Control circuit for fixed power phase sequence

Installing a three-phase power supply phase sequence protection circuit in the electrical circuit of a motor powered by a three-phase 380V, 50Hz power supply can prevent
mechanical damage or production accidents caused by the motor running in the opposite direction when the power supply phase sequence is connected incorrectly. ．
The motor control circuit with fixed three-phase power supply phase sequence is shown in Figure 4-44.
    The three-phase power supply Ll phase and L2 phase are connected with resistor RI
and capacitor c1 respectively. The common terminal is 0. The L3 phase is connected to
a series circuit of resistor Rz and bridge rectifier uc, and then also intersects at the common terminal
o. The DC output voltage of the bridge rectifier uc
is supplied to the coil of the relay KA through the voltage stabilizing tube VS, and the capacitor C2 plays
the role of smoothing the DC voltage on the coil. The varistor RV
does not conduct under normal circumstances. When an overvoltage occurs at the output end of the bridge rectifier UC
. RV conducts and absorbs over-voltage to protect the voltage regulator tube
VS, relay KA and capacitor C2 from damage, thereby ensuring that the relay
KA does not malfunction.
    When the three-phase power supplies L1, L2, and L3 are in forward phase sequence,
the voltage on the relay KA is about 23V, and KA is electrically closed, and its moving platform contact is closed, and the control circuit power is turned on, and the motor can be started.
    When the three-phase power supply L1, 12 and L3 are connected in wrong phase sequence, the voltage on the relay KA is about 5v and cannot be electrically closed. Its moving and closing contacts
remain open, and the control circuit power cannot be connected and the motor cannot be started.
    This phase sequence protection circuit also has phase failure protection function. When the L3 phase is disconnected, the bridge rectifier uc will have no input AC voltage, and will have a DC
voltage output, and the relay KA will have no power. Its moving and closing contacts remain open.