How large is the motor air gap?

30/10/2019

A certain length of air gap should be maintained between the outer surface of the rotor of the motor and the inner surface of the stator to ensure that the rotor can rotate freely within the stator cavity. Although there is no direct electrical connection between the rotor and the stator, when the stator windings are energized, there is electromagnetic connection between the rotors through the air gap. Similar to the transformer principle, the stator magnetic field and the rotor magnetic field are coupled to each other, thereby realizing electrical energy and Energy conversion between mechanical energy.

The length of the air gap has a great influence on the performance and operational reliability of the asynchronous motor. If the air gap is too large, the magnetic resistance will increase greatly. The excitation current required to achieve the same magnetic field strength will increase greatly, and the excitation loss will also increase greatly. The motor power factor will be significantly reduced, and the performance of the motor will deteriorate. In order to reduce the excitation current and improve the power factor, the air gap should be minimized. However, if the air gap is too small, the air gap harmonic magnetic field will increase, the motor stray loss and noise increase, and the maximum torque and starting torque will be reduced. On the other hand, if the air gap is too small, it is easy to wipe the rotor and the stator in operation, a broom failure occurs, and even the stuck rotor does not rotate, which reduces the reliability of operation and brings difficulties to the assembly. Generally, the air gap of small and medium-sized asynchronous motors is about 0.25~1.5mm, and the large and medium asynchronous motors are about 0.75~2mm.

01 Basic structure of three-phase asynchronous motor

The three-phase asynchronous motor mainly consists of a stator and a rotor and a bearing. The stator is mainly composed of a core, a three-phase winding, a base and an end cover. The stator core is generally formed by laminating and laminating a silicon steel sheet having an insulating layer on the surface of 0.35 to 0.5 mm thick, and has a uniformly distributed groove in the inner circle of the core for embedding the stator winding. The three-phase winding is formed by three identical windings that are symmetrically arranged at 120° electrical angles apart from each other. The respective coils of these windings are respectively embedded in the slots of the stator according to a certain regularity. Its function is to pass three-phase alternating current to generate a rotating magnetic field. The base is usually cast iron. The large asynchronous motor base is usually welded with steel plate. The base of the micro motor is made of cast aluminum. Its function is to fix the stator core and front and rear end caps to support the rotor, and to protect and dissipate heat.

There is a heat dissipation rib on the outer surface of the closed motor to increase the heat dissipation area. The end caps of the protective motor have ventilation holes at both ends, so that the air inside and outside the motor can be directly convected to facilitate heat dissipation. The end cap is mainly used to fix the rotor, support and protection. The rotor consists mainly of a core and a winding. The material of the rotor core is the same as that of the stator. It is made of 0.5 mm thick silicon steel sheet and laminated. The outer surface of the silicon steel sheet is punched with evenly distributed holes for the rotor winding. The inner core of the silicon steel sheet that is behind the stator core is usually used to punch the rotor core. Generally, the rotor core of a small asynchronous motor is directly press-fitted on a rotating shaft, and the rotor core of a large and medium-sized asynchronous motor (with a rotor diameter of 300 to 400 mm or more) is pressed against the rotating shaft by means of a rotor bracket.

The rotor windings are divided into a squirrel cage rotor and a wound rotor.

(1) Squirrel-cage rotor: The rotor winding consists of a plurality of bars inserted into the rotor slots and two end rings of the ring. If the rotor core is removed, the entire winding is shaped like a squirrel cage, so it is called a cage winding. The small cage motor uses a cast aluminum rotor winding, and is made of a copper strip and a copper end ring for a motor of 100 kW or more. The squirrel cage rotor is divided into: impedance type rotor, single squirrel cage type rotor, double squirrel cage type rotor, deep groove type rotor, and the starting torque and other characteristics are different.

(2) Wire-wound rotor: The wound rotor winding is similar to the stator winding, and is also a symmetrical three-phase winding, which is generally connected in a star shape, three outgoing heads are connected to the three collecting rings of the rotating shaft, and then passed through the brush. Connected to an external circuit.

02 induction motor working principle

The induction motor is also called "asynchronous motor", that is, the rotor is placed in a rotating magnetic field, and under the action of the rotating magnetic field, a rotational moment is obtained, and thus the rotor rotates.

The rotor of the motor is a rotatable conductor, usually in the form of a squirrel cage. The stator is the part of the motor that does not rotate. The main task is to generate a rotating magnetic field. The rotating magnetic field is not achieved mechanically. Instead, it is connected to a pair of electromagnets by alternating current, so that its magnetic pole properties change cyclically, so it is equivalent to a rotating magnetic field. Such a motor does not have a brush or a collector ring like a DC motor. There are single-phase motors and three-phase motors depending on the type of AC power used. Single-phase motors are used in, for example, washing machines, electric fans, etc. Three-phase motors are used as power for the factory. device.

Through the relative motion of the rotating magnetic field generated by the stator and the rotor winding, the rotor winding cuts the magnetic induction line to generate an induced electromotive force, thereby generating an induced current in the rotor winding. The induced current in the rotor winding interacts with the magnetic field to generate electromagnetic torque that causes the rotor to rotate. Since the induced current gradually decreases as the rotor speed gradually approaches the synchronous speed, the generated electromagnetic torque also decreases accordingly. When the asynchronous motor operates in the motor state, the rotor speed is less than the synchronous speed.