Selection of three-phase asynchronous motors

Three phase asynchronous motors are widely used and are a major power source. Here, it is particularly important to emphasize the reasonable selection of the rated power of the motor. If the rated power is chosen too large, it not only increases equipment investment costs, but also causes the motor to operate at a low efficiency and low power factor point for a long time, which is very unreasonable and uneconomical.

1. Key points for selecting three-phase asynchronous motors

(1) Reasonably select the type of electric motor based on comprehensive indicators such as mechanical load characteristics, production processes, power grid requirements, construction costs, and operating costs. (2) Reasonably select the power of the electric motor based on the overload capacity, starting torque, working mode, and operating conditions required by the mechanical load, so as to match the power reasonably and have appropriate backup power, striving for safe, reliable, and economical operation. (3) Choose the protection level and structural form of the electric motor according to the environment of the usage site. (4) Select the speed of the electric motor based on the maximum mechanical speed of the production machinery and the requirements of the transmission speed control system. (5) Select the insulation level and installation method of the motor according to the requirements of the operating environment temperature, easy maintenance and inspection, safety and reliability. (6) Select the rated voltage and frequency of the motor based on the grid voltage and frequency.

2. The selection steps of three-phase asynchronous motor: select motor type → select motor capacity → verify the maximum starting torque → verify the equivalent heat generation → verify the comprehensive economic indicators → compare the mechanical and load characteristics of the motor → determine the voltage level and frequency of the motor → compare the mechanical and load characteristics of the motor → determine the voltage level and frequency of the motor → determine

3. Preparation and inspection for maintenance and start-up of three-phase asynchronous motors

(1) Check whether the grounding of the motor and starting equipment is reliable and complete, and whether the wiring is correct and good. (2) Check if the rated voltage and frequency indicated on the motor nameplate match the power supply voltage and frequency. (3) Newly installed or long-term unused motors (unused for more than three months) should have their insulation resistance values relative to phase and ground checked before starting. (Measured with a 1000 volt megohmmeter). The insulation resistance should be greater than 0.5 megohms. If it is lower than this value, the winding should be dried. (4) For wound rotors, it is necessary to check whether the electric brushes on the collector ring and the brush lifting device are working properly, and whether the pressure of the electric brushes meets the requirements. The brush pressure is 1.5N/cm-2.5N/cm. (5) Check that the rotor of the electric motor rotates flexibly and reliably, and that the oil level in the sliding bearing reaches the specified level. (6) Check if the rated current of the fuse used in the motor meets the requirements. (7) Check whether the fastening bolts and installation bolts of the motor are firm and meet the requirements.

4. Troubleshooting during operation

(1) Fault during startup: When the circuit breaker or automatic switch is closed, the motor does not rotate, only a buzzing sound is heard, or it cannot be turned to full speed. The cause of this fault may be: one phase of the stator circuit is disconnected, such as the low-voltage motor fuse melting one phase, or the high-voltage motor short circuit and isolation switch have poor contact with one phase, which cannot form a three-phase rotating magnetic field. The disconnection or poor contact of the rotor circuit causes no current or a decrease in current in the rotor winding, resulting in the motor not rotating or rotating very slowly. In transmission machinery, there is a mechanical jamming phenomenon, and in severe cases, the motor does not rotate and produces abnormal noise. Low voltage reduces the torque of the motor, making it difficult or impossible to start. The friction between the stator and rotor iron core of the electric motor increases the load, making it difficult to rotate. When the operating personnel discover the above-mentioned faults, for high-voltage motors, they should immediately open the circuit breaker and isolation switch of the motor, and check its stator and rotor circuits. (2) Stator winding single-phase grounding fault. The insulation level of the motor winding is reduced due to erosion caused by various factors. In addition, due to long-term overload operation of the motor, the insulation of the winding will become brittle or fall off due to long-term overheating. This will cause single-phase grounding of the motor stator winding. (3) Faults in single-phase operation of three-phase motors: During operation, if one phase fuse is burned out or has poor contact, the isolating switch, fuse, cable head, and wire are loose in contact, and the stator winding is disconnected in one phase, it will cause single-phase operation of the motor. When the operator confirms that the motor is operating in single-phase mode based on the abnormal phenomena generated by the motor, the power supply should be cut off to stop it from running. Measure the resistance value of the stator circuit with a megohmmeter. If the resistance value is very large or infinite, it indicates that the phase is disconnected. Then check whether the fuses, circuit breakers, isolation switches, cable heads, and wiring connections in the stator circuit are in good contact.