When talking about the current motor starting control device, I believe that the first thing that the majority of electricians think of is the frequency converter and the Y – △ step-down starting circuit with the highest appearance rate as the subject of electrician grade examination. In fact, between the two, there is a widely used device dedicated to motor starting control – soft starter, which is often forgotten by everyone.

The soft starter uses the phase-shifting trigger signal to control the conduction of the bidirectional thyristor (in practice, two unidirectional thyristors are reversed and equivalent to a bidirectional thyristor), so as to achieve the function of gradually regulating the voltage and starting the motor, but does not have the frequency modulation function of the frequency converter. Because the soft starter has the advantages of relatively simple structure, simple operation and moderate price, it occupies a certain market share in some fields that only need to control the motor starting process. Over the years of working experience, the author has handled more than ten failures of six categories of soft starters of various brands and models. Now three maintenance examples are selected to share with you.

1、 A 55kW soft starter of Huifeng (now eurui) brand used by a unit failed to start due to leakage fault from time to time during the starting process. Since the fault of the machine occurred during startup, it can be preliminarily eliminated that the six channel phase-shifting signal of the main board is wrongly transmitted, which is likely to be caused by the deterioration of the insulation performance of a thyristor to the ground. After detection by the pressure tester, it is found that the insulation resistance of one a pole of the V-phase two reverse unidirectional thyristor to the heat dissipation base plate is abnormal! After replacing the thyristor, the fault was eliminated.

2、 A 100kW Chint brand soft starter has overtemperature fault during use after two years of operation. After eliminating the suspicion that the ventilation and cooling holes are blocked (the model is not equipped with a cooling fan), The author points the spearhead at the bypass contactor configured inside the soft starter (in order to prevent the thyristor from always running on power after the start-up process, resulting in damage to the thyristor, a considerable number of soft starters are equipped with bypass thyristor contactors in practice). After detection, the author found that although the control bypass contactor pulled in the relay (DC24V) on the main board, its normally open contact closed in a high resistance state! Therefore, the author infers that the fault reason is that after long-time use, the normally open contact of the relay has ablation fault, resulting in the failure of normal closing of the bypass contactor, resulting in large current flow through the thyristor for a long time!

3、 The miscellaneous 45KW soft starter used for the tap water pump in a local village suddenly had a serious imbalance fault of three-phase current (very similar to phase loss fault). After eliminating the suspicion of water pump and load cable, the author naturally detected the thyristor and six phase-shifting signals, but the results were normal, resulting in the stagnation of maintenance work for a time. After the reverse drawing of the line, the author adjusts the starting point to lock the fault on the isolated optocoupler of the intermediate line of the phase-shifting signal transmission to the thyristor. After testing, the author found that the output conduction internal resistance of a w-phase optocoupler is significantly larger than that of the other five, indicating that its conduction capacity is significantly worse, resulting in great attenuation of the signal transmitted to the thyristor, and the thyristor can not be controlled normally, resulting in failure! After the optocoupler is replaced, the fault that has plagued the author for a long time can be eliminated.

Leave a Reply

Your email address will not be published. Required fields are marked *