Improvement of a control circuit for step-down starting of autotransformer

Improvement of a control circuit for step-down starting of autotransformer

Autotransformer step-down starting, also known as compensator step-down starting, can use taps to adjust the transformation ratio of the autotransformer to change the starting current and starting torque. Traditional autotransformer starting is mostly controlled by adding time relays. The following is an improvement based on the shortcomings of the autotransformer step-down starting control circuit in a certain intermediate electrician training guide. Since the improved control circuit was put into use, it has been running stably and reliably without any failure.

1 Original action principle

The control principle of the original circuit is shown in Figure 1

The original intention of the control circuit is that when the start button SB2 is pressed, the coils of the AC contactors 1KM and 2KM are energized, the contacts 1KM and 2KM are closed, and the autotransformer is connected in series to the motor for voltage reduction starting; at the same time, the coil of the time relay KT is also energized, the contact of KT is delayed, the normally closed contact of KT is delayed and disconnected first, the coils of 1KM, 2KM and KT lose power successively, the main contacts of 1KM and 2KM are disconnected, the transformer is disconnected from the motor circuit, and the normally open contact of KT is closed later, the normally closed contact of 1KM is closed, the coil of 3KM is energized after the loss of power of 1KM and 2KM, the main contact of 3KM is closed, and the motor enters full-voltage operation. Then press the stop button to stop the motor. With this control circuit, the "start-automatic delay-operation" of the motor is completed in one operation, which is very convenient and safe. However, during formal operation, this phenomenon will occur: when the wiring is completely correct, the circuit can sometimes operate normally, and sometimes cannot operate normally. That is, after pressing the start button SB2, the motor starts at reduced voltage, and when it switches to full voltage operation, it stops and the 3KM coil cannot be energized.

2. The drawbacks of the line - the phenomenon of competitive risk-taking

The disadvantage of the control circuit in Figure 1 is that it encounters the problem of "contact competition" between electromagnetic components, that is, competition and risk occur, which makes the whole circuit unreliable. During the operation of the circuit, when the KT delay is reached, its delayed normally closed contact is always disconnected first due to mechanical movement and then closed after the delayed normally open contact. When the delayed normally closed contact is disconnected first, the 1KM coil is immediately de-energized, and the 1KM1 normally closed contact is closed to prepare for the 3KM coil to be energized. At the same time, the 1KMr normally open contact is disconnected, and the KT coil is immediately de-energized. Since the magnetic field cannot suddenly change to zero and the armature reset requires time, sometimes the delayed normally open contact has time to close, and then the 3KM coil can be energized, and the 3KM normally open contact closes and self-locks, and the motor enters full-voltage operation. But sometimes it loses control due to some interference, the KT delayed normally open contact does not have time to close, the KT magnetic field has disappeared and the armature has reset, the 3KM coil cannot be energized, resulting in the fault problem mentioned above. The main reason for the competition hazard caused by this circuit is that only the logical connection between the electromagnetic system and the contact system is considered during the design process, while the influence of the contact system action time and hysteresis on the system is ignored, resulting in competition hazard.

3 Improved wiring method

After analysis, it was found that the main reason for the imperfect circuit design was the unreasonable use of auxiliary contacts in the control circuit. Aiming at the shortcomings of this circuit, the original control circuit part was improved. The wiring method is shown in Figure 2.

4 Improved working principle

After the power is turned on, press the start button SB2, the AC contactor 1KM, 2KM coils are energized, the 1KM and 2KM main contacts are closed, and the autotransformer is connected in series with the motor to start at reduced voltage; at the same time, the time relay KT coil is also energized, and the KT instantaneous normally open contact closes and self-locks. After a certain time delay, the KT delayed normally open contact closes, the KT delayed normally closed contact opens, the 1KM coil is de-energized, the 1KM1 normally closed contact closes, the 3KM coil is energized, the 3KM1 normally open contact closes and self-locks, and the 3KM1 normally closed contact opens and interlocks, so that the 2KM and KT coils are de-energized and reset, and the motor is powered on.