When a three-phase power supply experiences phase failure, the system automatically disconnects the power to protect the motor from damage. This phase failure protection mechanism is illustrated in Figure 5-11. The circuit incorporates a zero-sequence voltage-based protection method into a standard self-locking control circuit.
Figure 5-11: Schematic of the Voltage Loss Protection Circuit
When the power switch Q is turned on and the start button SBT is pressed, the contactor KM is energized and latched in place. This closes the main contacts, allowing the motor to operate normally. However, if one of the phases fails, the voltage across the motor’s three-phase windings becomes unbalanced. This imbalance creates a zero-sequence voltage that is transmitted to the neutral line, triggering the relay KA to activate.
Once the relay KA is activated, its normally closed contact—connected in series with the contactor KM—opens, cutting off the power to the contactor. As a result, the motor stops immediately, preventing potential damage caused by phase loss.
It's important to note that for optimal sensitivity, relays with different voltage ratings should be used depending on the motor being protected. This ensures that the system responds effectively to phase failures under various operating conditions.
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