Competition and hazard issues in electrical control circuits

Electrical control If there are competition and risk problems in the circuit, it will directly endanger the safety of people and machines, cause major accidents, and have disastrous consequences. Therefore, it should be taken seriously in the design and application of electrical control circuits. In the design of electronic circuits, the problem of competition and risk is in an important position, because the existence of this problem is related to whether the system can work reliably. In addition, since the electrical control composed of strong electricity is generally more intuitive and easy to read, competition and risk problems rarely occur and are easily overlooked. If the design is not well considered or the control circuit wiring is wrong during production and maintenance, this problem will occur.

1 Circuit Analysis

After the soft starter starts the motor in bypass operation, the control principle diagram of the motor free stop and delayed stop is shown in Figure 1. The motor is started by the closure of the intermediate relay KA1; after starting and reaching speed, the intermediate relay KA2 is energized, the bypass contactor KM is energized, and the motor is bypassed; when SB1 is pressed, the KA2 and KM coils are de-energized, and the motor is free to stop; when SB2 is pressed, the time relay KT coil is energized, the normally open contact is instantaneously closed and self-maintained until the normally closed contact of KT's delayed disconnection is opened, the KA2 and KM coils are de-energized, and the motor stops. There is no problem with this circuit from the control principle analysis, but a careful observation will reveal that there is a problem with the delayed stop circuit. If the intermediate relay KA2 and the time relay KT are different types of products (such as one is an electromagnetic relay and the other is an electronic relay), due to the large difference in the coil action time, KA1 and KT have a time-competition problem in action, that is, when SB2 is pressed, KT is energized and self-retained, but when the normally closed contact of KT's delayed disconnection opens, KT's coil is de-energized, the self-locking contact opens, and KT's delayed disconnection normally closed contact will close instantaneously; during the time from the delayed opening to the instantaneous closing of KT's delayed disconnection normally closed contact, if KA2 is disconnected, the motor can stop; otherwise, the motor cannot be delayed to stop (because before KA2 is disconnected, KT's delayed disconnection normally closed contact is closed again, resulting in KA2 being unable to be de-energized). This delayed parking sometimes succeeds and sometimes fails, and the result is that the control circuit cannot work reliably, which is a typical competition and risk problem. If KT is an electronic time relay (such as ST3PC-D) and KA2 is an electromagnetic intermediate relay (such as JZC3-22d), then KT always completes the action first, and KA2 is released slowly due to the residual magnetism of the coil, causing KA2 to fail to reliably cut off the power. If KT and KA2 are changed to the same type of relay (such as KT in this example is changed to JSK4-224d), this problem can be solved.

Figure 1 Motor free stop and delayed stop control principle.

2 Improved design

Figure 2 is the control circuit diagram of the contact cabinet before improvement. In Figure 2, the manual and automatic control frame-type circuit breakers in the contact cabinet are eliminated. Due to the principle design defects, when the switch is in the automatic position, the undervoltage coil Q and the closing coil X are energized, and there is a competition and risk problem, which causes the circuit breaker to fail to close reliably. In order to highlight the main problem, other auxiliary circuits are omitted. When the energy storage motor has stored energy, if the 300# line is energized, the undervoltage coil Q is first attracted, and the automatic closing is successful, otherwise it fails, which is the reason why the automatic closing is sometimes successful and sometimes failed.

In order to close the circuit breaker reliably, a time delay relay BSJ is added to the line, as shown in Figure 3. When the 300# line is energized, the undervoltage coil Q is energized and the closing coil X is energized after the energized time delay relay BSJ is pulled in after a delay, and the circuit breaker is closed.

Figure 2: Control circuit diagram of the communication cabinet.

Figure 3 is the control circuit diagram of the contact cabinet after adding BSJ.

3 Conclusion

Competition and risk issues in electrical control loops are somewhat hidden and often encountered in the design of electrical control principles. Designers should carefully consider every detail of the control principle to avoid such situations. If there are doubts about the control principle of a new design, it is best to verify it through experimental means.

In short, competition and risk issues in electrical control circuits pose a great threat to human and machine safety and should be avoided as much as possible.