I was asked by a mechanical engineer why we only have just 3 phases, why not 4,5 or more? I know 6 is quite common for rectifier circuits but they are derived from 3. Any advance on 6? Is this a silly question or what?

Roy

 

G'day Roy,

My first thoughts were that three is the least required to easily get a motor to rotate. Then thinking some more, how many times does somebody get the phase rotation wrong when initially wiring a motor, imagine if the phases got crossed with a 4+ phase rotational machine.... I don't think it would be as simple as 'oops it's going backwards'. It does make good sense with rectifiers using hex phase and phase sequence would not be as common a problem.

Interesting discussion point though.

Cheers,

Trevor

 

Probably because three is the minimum and therefore the most economic number required to produce a stable rotating magnetic field in a stator?

Vince

 

Responding to Roy Matson's 21-Dec (00:43) question:

For a fixed amount of power there is no difference how many phases are used. However, when you take into account the weight, hence the cost, of the conductor material, then, the 3-phase system is more economical than any other N-phase system!

To illustrate the above, a 3-phase system requires 3/4 of the weight of a single-phase system. For a 4-phase system the comparable ratio is also 3:4.

This topic was covered before, so I suggest a search of the Control List Archives. If you can't find it, and you can stand the math (trig is all that' needed) contact me... on or off-list!

Regards, Phil Corso ([email protected])