Three-Phase Induction Machines

Three-Phase Induction Machines - EE 4420 Electric Machine...

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1 EE – 4420 Electric Machine Analysis Fall 2005 THREE-PHASE INDUCTION (ASYNCHRONOUS) MACHINES 1. CONSTRUCTION AND PRINCIPLE OF OPERATION There are two types of induction machines: slip-ring and squirrel cage machine. 1.1 Induction machine with slip-ring rotor (wound-rotor type) The motor structure is shown in Fig. 1. A stator has a 3-phase winding supplied from a 3-phase source. A rotor winding is of the same form as the stator winding and is embedded in the slots. The stator and the rotor cores are composed of laminations of high-grade sheet steel. The wire is insulated from the lamination. The rotor winding is connected in star and its left 3 free terminals are conducted through the hollow shaft and connected to the 3 copper rings (insulated from the rotor shaft). Using stationary brushes pressing against the slip rings, the rotor terminals can be connected to an external circuit e.g. external three-phase resistor. The three-phase windings on the stator and on the rotor are distributed windings. The winding of each phase can be distributed over several slots. When the current flows through the distributed winding it produces an essentially sinusoidal space distribution of mmf. X X A 1 A 2 B 1 C 2 variable resistance slip rings brushes 3-phase rotor winding 3-phase stator winding C 1 B 2 circuit diagram of 3-phase rotor winding connected in star Fig.1 Construction scheme of a slip-ring machine
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2 1.2 Machine with squirrel-cage rotor There is no any difference in the stator construction of both types of machines. The difference is in the rotor winding. The squirrel-cage winding consists of aluminum or copper bars embedded in the rotor slots and short-circuited at both ends by aluminum or copper end rings as shown in Fig.2. Fig.2 Construction scheme of rotor squirrel-cage winding 1.3 Principle of Operation If the three-phase stator winding is connected to a three-phase supply, a rotating magnetic field will be produced in the air gap. The field rotates at synchronous speed n s = n 1 (given by equation (13) – notes: Rotating Magnetic Field ). The rotating field when moving with respect to the copper bars of stator and rotor windings induces voltages. If the rotor circuit is closed, the induced voltages (in the rotor winding) cause the currents to flow. Their interaction with the air gap magnetic field produces a torque. The rotor if free to do so will then starts rotating. According to Lentz’s law, the rotor rotates in the direction of the rotating field such that the relative speed between the rotating field and the rotor winding decreases. The rotor will eventually reach the steady-state speed n that is less than the synchronous speed n s at which the stator rotating field rotates in the air gap. At rotor speed n = n s there will be no induced voltage and currents in the rotor circuit and hence no torque.
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This note was uploaded on 02/26/2012 for the course EE 4150 taught by Professor Wu during the Fall '10 term at LSU.

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Three-Phase Induction Machines - EE 4420 Electric Machine...

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