In modern industrial production, electric motors are widely used. Various types, voltage forms and voltage levels of electric motors emerge in endlessly. The following is a brief explanation of the reasons for single-phase operation and preventive measures.
Classification of motors
Electric motors can be divided into DC motors, asynchronous motors and synchronous motors according to different structures and working principles. Synchronous motors can also be divided into permanent magnet synchronous motors, reluctance synchronous motors and hysteresis synchronous motors. Asynchronous motors can be divided into induction motors and AC commutator motors. Induction motors are further divided into three-phase asynchronous motors, single-phase asynchronous motors and shaded pole asynchronous motors. AC commutator motors are further divided into single-phase series motors, AC and DC dual-purpose motors and repulsion motors.
Hazards caused by single-phase operation of three-phase asynchronous motors
Three-phase asynchronous motors have two wiring methods: Y-type and Δ-type. When a Y-connected motor operates in a single phase, the current in the disconnected phase is zero. The phase currents of the other two phases become line currents. At the same time, it will cause zero point drift and its phase voltage will also increase.
When the motor with Δ-type wiring is disconnected internally, the motor changes into V-type wiring under the action of the three-phase power supply, and the two-phase current increases by 1.5 times. When the motor with Δ-type wiring is disconnected externally, it is equivalent to the two-phase windings being connected in series and the third group of windings being connected in parallel between the two-line voltages. The current in the two windings connected in series remains unchanged. The additional current of the third group will be increased by 1.5 times.
To sum up, when a motor operates in a single phase, its winding current increases rapidly, and the winding and metal casing heat up rapidly, burning the winding insulation and then burning the motor winding, affecting the normal production activities. If the on-site environment is not good, the surrounding environment will accumulate. There are flammable items that can easily cause fires and cause more serious consequences.
Causes of motor single-phase operation and preventive measures
1.When the motor cannot start, there is a buzzing sound, and the shell has a temperature rise or the speed decreases significantly during operation, and the temperature rise increases, the power supply should be cut off immediately and the cause of the failure should be carefully found. Determine whether the above condition is caused by the lack of phase.
2.When the power line of the main circuit is too thin or encounters external damage, the three-phase power supply of the motor will cause single-phase operation due to a phase burning or external force hitting. The safe carrying capacity of the main power line of the motor is 1.5 to 2.5 times the rated current of the motor, and the safe carrying capacity of the power line is closely related to the laying method of the power line. Especially when it is parallel or intersecting with the heat pipeline, the interval must be greater than 50cm. The safe carrying capacity of the power cord that can run for a long time at 70°C temperature rise can generally be checked by the electrician's manual. According to past experience, the safe carrying capacity of copper wires is 6A per square millimeter, and that of aluminum wires is 4A per square millimeter. In addition, copper-aluminum transition joints should be used when copper-aluminum wire joints, so as to avoid oxidation between copper-aluminum materials and affect the joint resistance.
3.Improper configuration of air switch or leakage protector may cause single-phase operation of motor. If the air switch configuration is too small, it may be due to the power supply current is too large to burn the internal contacts of the air switch, resulting in a phase contact resistance is too large, forming a single-phase motor operation. The rated current of the air switch should be 1.5 to 2.5 times the rated current of the motor. In addition, during the operation of the motor, it should be monitored that the air switch configuration is too small, or the quality of the air switch itself is problematic, and the appropriate air switch should be replaced.
4.The connection line between the components in the control cabinet is burned off, which may cause the motor to run in single phase. The reasons for the connection line burning are as follows:
① The connection line is too thin, when the motor overload current increases, it may burn the connection line. ② The connectors at both ends of the connection line are in poor contact, causing the connection line to overheat, thus burning the connection line. There are small animal damage, such as mice climbing between the two lines, causing a short circuit between the lines and burning off the connection line. The solution is: before starting each operation, the control cabinet should be opened to carefully check whether the color of each connection line has changed, and whether the insulation skin has burn marks. The power line is reasonably equipped according to the load current of the motor, and the connector is connected according to the process requirements.
Peroration
In the construction, we must strictly comply with the specifications of various construction processes to ensure the quality of installation. Regular maintenance of various equipment and regular inspection and repair during operation will certainly avoid unnecessary losses and hazards caused by single-phase operation of the motor.
Post time: May-30-2024