Cause Analysis and Treatment of Motor Trip Caused by Power Supply Voltage Decrease

1 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, causing the dust collector at the top of the warehouse to fail to work. Field staff feedback is as follows.

(1) The dust collector occasionally trips 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 collection motor trips.

(3) When the dust collection motor is tripped due to fault, the operating current displayed by the motor protector is 40A.

(4) The on-site dust collector is of PPCS32-6 type, and the main nameplate data are as follows: the centrifugal fan is of 9-26 8D type, with a flow rate of 8792~11320 m3/h and a full pressure of 3834~3638 Pa; The dust collection motor is Y 2 180 M-4 type with rated power of 18.5kW and rated current of 36A A.

2 Root cause search and data recording

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

2.1 maintenance part inspection

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

(2) If the rotor of the fan is moved, whether the fan is scratched and rubbed;

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

(4) Whether the dust collector bag is damaged;

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

2.2 electrical inspection

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

(2) Check whether the cable wiring is firm and whether there is any poor contact;

(3) Check the parameter setting 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 of the cable and motor insulation, cable wiring, etc. In view of the occasional fault trip of the dust collector during the starting process, in order to make the dust collector start smoothly and record the operation data, the operating current of the motor protector is changed from 36A to 40A (I .e. 1.1 times the rated current of the motor). Record data during dust collector operation as follows:

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

(2) Data of 4h no-load operation of dust collector: phase A current 34.1A, phase B current 34.6A, and phase C current 33.9A;AB phase voltage 388V,AC phase voltage 386V,BC phase voltage 387V; The maximum temperature of the motor body is 73.2 ℃, and the maximum temperature of the motor bearing is 70 ℃.

(3) Data of dust collector running for 90 min during circulating discharging outside the steel plate warehouse: phase A current 40.2A, phase B current 39.5A, and phase C current 39.8A;AB phase voltage 354V,AC phase voltage 351V,BC phase voltage 356V; The maximum temperature of the motor body is 81.4 ℃, and the maximum temperature of the motor bearing is 77 ℃.

3 Cause analysis

Through the above data analysis and clamp meter test, it is found that when the steel plate warehouse is out of circulation, the three-phase power supply voltage drops from about 398V no-load voltage when the equipment is not running to about 354V when the equipment is running, and the current and temperature of the dust collecting motor are slightly higher than those when the equipment is running no-load. According to GB 50052-2009 Code for Design of Power Supply and Distribution System, under normal operation, the allowable voltage deviation at the motor terminal is ± 5% of the rated voltage of the motor. From the above, it can be seen that the actual operating voltage of the on-site dust collection motor is far lower than the rated voltage of the motor, and the voltage deviation is about-11%, which cannot meet the requirement of 5% of the rated voltage of the motor in GB 50052-2009. According to the power calculation formula P = √ 3UIcosφ, the reduction of the power supply voltage to 354V will directly cause the motor current to increase to about 40A. Since the actual current of the motor at this time is higher than the set value 36A of the motor protector, the overcurrent protection action of the motor protector trips. Note: When the material of the steel plate warehouse is circulated, the dust collecting motor is controlled by the motor protector, and the other equipment is controlled by the frequency converter.

After inspection, it is found that the reasons for the low power supply voltage of the dust collector 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 provided by the electrical room of the steel plate warehouse meets the equipment power requirements. However, the equipment involved in the operation of the external circulation discharging of the steel plate warehouse includes 1 18.5kW dust collection motor, 1 75kW Roots blower and 2 90kW Roots blowers. At this time, the power supply provided by the electrical room of the steel plate warehouse cannot meet the power consumption requirements of the equipment.

(3) This project is a construction in progress. There are other temporary electrical equipment on site to take power from the electrical room of the steel plate warehouse. It is more likely that the temporary electrical equipment and the material external circulation work at the same time.

4 Response measures and effects

Replace the source point of the total incoming line 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 work at the same time.

In November 2014, the newly built transformer and distribution room of this project was officially put into use. The power supply of the electrical room of the steel plate warehouse is changed to be introduced from 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 of the same model and specification as the temporary power supply cable, the power supply voltage during circulating discharge outside the steel plate warehouse is stable between 390~399V, the field equipment operates normally, and the dust collector motor does not have overcurrent trip phenomenon.

Source: Electric World

Author: Du Pu's Wang Qinghua Guo Xiangquan