Circuit diagram I of voltage reducing start control of autotransformer

Operation principle of the voltage reducing start control circuit of the autotransformer:


Close the switch, press the start button SB2, the KM1 coil is powered on and self-protection, the autotransformer t is connected, the stator winding is powered by the autotransformer and started by decompression; at the same time, the KT coil is powered on and self-protection, and the timing starts. When the KT set delay time is over, the closing contact of the energizing delay is closed, which makes the coil of Ka electrify and absorb and self protect, KM1 power off and release, and its main contact is open; km2 coil electrify and absorb, its main contact is closed, the autotransformer is cut off, and the motor operates at full voltage.

Circuit diagram II of voltage reducing start control of autotransformer;

Starting control circuit diagram of autotransformer

As shown in the right figure, it is the control circuit diagram of qj3 manual control compensator step-down start. Its working principle is as follows:

When the handle is pulled to the “stop” position, the moving contact installed on the main shaft does not contact with the two rows of static contacts, and the motor is in the state of power failure and stop; when the handle is pushed forward to the “start” position, the moving contact contacts with the upper row of starting contacts, and the three-phase power supplies, L1, L2, L3, are connected to the autotransformer through the three moving and static contacts on the right, and then through the three 65% of the autotransformer The tap (or 80%) is connected to the motor for step-down starting; the left two contacts of the dynamic and static diagram connect the autotransformer into a star.

When the rotation speed of the motor rises to a certain value, the handle is quickly pulled back to the “operation” position, so that the three moving contacts on the right side contact with the three running static contacts in the lower row. At this time, the autotransformer is separated, and the motor is directly connected to the phase power L1, L2, L3 for full voltage operation. When stopping, as long as the stop button sb is pressed, the kV coil of undervoltage release will lose power, the armature will fall and release, the compensator will be tripped through the mechanical operation mechanism, the handle will return to the “stop” position, and the motor will be shut down.

From the above right figure, we can see that the moving contact of the thermal relay fr, the undervoltage tripper coil kV, and the stop button SB are connected in series to the two-phase power supply, so when the power supply voltage is insufficient, the power is cut off suddenly, the motor is overloaded and stopped, the compensator can be tripped, and the motor can be cut off and stopped.

Circuit diagram III of voltage reducing start control of autotransformer:

As shown in the figure is the control circuit diagram of three-phase motor step-down start of button and contactor control compensator. The working principle of the line is as follows: turn on the power switch QS first:

Starting control circuit diagram of autotransformer

Step down start: press button SB1 → SB1 moving contact first opens to km2 interlock, SB1 moving contact then closes → KM1 coil is powered on → KM1 interlocking contact opens to km2 interlock, KM1 self-locking contact closes to self lock, KM1 main contact closes → motor M is connected to TM step-down start.

Full voltage operation: when the motor speed rises to a certain value, press SB2 → SB2 to make contact and then close, SB2 to break contact first → KM1 coil is powered on → KM1 self-locking contact breaking contact self-locking, KM1 interlocking contact closing, KM1 main contact breaking, TM cutting → km2 coil is powered on → km2 self-locking contact self-locking, km2 main contact closing, km2 interlocking contact breaking to KM1 interlocking, km2 dynamic contact breaking Disconnect and release the star connection of TM → motor M operates at full voltage. To stop, press Sb3.

The disadvantage of this circuit is that if the operator presses SB2 directly in error during start-up, it will cause the motor to start directly. To overcome its shortcomings, the control circuit shown in the right figure can be used. The control circuit has the following advantages:

During startup, if the operator presses SB2 by mistake directly, the coil of contactor KM3 will not be powered up, so as to avoid full voltage startup of motor.

Since the normally open contact of the contactor KM1 is connected in series with the coil of km2, when the step-down starting is completed, the contactors KM1 and km2 lose power. Even if the contact cannot be closed due to the fault of the contactor KM3, the motor will not operate under low voltage.

The closing time of contactor KM3 is ahead of the release time of contactor km2, so there will be no gap outage of motor during startup, and no second startup current.

The disadvantage of the line is that it needs to press the button twice from the step-down start to the full pressure operation, so it is inconvenient to operate and the interval time can not be accurately grasped.

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