The single-phase disconnection relay with low high voltage and imbalance, described here, guarantees complete safety for three-phase machinery.Generally the induction motor burns due to excessive voltage during shutdown time, very low voltage during peak hours, uneven phase voltage and single phasing (absence of a fully phase three phase supply). Therefore, it is essential to install a single phase preventor, in addition to the starter. Starter only provide protection against overloads and, therefore, can not prevent the motor from burning under the aforementioned supply voltage conditions. But this protector protects the motor by stopping it automatically under such conditions.
Circuit and Single Phase Prevention Work
For simplicity, this circuit can be divided into three parts, namely power supply, unbalanced sensor and cut-off relay.
As shown in Figure 1, the primary of transformer X1 is connected to a 400V power line. The 12 V AC current induced in the transformer secondary is rectified by the bridge rectifier comprising the diodes D1 to D4 and the smoothing capacitor C1. The capacitor C1 is deliberately chosen to be of low value to obtain misregulated rectified voltage to "detect" the variation in line voltage from the specified voltage level.