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# Phase Sequence | Explained

Phase Sequence | Explained 1024 576 Gaurav J

Phase sequence of three phase system determines the direction of rotation of 3 phase induction motor. Phase sequence also decides, if we can connect one 3 phase system parallel with another or not. And hence, it is very important to understand what is phase sequence So, to get the details about phase sequence, watch the video.

## What is Phase Sequence?

Consider a winding of a 3 phase alternator as shown in the above figure. We know that phase difference is 120 deg in 3 phase system. Why 120 deg? That because the windings are placed 120 deg apart from each other. So, if you would like to have phase difference of 90 deg, just place the windings 90 deg apart from each other. It’s all about how you place the windings.

Let’s say the magnet is rotating in clockwise direction (as shown above), that means, winding Y will reach it’s peak value first, after that winding R and then winding B will have it’s peak. As you might have guessed correctly, winding Y reached it’s peak value right after the magnet has rotated 120 deg from it’s initial position. Similarly, when the magnet will rotate 240 deg from it’s initial position, winding B will reach it’s peak value. We can also say that, waveform of winding Y is leading the waveform of winding B by an angle of 120 deg, or we can say that waveform of winding Y is lagging the waveform of winding R by 120 deg.

So, if you observe the sequence of reaching peak values, you’ll find that winding R is reaching it’s peak value first, and then winding Y and then winding B. And this sequence will continue till the magnet is rotating in clockwise direction.

In polyphase system, the order in which voltage waveform reaches their respective peak values is called as phase sequence.

So, in this case our phase sequence would be R-Y-B. This is because, R phase reaches it’s peak value first and then Y and then B. Phase sequence can sometimes also called as phase rotation. So, you need not to get confused between phase sequence and rotation as both are one and the same.

## How to reverse phase sequence?

Now, if I would like to reverse this phase sequence what should I do? I only need to change the direction of rotation of my magnet. Now we’ll rotate it in anti-clockwise direction. As we have changed the direction of rotation of magnet the sequence of reaching the peak values will also change. Now, winding B will reach it’s peak value first and then winding Y and then winding R. So, my phase sequence now is B-Y-R.

Of course, changing the direction of rotation of magnet is not an option for end user. You can change the phase sequence at your end also by interchanging the phase conductors. So, for example, this our 3 phase load let’s say induction motor. If you connect your respective phase in their respective slots the phase sequence would be R-Y-B.

If you want to change the phase sequence just interchange any of the phase, and the phase sequence would also change. Why phase sequence is important for induction motor? Because the direction of rotation of motor can change with change in phase sequence. So, if your motor is running in opposite direction, phase sequence could be one of the cause.

If your 3 phase load is purely resistive then change in phase sequence will not affect anything. But, it may affect the unbalanced inductive or capacitive loads. So, I hope you have understood what is phase sequence.

###### Gaurav J

Electrical Engineer. Content Creator. Currently working with a High & Extra High Voltage Switchgear Industry.

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