Because both three-phase and single-phase asynchronous motors use the principle of electromagnetic induction, they transfer electrical energy from the stator to the rotor through coupling of rotating magnetic fields, and output mechanical energy as a rotating machine. It has many similarities with transformers, so some analysis methods for transformers are also applicable to analyzing its stator and rotor.
★ Eddy currents in AC electricity exist in equipment such as power transformers, three-phase AC induction motors, generators, etc. Although eddy currents cannot be seen, they do exist.
Any electrical equipment that utilizes the principle of electromagnetic induction is affected by eddy currents, which mainly affect the utilization rate of electric energy in AC circuits. This is because eddy currents can generate Joule heat, and the amount of heat generated is proportional to the square of the current. For applications such as electric motors, generators, transformers, etc., this kind of heat generation generally results in wasted electrical energy. In three-phase AC motors, for example, since the stator coil and rotor rotating part use an air gap to transmit induced electromotive force between them, if there is too much space between them then the motor's no-load operating current will increase. If left unchecked it could cause serious overheating and burn out the stator coil. This is caused by excessive eddy currents.
★ Eddy current in AC is the phenomenon where, under the action of an AC magnetic field, a magnetic core attached to an AC coil will have magnetic lines passing through it (or silicon steel sheets) due to electromagnetic induction. An electric current will pass through the closed loop conductor in the coil at the same time as there are magnetic lines passing through the silicon steel sheet. Induced currents are generated on planes perpendicular to these magnetic lines, forming a closed loop automatically (i.e., eddy currents). Therefore, they are called eddy currents. Any substance whose conductivity changes with respect to either direction or intensity of a magnetic field can produce eddy currents. The magnitude of eddy currents is directly proportional to that of the magnetic field and changes in flux linkage area while inversely proportional to electrical resistivity (see Lenz's law).
Furthermore, since there exists some resistance within silicon steel sheet cores, their presence causes heating and loss of part of electrical energy due to eddy current flow. Therefore, when manufacturing motors, appropriate gaps must be left between stator and rotor; excessively large gaps would not work.
The structure of three-phase asynchronous AC motor
The main components of a three-phase asynchronous AC motor are shown in the figure below:
It consists of two main parts: stator and rotor.
I. Stator
Stator refers to the stationary part of the motor. It mainly consists of a machine base, stator core, end cover, and three-phase symmetrical winding of the stator. The machine base is usually made of cast iron or cast steel. The stator core is pressed into the machine base. The stator core is a part of the motor magnetic circuit. In order to reduce iron loss, it is made up of 0.5mm thick silicon steel sheets stacked in a circular shape and pressed into the machine base.
There are several evenly distributed slots on the inner circumference N of the core for embedding the three-phase winding of the stator. The winding of small motors is generally made by enameled wire. There are six output terminals for each phase winding in an asynchronous motor, with U1,V1,W1 representing its starting end and U2,V2,W2 representing its ending end respectively; they are usually led out from junction box on top of machine base . Three-phase symmetrical windings can be connected in star or delta depending on line voltage and rated voltage; for example, if line voltage is 380V while rated phase voltage for motor's coil group is 220V , then windings must be connected in star; if rated voltage for coil group is 380V , then windings must be connected in delta as shown below:
First, this ensures that each phase winding works under its rated voltage. There are certain rules for the arrangement of the outgoing ends of each phase winding in the junction box.
Second, rotor
The rotor is the rotating part of the motor and is composed of a shaft, a rotor core, a rotor winding, and a fan.
The rotor core is also made up of 0.5mm thick silicon steel sheets stacked into a cylinder with the shaft pressed in the middle. Its outer circular surface has several evenly distributed slots where the rotor windings are placed. According to different structural forms of windings on rotors, asynchronous motors can be divided into two types: squirrel-cage type and wound-rotor type.
Squirrel-cage type: Bare copper bars are inserted into slots on the rotor core and welded at both ends to two copper rings (also called end rings). Due to its similar shape to a squirrel cage, it is called a squirrel-cage motor.
In order to save copper materials, currently for squirrel-cage motors below 100 kW, their rotors generally use cast aluminum rotors. Cast aluminum rotors melt aluminum using pressure casting or centrifugal casting methods and pour it into slots on the rotor core along with both end rings and internal fans during casting. This simplifies manufacturing processes while reducing motor costs.
Dongchun Motor: Your Partner in Efficiency
As a leading electric motor manufacturer based in China, Dongchun Motor is committed to producing motors that are not only efficient but also reliable. Our quality control and manufacturing processes are designed to mitigate the effects of eddy currents, thereby maximizing the performance of our motors. With a broad range of products to meet various industrial needs, we are your go-to source for motors that are engineered for excellence.
