AC Mains Phase Sequence Indicator and Working Principles

In our day-to-day life we frequently use three-phase AC power supply system for all electrical and electronic appliances. This three-phase supply consists of three phases, generally represented as R, Y and B or A, B and C. These three phases of a three-phase AC supply attain maximum voltage when they are in a particular sequence. This sequence of three phases while they attain their maximum voltage is called as Phase Sequence.

This phase sequence of a three-phase power plays a critical role in controlling the direction of rotation of the three-phase-electrical motors. If this sequence is altered, then the direction of the motor gets altered, which may cause temporary or permanent failure of motor. So, it is important to keep the phase in sequence or to maintain the proper phase sequence.

Hence, for checking phase sequence, there is a device called as a Phase- sequence indicator or Phase-sequence checker for a three-phase supply.

What is AC Mains Phase-Sequence Indicator?

The phase-sequence indicator or phase-sequence checker for a three-phase supply is a device used for testing the three-phase sequence of a supply in an electrical circuit or at the input of the electrical motors such as a three- phase induction motor, a  three- phase-energy meter, etc.

Different types of Phase Sequence Indicators

There are different types of phase-sequence checkers, but only a couple of frequently used phase-sequence checkers, with their working principles are explained below.

Static type Phase Sequence Indicators with their Working Principles

Static type is again of two types based on the element used, along with one of the three phases such as an inductor or capacitor.

Consider the three phases as R, Y and B.

Static type Phase Sequence Indicator using Inductor

Connect two lamps, lamp1 to R-phase, lamp2 to Y-phase and inductor to B-phase as shown in the below figure. Resistors are connected in series with the lamps for protecting the lamps from over currents and breakdown voltages.

If the sequence of supply is RYB, then the lamp 2 will glow brighter than lamp 1; if the sequence of the supply is reversed or altered, then the lamp 1 will glow brighter than the lamp 2. We can understand this easily with the following description:

The three-phase-voltages of the three-phase supply are represented as VRY, VYB and VYB.

Now, from the above circuit diagram, we can get

VRY = V
VYB = V (-0.5-j0.866)
VBR = V (-0.5+j0.866)

For a balanced operation, we have VRY=VBR=VYB=V. Such that the algebraic sum of all phase currents is equal to zero. Thus, we have

IR + IY + IB = 0

Then, from the above equations, the ratio of the IR and IY can be obtained and is equal to 0.27.

From this ratio, we can say that, if the phase sequence is RYB, then the voltage across lamp 1 is only 27% of the voltage across lamp 2. So, lamp 2 will glow brighter than the lamp 1 indicating that the supply is in proper phase (i.e., RYB). Similarly, if the phase is reversed or altered, then the lamp 1 will glow brighter than the lamp 2.

Static type Phase Sequence Indicator using Capacitor

From the above circuit, by replacing the inductor with the capacitor, a static-type checker with a capacitor can be obtained, as shown in the below figure. Same as the above two lamps, the lamp 1 to R-phase and the lamp 2 to Y-phase are connected. Resistors are connected in series with the lamps for protecting the  lamps from over currents and breakdown voltages.

From the above circuit, we can observe that, whenever a three-phase supply is given, – if the phase sequence is RYB, then the lamp 1 will glow and the lamp 2 will be in off condition. Similarly, if the sequence is reversed or altered, then the lamp 1 will be in off condition and the lamp 2 will glow.

Rotating type Phase Sequence Indicator

It consists of coils and a rotatable aluminum disc. This checker works on the principle of three-phase electrical motors, especially induction motor. We know that, if the sequence of the supply given to the motor is altered, then the direction of the rotation of the motor will change or gets reversed.

Similarly, if a three-phase supply is given to the rotating-type phase sequence-checker, then its coils will produce a rotating magnetic field, which further produces eddy EMF in the aluminum disc. A torque is produced by the interaction of the eddy EMF produced on the disc and the rotating magnetic field. Due to this torque, the aluminum disc will rotate, and the direction of rotation of aluminum disc is based on the sequence of supply.

If the supply sequence is RYB, then the disc rotates in clockwise direction and if the supply sequence is changed or altered, then the disc rotates in anticlockwise direction.

To get a better idea regarding this article, a simple electrical and electronic project is described here as a phase-sequence checker,

Phase Sequence Indicator or Checker

The main intend of this project is to detect the phase sequence of the three- phase AC supply (which is given as input for electric motors). The phase- sequence indicator circuit is shown in the below figure, and it consists of a step-down transformer, a bridge rectifier, a regulator, a  NAND logic gates circuit, a timer and a LED indicator.

All these are connected to form a circuit,such that if the supply three phase is in a particular sequence (say RYB), then no triggering signal will be generated from the logic gate circuit, and thus, Light Emitting Diodes will run in the clockwise direction.

If the three-phase supply sequence is changed or altered, then the logic gate circuit generates a signal, This signal is fed to the 8051 microcontroller using a 555 timer and the output generated by a microcontroller is used to drive the LEDs. This causes the LEDs to run continuously clockwise direction for some time and counter clockwise for some time indicating the improper phase sequence.

The phase sequence checker project discussed above is used only to indicate the changes in the supply phase sequence. However, this  project can be implemented by using a relay to trip off the supply to (load) induction motor whenever the sequence is changed. For more information regarding this article and to create awareness among others about the phase indicator project, please post your ideas and queries as your comments in the below section.