Induction motor classification

Asynchronous motor is a kind of ac motor, also called induction motor, mainly for motor use. The stator phase number of asynchronous motor has single phase and three phase. The rotor structure of three-phase asynchronous motor has two kinds of squirrel cage and winding type, and the rotor of single-phase asynchronous motor is squirrel cage type. Squirrel cage motor is the most widely used asynchronous motor. Wound motor structure is complex, generally used for starting and speed control performance requirements of the occasion.

Overall structure of asynchronous motor

The squirrel-cage asynchronous motor as an example to introduce the overall structure of asynchronous motor, it is composed of static stator part and rotating rotor part. There is a uniform air gap between stator and rotor, the air gap is very small, generally 0.2~2mm. In addition, both ends of the stator are provided with a supporting end cover, bearing and bearing inside and outside cover. To form a cooling wind path, a fan and a hood are installed on one side of the stator. The stator winding end lead is connected to the terminal of the terminal box outside the frame. Besides rotor core and squirrel cage winding, there are shaft, fan and other parts on the rotor.

Each part of asynchronous motor structure

1, stator part

The stator is used to generate a rotating magnetic field. The stator of three-phase motor is generally composed of shell, stator core, stator winding and other parts.

A, the shell

Three-phase motor housing includes frame, end cover, bearing cover, junction box and lifting ring and other parts.

Frame: cast iron or cast steel, its role is to protect and fix the stator winding of the three-phase motor. The frame of medium and small three-phase motor has two end covers supporting the rotor, which is an important part of the mechanical structure of three-phase motor. Usually, the appearance of the frame requires good heat dissipation performance, so generally cast heat sink.

End cover: cast with cast iron or steel molding, its role is to fix the rotor in the stator cavity center, so that the rotor can rotate evenly in the stator, fixed bearing installation, support the rotor and cover the motor.

Bearing cover: also cast iron or cast steel molding, its role is to fix the rotor, so that the rotor can not move axially, in addition to the storage of lubricating oil and protect the bearing, prevent lubricating oil outflow and dust into, so as to protect the bearing.

Terminal box: generally cast with cast iron, its role is to protect and fix the winding lead terminal.

Lifting ring: generally made of cast steel, installed on the upper end of the frame, used to lift and carry the three-phase motor.

B, stator core

Induction motor stator core is the magnetic conductivity part of the motor, made of 0.35mm ~ 0.5mm thick silicon steel sheet with excellent magnetic conductivity coated with insulating paint. Because the silicon steel sheets are thin and insulated from each other, the eddy current loss of the core due to the passage of alternating flux is reduced. The inner circle of the core has evenly distributed slots for embedding stator loops.

C. Stator winding

The stator windings are placed in the inner circular groove of the stator core. It is the conductive part of the three-phase asynchronous motor. The three-phase asynchronous motor has the three-phase windings. The three-phase winding consists of three independent windings, and each winding is connected by several coils. Each winding is a phase, each winding in space difference of 120° electrical Angle. The coil is made of insulated copper wire or insulated aluminum wire. Medium and small three-phase motors mostly use round enameled wire, and the stator coils of large and medium-sized three-phase motors are wound with large section of flat insulated copper wire or flat aluminum wire, and then embedded into the stator core groove according to certain rules. The six outgoing ends of the stator three-phase winding are all connected to the junction box, with the first end marked as U1, V1, W1, and the end marked as U2, V2, W2. The arrangement of the six outgoing terminals in the junction box is shown in Figure 7, which can be connected into a star or triangle shape.

FIG. 7 Connection of stator windings

(a) star connection (b) triangle connection

2. Rotor part

The rotor of an asynchronous motor consists of a rotating shaft, a rotor core and a rotor winding.

A, a shaft

The rotating shaft is to transfer power, supporting the rotor core and output the role of mechanical torque, made of medium carbon steel, both ends of the journal and bearing matching, generally supported on the end cover, the shaft out of the end milling keyway to fix the pulley or coupling and is connected with the mechanical drag.

B. Rotor core

The rotor core is also a part of the magnetic circuit of the motor, which is generally made of 0.5mm silicon steel sheet after being pressed. The rotor core is fixed on the rotating shaft or rotor bracket. The rotor core is cylindrical.

C. Rotor winding

The rotor windings of asynchronous motors are used for induction of electromotive force, current flow and electromagnetic torque generation. The rotor windings are divided into two types: wound type and squirrel cage type, which can be divided into wound type asynchronous motors and squirrel cage type asynchronous motors.

Winding: The winding of the winding rotor is similar to that of the stator. It is embedded in the rotor core groove and connected into a three-phase symmetric winding. The star (Y) connection is generally adopted. The three outlet ends are connected to three collector rings (or slip rings) insulated from each other on the rotating shaft, and the current is elicited through a brush. The characteristic of the wound rotor is that additional resistance can be connected to the rotor loop through the slip ring brush to improve the starting performance of the motor and adjust its speed. The wiring diagram is shown in the figure.

1- collector ring; 2 - brush; 3- Additional resistance

Squirrel cage winding: the iron core of the squirrel cage rotor is evenly distributed with many slots, and there is a bare guide bar in each slot. At the notch at both ends, all the guide bars at both ends are connected with two ring end rings respectively. If take out iron core, the appearance of whole winding resembles a "squirrel cage", reason calls squirrel cage type rotor. When manufacturing, the guide bar and the end ring can be cast out of molten aluminum at a time, or the copper strip can be inserted into the rotor slot, and then the upper ring is welded at both ends. Small and medium asynchronous motors generally use cast aluminum rotors.

3. Other parts

Other parts include fans, hoods, etc. Fans are generally cast aluminum parts (or plastic parts), play a role in ventilation cooling; The air hood is made of thin steel plate, which mainly plays the role of air conduction and heat dissipation and fan protection. There is an air gap between the stator and rotor of the three-phase asynchronous motor. The air gap is uniform and the size is the minimum allowable by mechanical conditions. Generally, the air gap of small motors is 0.35 ~ 0.5mm, and the gap of large motors is about 1 ~ 1.5mm. If the air gap is too large, the power factor of the motor will be reduced. The air gap is too small, which makes assembly difficult, operation unreliable, high-order harmonic magnetic field enhanced, which increases the additional loss and makes the start-up performance worse.