Three Phase Induction Motor Construction

Induction motor, also known as asynchronous motor, is a kind of AC electric motor. According to the different power phase, it can be divided into single-phase and three-phase. The main construction of induction motor is composed of two parts - stator and rotor. In addition, there are end bells, bearings, motor frame and other components. The following will give more details about the main structure of three-phase induction motor or asynchronous motor.

1. Stator
Stator is a fixed part of induction motor, consisting of stator iron core, stator windings andmotor frame.

• Stator iron core
  As a part of the motor magnetic circuit, it is installed inside the motor frame. It is a hollow cylinder, outer wall of which is connected to the motor frame. And the stator windings are placed in the slot of the iron core inside. In order to reduce the loss of iron core, the stator iron core is stacked up with 0.5 mm thick silicon steel sheets.
• Stator winding
  It is a part of the motor electric circuit, generating the rotating magnetic field by inducing three-phase alternating current. Stator windings are wound with insulated copper wires and embedded in the stator slot, which are separated by insulating material between the windings and the slot.
  For the connection methods of stator windings of three-phase induction motor, not all of them are linked in the star connection(Y connection). But only under the circumstance of large capacity and high voltage will they connect in that way. In general, as for the small capacity and low voltage induction motor, six wire ends of three-phase  stator winding are pulled out to link in delta connection(Δ connection) or star connection(Y connection). In this way, the motor can be applied to two different levels of supply voltage, for example the star connection is introduced into the 380V power supply and the delta connection used for 220V power supply, which can satisfy the requirement of starting. In other words, it is designed as the delta connection for 380V power supply, and changed into the star connection at starting to achieve the purpose of reduced-voltage starting.
• Motor frame
  It fixes the stator core and stator windings and supports the rotor with two end bells. Meanwhile, it protects the electromagnet part of the entire motor and dissipates the heat generated during motor operation. The frame is typically made from iron or aluminum.

2. Rotor
Rotor is a rotating part of induction motor, including iron core, windings and shaft etc.

• Rotor iron core
  It is also a part of the magnetic circuit, generally stacked by silicon steels and fixed on the shaft.
• Shaft
  It plays a role of converting torque and support the rotor. It is generally made of medium carbon steel or alloy steel.
• Rotor winding
  It produces induced current as cutting the stator magnetic field, and under the effect of the rotating magnetic field, it forces the rotor to rotate. According to the different structure, it can be divided into two types: squirrel-cage rotor and wound rotor.
  The wound-type rotor windings can be connected into star or delta connection. In general, the small capacity rotor is connected into delta while the large and medium capacity one is connected into star. These three windings wire ends are connected to three slip rings fixed on the shaft by a set of electric brush. It can connect the external resistor to the rotor winding circuit. The purpose of string resistance is to improve the induction motor characteristics or adjust the rotate speed.
  The winding structure of squirrel-cage rotor is quite different from the one of stator. There are slots on the rotor iron core with a bar on each slot. Two ends of iron core that connect all the bars to the external of slots respectively form a short circuit. If the iron core is removed from the stator, the windings shape is like a squirrel cage. Some bars are made of copper, and some are aluminum. If the winding is made of copper, the bare copper bar prepared will be inserted into the iron core slot and then capped with copper rings on both ends followed by welding; if the winding is made of aluminum, the smelt aluminum liquid is directly cast on the slots of rotor iron core, cast with rings and fan blades at one time.