Picture an X/Y axis (positive/negative, time elapse) and put three sine waves 120 degrees apart and you will see a smaller drop off of the peak voltage than if you had only one phase. With a single phase motor you have 180 degrees of "positive" on the sine wave, this is the only time that you will enduce a voltage if I am correct. On the negative side of the sine wave nothing is going on. I guess you could almost look at it as on (positive) off (negative). 3 phase motors are always up in the positive side and most of the time right up around RMS (it has been a few years since I went over this in my apprenticeship, I hope it is not too confusing...Good luck)

Think about disponibility. Electricity is supplied as three phase. There is not two has network, afaik. 1-phase is directly obtained from a 3-phase network with a neutral wire. But 2-phase is not.

I have always thought that three phase would always be better because the effective value of voltage would be increased sinusoidally,i.e.,for a current that varies sinusoidally with time, the effective value of amperage is the product of a common factor of 0.707(effective electromotive force).So, the efficiency of each phase would be a product of 0.707,i.e.,-first phase=0.707 total effective electromotive force,etc.+ second phase (0.707)(0.293)emf,etc

I would like to hear what you electricrical engineering people have to say......

Technically seen, there is no 2 phase motors, they use a single phase supply to power two separate fields, shifting the vector of one field relative to the other with a starting capacitor to simulate 2 phases....otherwise they old not self-start as there would be no rotating field....They tend to be used where not to much motor power is needed.....

3 phase motors are obtainable in much larger and more varied sizes and will self-start without capacitors and other tricks...as the 3 phases "rotate" so to say, dragging the squirrel cage rotor along with it.....

Philadelphia Pennsylvania is another place that still has a number of true 2 phase systems installed from bygone days.

In a nutshell, the answer was given earlier. Many many people confuse 2-phase with 1-phase power that uses 2 hot legs. They are NOT the same thing. 1-phase motors must have something to make them spin in the first place. Polyphase motors do not. 2-phase is a true polyphase system, but true 2-phase requires 4 wires (maybe 3 if you know what you are doing). 3 phase requires 3 wires. So the benefits of smoother power and self-starting motors are compounded with the least number of wires to accomplish it.

http://www.3phasepower.org/2phasesystems.htm

In #4 it should read the direction of a three-phase electric motor is reversed by changing any two of its phases.

The direction of a two phase motor is determined by ? Maybe the relationship ( phase and or placement ) of the added coil ?

There are maybe three 1500kW or thereabouts two (2) phase generators driven by hydro wheels near Dubuque, IA operating today. They go thru three phase switchgear and into transformers for connection to the grid. I sold a turbine repair job there about 10 years ago.

There are many motor winding designs, look up EASA (Electric Apparatus Service Assn) www.easa.org and there links to there member sites should provide tons of info about every concievable electric motor. Another is www.Barks.com I think for Barks Publications which prints the monthly magazine "Electric Apparatus" which is the newsletter of the electric motor and switchgear industry.

You may see my ad in there also! Happy Holidays

Hi my friend , In general without going in depth , you should know that :

1. normally the generated power by a generatoe or transformer is 3 phase , then later it will be singlr phase when it reaches your home , so if you have already a 3 phase power , why you will chande it to single phase?? , just buy a 3 phase motor.

2. you have to know that single pase motors are used for low loads but for high loads you have to use 3 phase ones , ( for example you will not find a single phase motor 40 kw as an example ).

I am looking at my toe, but no electricity is coming out of it. Am I looking at the wrong toe?

By the way - Merry Christmas!

Hmmmmmmmmmm, is that why they call them kiltoewatts? OOOps, I am sorry it was kilowatts damn and to think I was trying to come up with a reason for the brown under my toenails . Happy New Years... 2008? Already?