Electrical Drives and control for Automation-Module IV THREE PHASE INDUCTION MOTOR The 3-phase induction motor has a stator and a rotor. The stator carries a 3-phase winding (called stator winding), while the rotor carries a short-circuited winding (called rotor winding). Only the stator winding is fed from 3-phase supply. The rotor winding derives its voltage and power from the externally energized stator winding through electromagnetic induction and hence the name. The Induction machines can be considered as a transformer with a rotating secondary in the sense that the power is transferred from the stator (primary) to the rotor (secondary) winding only by mutual induction and it can, therefore, be described as a “transformer type” a.c machine in which electrical energy is converted into mechanical energy. Advantages It has simple and rugged construction. It is relatively cheap. It requires little maintenance. It has high efficiency, good speed regulation and reasonably good power factor. It has self starting torque. Disadvantages It is essentially a constant speed motor and its speed cannot be changed easily. Its starting torque is inferior to d.c motors. CONSTRUCTION A three phase induction motor has 4 main parts: 1. Frame 2. Stator 3. Rotor 4. Shaft and Bearings The stator houses a 3-phase winding which is fed with electric power. The rotor is placed inside the stator and both are separated by an air gap. 1. Frame It is the outerKTUNOTES.IN body of the motor. Its function are to support the stator core and winding, to protect the inner parts of the machine and serve as a ventilating housing or means of guiding the coolant into effective channels. 2. Stator It consists of a steel frame which encloses a hollow, cylindrical core made up of thin laminations of silicon steel to reduce hysteresis and eddy current losses. A number of evenly spaced slots are provided on the inner periphery of the laminations. The insulated conductors are placed in the stator slots and are suitably connected to form a balanced 3-phase star or delta connected circuit. The stator winding is usually 3-phase winding which is supplied from a 3- phase supply mains. The 3-phase stator winding is wound for a definite number of poles as per requirement of speed. When 3-phase supply is given to the stator winding, a rotating magnetic field of constant magnitude is produced. This rotating field induces currents in the rotor by electromagnetic induction. 1 Aswathy Mohandas P Asst.Professor ,EEE,SNGCE Downloaded from Ktunotes.in Electrical Drives and control for Automation-Module IV 3. Rotor The rotor consists of a laminated core, with slots, cut on its outer periphery where windings are placed. The windings may be either of squirrel cage type or wound rotor type. The winding placed in these slots (called rotor winding) may be one of the following two types: (i) Squirrel cage type (ii) Slip ring type or Wound type (i) Squirrel cage rotor: It consists of a laminated cylindrical core having parallel slots on its outer periphery. One copper or aluminum bar is placed in each slot. All these bars are joined at each end by metal rings called end rings. This forms a permanently short-circuited winding which is indestructible. The entire construction (bars and end rings) resembles a squirrel cage and hence the name. The short circuited rotor conductors on both sides, constitute a closed path for the rotor current to flow. The rotor winding is not electrically connected to the supply, but by electromagnetic induction from the field created by the stator, a voltage is induced in the rotor conductors and a current is circulated through the end rings. Those induction motors which employ squirrel cage rotor are called squirrel cage induction motors. Most of 3-phase induction motors use squirrel cage rotor as it has a remarkably simple and robust construction However, it suffers from the disadvantage of a low starting torque. It is because the rotor bars are permanently short-circuited and it is not possible to add any external resistance to the rotor circuit to KTUNOTES.INhave a large starting torque. (ii) Slip ring rotor: It consists of a laminated cylindrical core and carries a 3- phase winding. The rotor winding is uniformly distributed in the slots on the outer periphery of a laminated cylindrical steel core and is usually star-connected. The open ends of the rotor winding are brought out and joined to three insulated slip rings mounted on the rotor shaft with one brush resting on each slip ring. The three brushes are connected to a 3-phase star-connected rheostat as shown in figure. At starting, the external resistances are included in the rotor circuit to give a large starting torque. These resistances are gradually reduced to zero as the motor runs up to speed. The external resistances are used during starting period only. When the motor attains normal speed, the three brushes are short-circuited so that the wound rotor runs like a squirrel cage rotor. 2 Aswathy Mohandas P Asst.Professor ,EEE,SNGCE Downloaded from Ktunotes.in Electrical Drives and control for Automation-Module IV 4. Shaft and Bearings Shaft couples mechanical energy developed in the rotor to mechanical load. It is made of carbon steel. Rotor is supported over the shaft through bearings to reduce friction. Rotating Magnetic Field Due to 3-Phase Currents When a 3-phase winding is energized from a 3-phase supply, a rotating magnetic field is produced. This field is such that its poles do not remain in a fixed position on the stator but go on shifting their positions around the stator. For this reason, it is called a rotating field. It can be shown that magnitude of this rotating field is constant and is equal to 1.5 m where m is the maximum flux due to any phase. To see how rotating field is produced, consider a 2-pole, 3-phase winding (3 phase windings are 1200 electrical apart). The three phases X, Y and Z are energized from a 3-phase source and currents in these phases are indicated as Ix, Iy and Iz as shown in fig. The fluxes produced by these currents (the flux produced by a current is in phase with the current that producesKTUNOTES.IN it) are given by: where m is the maximum flux due to any phase. The phasor of the three fluxes are shown in figure. It can be proved that this 3-phase supply produces a rotating field of constant magnitude equal to 1.5 m i. At instant 1, t = 0°. Therefore, the three fluxes are given by, The phasor sum of y and z is the resultant flux, r . 3 Aswathy Mohandas P Asst.Professor ,EEE,SNGCE Downloaded from Ktunotes.in Electrical Drives and control for Automation-Module IV ii. At instant 2, t = 30°. Therefore, the three fluxes are given by, The phasor sum of x, y and z is the resultant flux, r. Phasor sum of x and z, Phasor sum of 'r and y, Therefore, resultant flux is displaced 30° clockwise from position 1. iii. At instant 3, t = 60°. Therefore, the three fluxes are given by, KTUNOTES.IN The resultant flux r is the phasor sum of x and y z 0. Therefore, the resultant flux is displaced 60° clockwise from position 1. iv. At instant 4, t = 90°. Therefore, the three fluxes are given by, The phasor sum of x, y and z is the resultant flux r Phasor sum of z and y, Phasor sum of 'r and x, 4 Aswathy Mohandas P Asst.Professor ,EEE,SNGCE Downloaded from Ktunotes.in Electrical Drives and control for Automation-Module IV Therefore, the resultant flux is downward i.e., it is displaced 90° clockwise from position 1. It is proved that a 3-phase supply produces a rotating field of constant value (= 1.5 m, where m is the maximum flux due to any phase). PRINCIPLE OF OPERATION (i) When 3-phase stator winding is energized from a 3-phase ac supply, a rotating magnetic field is set up which rotates round the stator at synchronous speed, Ns (= 120 f/P), where f is the supply frequency and P is the number of poles on the stator. (ii) The rotating field passes through the air gap and cuts the rotor conductors, which are stationary initially at the instant of starting. When the rotating magnetic field sweeps past the stationary rotor conductors, an emf is induced in the rotor conductors just as emf is induced in the secondary winding of a transformer by the flux set up by the primary current. Since the rotor circuit is short-circuited, currents start flowing in the rotor conductors. (iii) Now the situation is exactly like current-carrying rotor conductors are placed in the magnetic field produced by the stator. Consequently, mechanical force acts on the rotor conductors. The sum of the mechanical forces on all the rotor conductors produces a torque which tends to move the rotor in the same direction as the rotating field. (iv) The factKTUNOTES.IN that rotor is urged to follow the stator field (i.e., rotor moves in the direction of stator field) can be explained by Lenz’s law. According to this law, the direction of rotor currents will be such that they tend to oppose the cause producing them. Now, the cause producing the rotor currents (induced current) is the relative speed between the rotating magnetic field and the stationary rotor conductors. Hence to reduce this relative speed, the rotor starts running in the same direction as that of stator field and tries to catch it.