It is known that the current of a three-phase asynchronous motor will increase when it is stuck, but many people may not know the reason

We know that when a three-phase asynchronous motor is blocked, the current will become very large. If it is not stopped in time, it will cause the motor to burn out. But why does the current increase when the rotor is blocked?We all know this phenomenon, so what is the cause of this phenomenon?

Also, why is it called an asynchronous motor?Why does the larger the number of poles, the slower the rated speed?Why does the motor burn out when the phase is missing?With these questions, this article tries to analyze it.

First, let’s understand the working principle of three-phase asynchronous motor.

Three-phase asynchronous motor mainly consists of two parts: stator and rotor.The stator consists of a stator core and a stator winding. The stator winding is composed of three-phase symmetrically distributed windings with an equal number of turns and an electrical angle difference of 120° from each other.When the stator winding is connected to a three-phase symmetrical power supply, a three-phase symmetrical current flows through the winding and a rotating magnetic field is generated.

Magnetic effect of electric current: If a straight metal wire passes through a current, a circular magnetic field will be generated in the space around the wire.The greater the current flowing in the wire, the stronger the magnetic field produced.The magnetic field forms a circle around the wire.The direction of the magnetic field can be determined by Ampere's rule (right-hand spiral rule).Right-hand spiral rule: The right-hand spiral rule is a rule that determines the direction of magnetic field lines around a conductor.For a current-carrying straight wire, hold the straight wire with your right hand, with your thumb pointing in the direction of the current, and your four fingers pointing in the direction of the magnetic field lines. The magnetic field lines are circular annular currents. Hold the ring coil with your right hand, with your four fingers pointing in the direction of the current, and your thumb pointing in the direction of the magnetic field lines (N pole).Regarding the rotating magnetic field of the motor, you can imagine a circular marquee. Some winding currents lead and some winding currents lag, just like the marquee constantly changing the position of the light and thus rotating.

The rotor of the motor consists of a rotor core, a rotor winding and a rotating shaft.The rotor winding generally consists of a conductor buried in the rotor core slot and end rings at both ends to form a closed coil.Because the stator generates a rotating magnetic field, the rotor winding will cut the magnetic field to generate an induced electromotive force. The direction of the induced electromotive force is determined by the right-hand rule. Because the rotor is closed, an induced current will be generated in the rotor winding of the closed loop.At this time, the induced current in the rotor winding interacts with the rotating magnetic field to generate electromagnetic force on the rotor. The direction of the electromagnetic force can be determined by the left-hand rule.The direction of the electromagnetic force is consistent with the tangent direction of the rotor cross-section, thus forming an electromagnetic torque, which drives the rotor automatically in the direction of the rotating magnetic field, and the motor rotates.

The above is the working principle of the three-phase asynchronous motor to rotate. Knowing this principle, we can analyze the previously raised questions.

1. Why is it called asynchronous motor?

Because although the direction of rotation of the rotor is consistent with the direction of the rotating magnetic field, the rotor speed always cannot reach the speed of the rotating magnetic field. According to the law of electromagnetic induction, once the rotor speed is consistent with the speed of the rotating magnetic field, there will be no relative motion, and the magnetic field lines will not be cut. There will be no induced electromotive force and induced current, and there will be no torque for the rotor rotation. Therefore, the rotor speed is consistent with the rotating magnetic field.