When 3 phases ona certain order are able to achieve their maximum point or their peak; that sequence can be expressed as phase sequence.
In case of 3 phase e.m.f. generation in consideration offield system the assumption is mainly dependent on clockwise rotation. This presumption is based on e.m.f. of phases.
Let us consider these 3 phases as‘l,’‘m’ and ‘n.’As per this, the phase lag between ‘l’and ‘m’ is observed to have an angular difference of 120°. Similarly, thephase lag between ‘m’ and ‘n’ do also have similar variation by 120 electrical degrees. Sofinally, it can be clearly seen that angular variation between ‘n’ and ‘l’ in clockwise movement is 240 electrical degrees.
So, the sequential pattern in which phases of these e.m.f. (m, n, and l) are placed to acquire their highest value will be denoted as lmn. This positioning and value are known as phase sequence or phase order.
This sequence follows as clockwise motion with the order as ‘l,’‘m’ and finally to ‘n.’In case this rotational movement of this field structure is forced to move in counter clockwise movement or reversed direction, its resultant will also follow the same path. Those 3 phases in that order would be able to achieve their maximum voltage ina reversed order, thereby relaying the new phase order as ‘l,’‘n’ and then ‘m.’
As per this counter rotation, the new e.m.f. of phase showcases the fact that in this new phase, ‘n’ will be lagging behind ‘l.’But in place of 120 electrical degrees, the angular difference would be of 240 electrical degrees.
In generalizedsense, this phase orderof voltages which on application to a load can help in determiningthis sequence. The purpose of this sequencing is based on the connected 3 phase lines. The reversal of this phase sequence is possible when any of the line pairs are interchanged with each other.
The outcome for reversal of sequence can be seen in an induction motor, where its motor rotation takes place in counter clockwise direction.
Explaining via examples
Let us consider these 3 phases to be either numbered or be taken as in the form of colors. For this purpose, let us stick to colors (primary colors). So, we will be using the initials for blue, red and yellow, taking it as BRY. So, in this sequence regarding any of the 3 phase system, 2 of the potential orders having phase voltages or 3 coils can pass via their peak value. It can be RBY (red, blue and then yellow) or it can be RYB (red, yellow and then blue).
So, in accordance with this convention,
- RBY can be taken to be negative
- RYE can be taken to be positive