Phase sequence indicator:
These instruments are used to measure the phase sequence of three phase supplies. They are of two types 1) Rotating type and 2) Static type.
1) Rotating type:
- Rotating type phase sequence indicator
The principle of rotating type meters is similar as three phase induction motor.
- They consist of three coils connected 120 degree apart in space.
- The three ends of the coils are brought out and connected to three terminals marked RYB as shown in figure.
- The coils are star connected and are excited by the supply whose phase sequence is to be determined.
- An aluminium disc is mounted on the top of the coils.
- The coils produces rotating magnetic field and Eddy emfs are induced in the disc.
- These emfs cause Eddy currents to flow in the aluminium disc .
- A torque is produced with the interaction of the Eddy currents with the field.
- The disc revolves because of the torque and the direction of rotation depends upon the phase sequence of the supply.
- An arrow indicates the direction of the rotation of the disc.
- If the direction of the rotation is same as that of indicated by arrow head, the phase sequence of the supply is the same as marked on the terminals of the instrument.
- However, if the disc revolves opposite to the direction indicated by the arrowhead, the sequence of the supply is opposite to that marked on the terminals.
2) Static type:
- Static type phase sequence indicator
It consists of two lamps and one inductor connected as shown in figure. If the sequence is RYB then lamp L1 will dim and lamp L2 will glow brightly. If phase sequence is RBY then lamp L1 will glow brightly and lamp L2 will be dim.
Thus the voltage drop across lamp 1 is only 27% of that across lamp 2.
Thus if the phase sequence is RYB lamp 1 glows dimly while lamp 2 glows brightly.
It can be shown that, if the inductor is replaced by capacitor of such value that Xc=XL, the ratio (Ir/Iy) is 3.7, which means in this case lamp 1 glows brightly and lamp 2 glows dimly if the phase sequence is RYB.
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