Register to reply 
Phasor rotation 
Share this thread: 
#1
Nov2311, 04:50 AM

PF Gold
P: 2,011

When phasors are used,for example to illustrate some of the properties of waves,all of the texts seem to emphasise that the phasor rotates anticlockwise.Why anticlockwise?I have always assumed that the direction is just a convention...or am I missing something?If it is a convention then does the choice of anticlockwise have any advantages over clockwise?Thanks to anyone who replies.



#2
Nov2311, 07:14 AM

P: 873

It's probably a convention, like so many other things.
By the way, why do degrees start at 3 o'clock and increase counterclockwise? Maybe these degrees are the ones to blame! In any case, there ARE phasors that rotate in the clockwise direction. I am an electrical engineer and learned about phasors and symmetric components in the power systems class a couple of decades ago...this is what I remember. Example, if you have a 3phase power system, it can be handled and represented with a single phasor per quantity when things are nice and balanced; but when something happens (a short circuit, lightning, etc) and the system becomes unbalanced, it is difficult (impossible with phasors) to handle and so it is decomposed into its 3 symmetric components...these symmetric components can once again be handled with phasors as "things" are balanced within the corresponding frame of reference...the thing is that I said they are 3 symmetric components, they are referred to as the positive sequence, negative sequence and zero sequence components...the positive one rotates counterclockwise, the negative rotates clockwise and the zero does not rotate at all. The linear combination of these symmetric components models the original system. A phasor represents a single pure sinusoidal quantity so that when there is a fault and things are momentarily not purely sinusoidal, an nphase system can be decomposed into its n symmetric components that when combined can represent any state of the original system via these symmetric components which CAN be handle with phasors. I hope this help and did not make it too confusing. 


#3
Nov2311, 10:18 AM

Sci Advisor
PF Gold
P: 11,942

I think it's probably because we originally chose Right and Up for our +X and +Y cartesian axes and gradient is the angle 'upwards from the horizontal'. That gives you angles that increase from zero to 90 in an anticlockwise direction. The rest just follows.



#5
Nov2511, 06:23 AM

Sci Advisor
P: 1,474

We can classify frequencies as being positive or negative depending on what way the phasor rotates; the direction we nominate as being positive is quite arbitrary.
Claude. 


#6
Nov2511, 06:56 AM

Sci Advisor
PF Gold
P: 11,942

Not quite arbitrary. For a frequency higher than the reference frequency (at which the axes are 'frozen', a higher frequency will be represented by a phasor which is rotating anticlockwise and vice versa because of the rate of change of phase relative to the reference.



#7
Nov2511, 08:17 AM

P: 873

It is possible to make higher frequency signals show up rotating counterclockwise respect to the frame of reference...this can be accomplished via linear combination of spatially shifted higher frequency signals. This is not as weird as it may seem...it is happening right now, somewhere in the world. I am talking about a 3phase electrical generator at some power plant that might not be working under perfect conditions and hence having nonpurely sinusoidal waves through its winding...under these conditions, the waves can be decomposed (fourier) into purely sinusoidal ones starting with one at the fundamental frequency (f) and the rest at higher frequencies (2f, 3f, 4f, 5f, 6f, 7f...). BUT, when we are talking about a 3phase system, we have 3 sets of these signals that are (electrically) phase shifted 120 degrees (360/3) and being applied to windings ("mechanically", physically) located 120 degrees (for 2pole generator) away from each other...when you add the signals vectorially, the resultant may rotate counterclockwise (CCW) or clockwise (CW) respect to the fundamental... ...for example, say the fundamental is rotating CCW at an angular velocity of 1f:



#8
Nov2511, 09:22 AM

Sci Advisor
PF Gold
P: 11,942




Register to reply 
Related Discussions  
About phasor voltage and phasor current.  Electrical Engineering  4  
Solving this partial fraction  phasors  Calculus & Beyond Homework  1  
What is a phasor  Introductory Physics Homework  3  
Phasor problem help  Engineering, Comp Sci, & Technology Homework  5  
Inductor resistor phasor help  Engineering, Comp Sci, & Technology Homework  7 