Cause analysis and treatment of motor tripping caused by power supply voltage reduction

1, the fault phenomenon

In September, 2014, during the external circulation discharging operation of a fly ash steel plate warehouse project, the dust collector at the top of the warehouse often tripped, which caused the dust collector at the top of the warehouse to fail to work. Feedback from field staff is as follows.

(1) The dust collector occasionally trips due to the fault of the dust collector motor during the starting process.

(2) The external circulation of materials in the steel plate warehouse runs for about 1 ~ 2h, and the dust collecting motor trips due to fault.

(3) When the dust collecting motor trips due to fault, the action current of the motor protector is 40A.

(4) The on-site dust collector is PPCS32-6, and its nameplate main data are as follows: the centrifugal fan is 9-268D, with a flow rate of 8,792 ~ 11,320 m3/h and a total pressure of 3,834 ~ 3,638 Pa; The dust collecting motor is Y2180m-4, with a rated power of 18.5kW and a rated current of 36A.

2 root cause search and data recording

According to the feedback from the site, our company immediately sent relevant professionals to the site to find out the cause of the failure from the following aspects.

2.1 maintenance part inspection

(1) Whether the installation of coupling between motor and reducer meets the standard;

(2) Turn the rotor of the fan, and whether the fan has scraping friction;

(3) Whether the oil level of reducer bearing is normal;

(4) Whether the bag of dust collector is damaged;

(5) Whether the parameters of the arrived equipment are consistent with the design parameters.

2.2 electrical part inspection

(1) Check whether the insulation of cable and motor meets the requirements with insulation resistance meter;

(2) Check whether the cable connection is firm and whether there is any bad contact;

(3) Check the parameter settings of the motor protector.

2.3 Record relevant operation data.

After inspection by the equipment engineer, there is no problem in the maintenance part, and there is no problem in the electrical part, such as cable and motor insulation, cable wiring, etc. In view of the occasional fault tripping of the dust collector during the starting process, in order to make the dust collector start smoothly and record the operation data, the action current of the motor protector is changed from the original 36A to 40A (i.e. 1.1 times the rated current of the motor). Record the running data of the dust collector as follows:

(1) when the equipment is not running, the incoming voltage of power supply: AB phase voltage is 399V, AC phase voltage is 397V, BC phase voltage is 398V.

(2) Data of dust collector running for 4 hours without load: A phase current 34．1A, B phase current 34.6A, C phase current 33.9A; AB phase voltage 388 V, AC phase voltage 386V, BC phase voltage 387V; The higher temperature of the motor body is 73.2℃, and the higher temperature of the motor bearing is 70℃.

(3) Data of the dust collector running for 90 min when the steel plate is discharged from the warehouse in circulation: A phase current 40．2A, B phase current 39.5A, C phase current 39.8A; AB phase voltage is 354V, AC phase voltage is 351V, BC phase voltage is 356V; The higher temperature of the motor body is 81.4℃, and the higher temperature of the motor bearing is 77℃.

3 Cause analysis

Through the above data analysis and pincer-like meter test, it is found that when the steel plate is discharged outside the warehouse, the voltage of the three-phase power supply drops from about 398V when the equipment is not running to about 354V when the equipment is running with load, while the current and temperature of the dust collecting motor are slightly higher than those when the equipment is running without load. According to GB 50052—2009 Code for Design of Power Supply and Distribution System, under normal operation, the allowable value of voltage deviation at motor terminals is 5% of the rated voltage of the motor. It can be seen from the above that the actual running voltage of the field dust collecting motor is far lower than the rated voltage of the motor, and the voltage deviation is about-11%, which can no longer meet the requirements of GB 50052—2009 that the allowable voltage deviation is 5% of the rated voltage of the motor. According to the power calculation formula P= √3UIcosφ, a 354V decrease in power supply voltage will directly lead to a 40A increase in motor current. At this time, the actual current of the motor is higher than the set value 36A of the motor protector, so the overcurrent protection of the motor protector trips. Note: During the external circulation of materials in the steel plate warehouse, the dust collecting motor is controlled by motor protector, and other equipment is controlled by frequency converter.

After inspection, it is found that the reasons for the low power supply voltage of the dust collecting motor are as follows.

(1) The incoming power supply of the electrical room of the steel plate warehouse adopts temporary power supply, and the distance from the source point of the power supply to the electrical room of the steel plate warehouse is about 500m.

(2) If only a single equipment works, the power supply provided by the electrical room of the steel plate warehouse meets the requirements of equipment power consumption. However, there are one 18．5kW dust-collecting motor, one 75kW Roots blower and two 90kW Roots blowers that are involved in the operation during the circular discharging outside the steel plate warehouse. At this time, the power supply provided by the electrical room of the steel plate warehouse can no longer meet the requirements of equipment electricity consumption.

(3) This project is a construction in progress, and there are other temporary electrical equipment on the site to take power from the electrical room of the steel plate warehouse. It is more likely that the temporary electrical equipment will work simultaneously with the external circulation of materials.

4 countermeasures and effects

Replace the source point of the main incoming power supply in the electrical room of the steel plate warehouse as soon as possible. Before the new power supply is connected, it is forbidden to operate the electrical equipment at the same time.

In November 2014, the newly-built transformer room of this project was officially put into use. The power supply of the electrical room of the steel plate warehouse is replaced by the grinding electrical room, and the distance from the grinding electrical room to the electrical room of the steel plate warehouse is about 60m. After the formal power supply is connected to the electrical room of the steel plate warehouse by using the cable with the same model and specification as the temporary power supply cable, the power supply voltage of the steel plate warehouse is stable between 390 ~ 399 V during the circulating discharge, and the field equipment runs normally, and the dust collector motor has no overcurrent tripping phenomenon.