A single-phasing preventer is an electrical device used to protect three-phase electrical equipment and motors from damage in case one phase of the power supply is lost or fails to operate. In a three-phase power system, each phase has a voltage that is 120 degrees out of phase with the others, creating a balanced system. If one of the phases fails, it can lead to various issues, including equipment damage, motor failure, and unbalanced operation.
The working principle of a single-phasing preventer involves monitoring the voltage in all three phases and detecting any imbalance. When the voltage in one of the phases drops significantly or disappears altogether, the preventer takes action to protect the equipment. Here's a basic overview of how it works:
Voltage Sensing: The single-phasing preventer continuously monitors the voltages of all three phases of the power supply using voltage sensing circuits.
Reference Voltage: The device establishes a reference voltage based on the average voltage of all three phases. In a balanced system, the average voltage should be constant.
Comparison: The voltage of each phase is compared to the reference voltage. If the voltage in any phase drops below a certain threshold, typically around 70-80% of the reference voltage, it indicates a single-phasing condition.
Time Delay: To avoid false tripping due to momentary voltage fluctuations, the single-phasing preventer often incorporates a time delay. This delay allows the system to differentiate between a temporary voltage dip and a genuine single-phasing event.
Relay Operation: If the voltage in one phase remains below the threshold for the specified time delay, the preventer activates a relay. This relay is typically connected in series with the control circuit of the equipment or motor.
Motor Disconnect: The relay's operation results in the control circuit being opened, effectively disconnecting the motor or equipment from the power supply. This action prevents the motor from running under single-phase conditions, as it can lead to overloading and damage.
Visual/Alarm Indication: In some preventers, there may be additional features such as visual indicators or alarms to signal the single-phasing condition, making it easier for maintenance personnel to identify and rectify the issue.
It's important to note that the single-phasing preventer doesn't correct the voltage imbalance; rather, it provides protection by preventing the equipment from operating in such conditions until the power supply is restored to normal three-phase operation. This protection helps extend the life of the equipment and ensures the safety of the connected systems.