Single Phasing and Single Phasing Preventer
In this post, we are going to learn a very important topic in electrical engineering that is What is Single Phasing? Working and connection of Single Phasing Preventer.
Before going to read about Single Phasing Preventer, you must be known about What is single Phasing?
What is Single Phasing?
When a three-phase motor is running, if one phase among three phase open circuited then it is called Single Phasing.
We know that a three-phase motor is more efficient than a single phase motor. Each three-phase motor has either star or either delta connection. So if one of phase is open they will continue to run but they will be drawn approximately 2.5 times the normal current during single phasing condition.
So basically, absent of one phase among the three phase is called Single Phasing.
Why we need Single Phasing Preventer?
You may think that as the motor will continue to run after single phasing, then why we need a Single Phasing Preventer.
We need Single Phasing Preventer because as after single phasing the motor will draw current 2.5 times of the normal current so motor windings may get damaged even may burn.
There are many bad effects of single phasing, so we need a Single Phasing Preventer.
How a Single Phasing Preventer works?
Basically, a Single Phasing Preventer works like a switch, but it automatically closes the circuit in normal condition and opens the circuit in the fault condition that is single phasing.
For better understanding see the below circuit diagrams. There two circuit diagram is given. One diagram for normal condition and other for the fault condition.
Types of Single Phasing Preventer:
(1) Voltage sensing Single Phasing Preventer.
(2) Current sensing Single Phasing Preventer.
What inside a Single phasing Preventer?
Here a voltage sensing single phasing preventer is shown. as you see in the below picture it consists of a 12V Relay, a 440V/12V transformer for operating the Relay, PN junction diodes for the bridge rectifier, some capacitors, transistors, Zener diodes, resistors, variable resistors.
Connection diagram of The Single Phasing Preventer:
The Single Phasing preventer which is shown in pictures has Five terminals, three terminals red, yellow, and blue for three phase supply which are to be connected before the motor and after the contactor. The remaining two gray terminals are the 'Common terminal' of the relay and 'Normally closed (NC) terminal' of the relay. As you see in the below connection diagrams the Contactor having the rating 230V. So an auxiliary supply of 230V is given to the Contactor through the two terminal of the Single Phasing Preventer.
Under Normal Condition:
In normal condition the preventer act as a closed switch therefor the contactor is connected to 230V supply and holds the circuits closed so the motor will be run.
Click on the image to enlarge.
Under Fault Condition:
When the 'R' phase gets disconnected or single phasing occurred the preventer sens the fault and act as an opened switch therefor the Contactor not get supply and it breaks the circuits so the motor will be stopped.
Click on the image to enlarge.
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Mr. Muhammad Saqib, M.Sc. (PAK), Research Fellow (PAK), a member of IEEE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from University Of Engineering And Technology Taxila, PAKISTAN. 
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