An HBT60A concrete pump stopped and restarted after pumping, the motor can not start, listening to sound seems to be insufficient power, while the star-connected motor power AC relay "flip-flop" keep jumping, the first card Suspected that the power supply voltage is less than or less caused. After the inspection, the three-phase voltage is very stable; when we exchange the new relay, the relay no longer beating, but the motor start sound is still abnormal; the motor power converted to a triangle connection, the motor starts sound back to normal, but the machine owner High pressure on the oil circuit, and pumping operation is normal, stop pumping, the main oil pressure is still high, indicating that the main pump is not unloading. From this speculation, the motor may start abnormal because the motor starts with a load, the fault is not on the electrical. When the motor is started normally, it is impossible for the main oil line to be pressured. If the hydraulic oil is not clean, the main oil-hydraulic valve is blocked, will lead to poor communication, causing the motor with a load to start. Then, the main oil solenoid valve and hydraulic reversing valve were removed and cleaned, when the hydraulic valve is opened, the valve cavity at both ends of the return spring has found that one end of the spring has broken, so that the valve core loss Balance, moving to the end of the broken spring, which will block the T-shaped valve, the hydraulic oil into the hydraulic valve can not flow back into the tank with the T-shaped cavity, and into the push cylinder hydraulic cylinder, resulting in the main The pump can not relieve pressure, causing the motor to start with a load. Put on a new spring, the motor starts normally, the pump is also working properly.