Design and manufacture of three-phase motor protection circuit

The working principle of this protection circuit is as follows:

In the attached figure, the operational amplifier ICLM324 is connected as a voltage comparator. The current transformers TA1, TA2, and TA3 are used to detect the magnitude of the motor's operating current.

When the motor is missing a phase, the current in two phases will increase a lot (depending on the load). The induced current in the corresponding transformer secondary winding increases and is rectified by D1-D3.

After filtering by C1~C3, when the potential of two points A, B, and C is higher than that of point D, that is, the op amp in-phase input terminal is higher than the inverting input terminal, at this time, op amps a, b, and c have two op amp comparators outputting high level, and then point E becomes high level after passing through the OR gate composed of D4-D6, and after delay and current limiting by R1~R4, c5, and C4, transistors V1 and V2 are turned on, and relay K is powered on, so that the contactor or subsequent alarm device is activated to achieve the protection purpose. Among them, R1-R4 and C4, c5 also have a delay and avoidance effect on the startup current of the power on? The delay length can be set according to the RC time constant.

debug

The three phase lines of the motor pass through the cores of the transformers TA1, TA2, and TA3 respectively, and the number of turns of the transformer is determined according to the power of the protected motor (the author uses two 370W motors in parallel and no-load for debugging), and then install a single-connection switch at the motor's incoming line for phase loss experiment (no-load phase loss... generally will not burn the motor), and use the wire to pass through the transformer core for 3 turns. First adjust VR4 to the maximum position, start the motor, adjust VR1, VR2, and VR3 to make the potentials of A, B, and C equal and maximum. Then adjust VR4 to make the potential of point D slightly higher than that of points A, B, and C. Then disconnect the incoming line switch of the motor to make it run in phase loss. After a few seconds, if relay K acts, the production is successful.

Note: One end of the transformer secondary winding must be grounded or zeroed to ensure safety. R6 is set to protect the set voltage from being too high to avoid damaging the IC and cannot be cancelled.