Diagnosis and elimination of start-up faults in rotary drilling RIGS

I. Fault Phenomenon: A XR200 rotary drilling rig of a certain company experienced a start-up failure. During on-site inspection, it was found that when the key switch was set to the start position, the start motor did not act.

Ii. Basic Principles of the Starting Circuit

When starting the vehicle, rotate the key switch S2 to position II (start position). The start relay Kl is energized, the normally open contact of K1 closes, and the connection between terminal 50 of the start motor and the positive terminal of the battery is connected. The pull-in coil of the start motor is connected, and the start motor begins to rotate, with the battery providing the starting power. At this point, release your hand to automatically return the key switch to position I (power position), K1 opens, the power relay K2 is energized, and the normally open contact of K2 closes. The 24V DC power generated by the generator supplies energy to the entire vehicle's power system and simultaneously charges the battery.

Iii. Fault Cause Analysis and Elimination: Based on the principle analysis, it is believed that when the vehicle starts, if the starting motor does not act, there are usually the following reasons: 1. The battery is severely discharged or internally sulfated; 2. The key switch or starting relay is damaged; 3. Poor contact between the battery and the starter motor, such as loose connection or oxidation of the terminals; 4. The pull-in coil for starting the motor is burned out.

Given that such faults have occurred many times before, it is suspected that it might be due to: 1. The matching issue of the starter motor selection, and whether there is a phenomenon of a small horse pulling a big cart; 2. Are there any unreasonable aspects in the circuit design that cause repeated burnout? 3. The same batch of components has quality issues, resulting in the recurrence of similar faults. After checking the drawings and technical data and consulting the engine manufacturer, it was confirmed that the motor selection and matching were correct, and the electrical principle was also confirmed to be correct. After further inspection, it was found that the battery and the key switch were fine. Finally, it was determined that the pull-in coil of the starter motor was burned out. What causes the pull-in coil of the starter motor to be burned out? Based on the comprehensive judgment of previous phenomena such as the burning out of the starting relay, it is believed that the possibility of a problem with the starting relay is the greatest. However, this type of relay is an imported component. According to its specification parameters, it fully meets the usage requirements of the vehicle. Moreover, it has been stably used on a certain model of pump truck without any issues. We have no choice but to troubleshoot first and put this issue aside for now.

After replacing the pull-in coil of the starter motor, the vehicle was restarted and started successfully. But at the same time, it was found that the terminals of the starter motor were getting redder and redder. Upon inspection, the normally open contact of the starting relay Kl cannot automatically disengage after the engine starts, that is, when the key switch returns to position I. Due to the very low resistance of the motor's pull-in coil, the current passing through the coil during startup is relatively large (generally 30 to 40A). As the normally open contact of K1 cannot be disengaged in time, the pull-in coil heats up severely for a long time, eventually burning out the coil. To prevent the motor suction coil from being burned out again, turn off the vehicle in time first. After replacing the starting relay from another manufacturer and conducting a starting test, the engine started and stopped normally. However, upon observation, it was found that there was still a relatively bright red dot at the terminal post.

The machine was shut down and the power cord was removed for inspection. It was found that the power cord and its copper connector were not firmly crimped. After inquiry, it was learned that since the electrician did not have a dedicated crimping pliers, a bench vise could only be used as the crimping tool. However, the teeth of the bench vise were flat, which led to the problem of not being firmly crimped. It was re-compacted with a wire crimping pliers and then installed on the vehicle. The engine started normally. To further test whether the original starting relay had any problems, the original relay was used for starting several times. It was found that once the engine started, the relay contact could not disengage. Eventually, it was determined that there was a quality issue with the starting relay. The starting relay was replaced. After dozens of tests, the engine started normally, thus eliminating this fault.

Iv. Conclusion: The main cause of the start-up failure of this rotary drilling rig is the quality issue of the start-up relay, which led to the long-term heating and damage of the start-up motor's suction coil. The improper connection of the power cord caused a sudden change in local resistance, accelerating the occurrence of the fault. The reason why there was no problem with using this type of relay on the pump truck is that the starting current of the pump truck is smaller than that of the drilling rig. If the current reaches the upper limit of its rated value, similar problems may occur. After resolving the issue with this vehicle, the starting relays of the sold drilling RIGS and those that have not yet been manufactured will be replaced subsequently. It has been a year since this modification, and the same malfunction has not occurred again.