- Phase Sequence Indicator Definition: A phase sequence indicator is a device used to determine the phase order of a three-phase electrical supply.
- Types of Indicators: There are two types—rotating type and static type, each with a distinct working principle.
- Rotating Type Working Principle: It operates on induction motor principles, using a rotating magnetic field to move an aluminum disc and indicate phase sequence.
- Static Type Working Principle: It uses lamps that vary in brightness depending on the phase sequence, with some variants using capacitors and neon lamps.
- Determining Phase Sequence: These indicators help identify if the phase sequence is RYB or reversed, crucial for the correct operation of three-phase systems.
In a three phase system, the phases are conventionally labeled as RYB. A phase sequence indicator is a device that determines the order of these phases in a three-phase supply.
When we apply a conventional RYB three-phase supply to an induction motor, the rotor rotates clockwise.
Now what will happen to direction of rotation of rotor if the phase sequence is reversed, the answer to this question is that the rotor will rotate in the anticlockwise direction. Thus, we see that the direction of rotation of rotor depends on the phase sequence. Let us study how these phase instruments work and on what principle they work.
Now there are two types of phase sequence indicators and they are:
- Rotating type
- Static type.
Let us discuss one by one each type.
Rotating Type Phase Sequence Indicators
It works on the principle of induction motors. In this coils are connected in star form and the supply is given from three terminal marked as RYB as shown in the figure. When supply is given the coils produces the rotating magnetic field and these rotating magnetic fields produces eddy emf in the movable aluminium disc as shown in the diagram.
The eddy EMF produces eddy current in the aluminum disc, which interact with the rotating magnetic field to create torque, moving the disc. If the disc rotates clockwise, the sequence is RYB; if it rotates counterclockwise, the sequence is reversed.
Static Type Phase Sequence Indicators
Given below is the arrangement of static type indicator:
When the phase sequence is RYB then the lamp B will glow brighter than the lamp A and if the phase sequence is reversed then the lamp A will glow brighter than the lamp B. Now let us see how this happens.
Here we assume that the phase sequence is RYB. Let us mark voltages as Vry, Vyb and Vbr as per the diagram. We have
Here we have assumed balance operation such that we have Vry=Vbr=Vyb=V. Since algebraic sum of all the phase currents is also equal, therefore we can write
On solving the above equations we have ratio of Ir and Iy equals to 0.27.
It implies that the voltage across the lamp A is only 27 percent of that of lamp B. Hence from this we can conclude that the lamp A will glow dimmer in case of RYB phase sequence while in case of reversed phase sequence we have lamp B is dimmer than lamp A.
Another type of phase inductor works similarly but uses a capacitor instead of an inductor, as shown in the diagram.
Two neon lamps are used, along with them two series resistor are also used to limit the current and to protect the neon lamp from breakdown voltage. In this indicator if the supply phase sequence is RYB then lamp A will glow and lamp B will not glow and if the reversed sequence is applied then the lamp A will not glow while lamp B will glow.
