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Unit-3. Three Phase Induction Motor

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Unit-3. Three Phase Induction Motor Unit-3. Three Phase Induction Motor. UNIT – III THREE PHASE INDUCTION MOTOR THREE PHASE INDUCTION MOTOR Constructional details – Types of rotors – Principle of operation – Slip – Equivalent circuit – Slip-torque characteristics - Condition for maximum torque – Losses and efficiency – Load test - No load and blocked rotor tests - Circle diagram – Separation of no load losses – Double cage rotors – Induction generator – Synchronous induction motor. Construction and working principle of three phase induction motor. Construction An induction motor has two main parts. There are, Stator. Rotor. Stator. Constructed by laminated silicon steel plates by stamping for the purpose of reducing the Eddy current losses and Hysteresis losses. Stator has inward projected stator poles; if we increase the no of poles then speed will be decreased. It consists stator windings, which are placed in stator slots. Three types of slots are shown here, N.Balamurugan AP. EEE Page 2 Unit-3. Three Phase Induction Motor. No of poles and types of slots are chosen corresponding to our requirements. Normally number of poles P = 2*n. Where, n = number of stator slots/pole/phase. Stator winding is wounded in two types, are as follows, * Lap winding. * Wave winding. Rotor. Two basic design types of the rotor are as follows, o squirrel-cage o slip ring Squirrel-cage Squirrel-cage rotor consist of copper bars slightly longer than the rotor, which are pushed into the rotor N.Balamurugan AP. EEE Page 3 Unit-3. Three Phase Induction Motor. The ends are welded to copper end rings, so that all the bars are short circuited permanently. In this type, we can‟t add any more external resistance to improve the starting torque. So to improve the Tst, “Skewed rotor” has introduced. In small machines bars and end rings are die cast in alluminium Skewed rotor. Slip Ring N.Balamurugan AP. EEE Page 4 Unit-3. Three Phase Induction Motor. It is usually for large 3 phase induction motors. Rotor has winding, same as stator and the end of each phase is connected to a slip ring. In phase wounded rotor always double layer, distributed windings are used in alternators even the stator has two phases. The three phase winding stared internally and other three winding terminals are brought out and connected to three insulated sliprings, which are mounted on the shaft with brushes resting on them. Yoke Made by Alloy cast iron steel. It provides mechanical support to all parts of the motor and restricts the magnetic leakage. Shaft and Bearings Shaft - cylindrical, solid core type. Bearings – Ball bearings and roller bearings are used. Airgap Should design uniform airgap between rotor and stator. Working principle. Rotating Magnetic Field Balanced three phase windings, i.e. mechanically displaced 120 degrees from each other, fed by balanced three phase source A rotating magnetic field with constant magnitude is produced, rotating with a speed N.Balamurugan AP. EEE Page 5 Unit-3. Three Phase Induction Motor. Where f is the supply frequency and p is the no. of poles and Ns is called the synchronous speed in rpm (revolutions per minute) Principle of operation The stator is connected to a 3-phase AC power supply. The number of poles is determined by how many times a phase winding appears. In this example, each phase winding appears two times. This is a two- pole stator When AC voltage is applied to the stator, current flows through the windings. The magnetic field developed in a phase winding depends on the direction of current flow through that winding. N.Balamurugan AP. EEE Page 6 Unit-3. Three Phase Induction Motor. Start It is easier to visualize a magnetic field if a start time is picked when no current is flowing through one phase. In the following illustration, for example, a start time has been selected during which phase A has no current flow, phase B has current flow in a negative direction and phase C has current flow in a positive direction. Based on the above chart, B1 and C2 are south poles and B2 and C1 are north poles. A magnetic field results, as indicated by the arrow. N.Balamurugan AP. EEE Page 7 Unit-3. Three Phase Induction Motor. Time 1 At Time 1 phase C has no current flow, phase A has current flow in a positive direction and phase B has current flow in a negative direction. Following the same logic as used for the starting point, windings A1 and B2 are north poles and windings A2 and B1 are south poles. Time 2 Phase B has no current flow. Although current is decreasing in phase A it is still flowing in a positive direction. Phase C is now flowing in a negative direction. At start it was flowing in a positive direction. Current flow has changed directions in the phase C windings and the magnetic poles have reversed polarity. Advantages Cheaper, Light weight, High efficiency, Require less maintenance. Disadvantages Moderate starting torque. External resistance can‟t be added, so Tst can‟t be controlled. Applications o Generally Conveyer line (belt) drives, o Roller table, Paper mills, o Traction, Electric vehicles, o Elevators, pulleys. (End of the answer.) N.Balamurugan AP. EEE Page 8 Unit-3. Three Phase Induction Motor. Slip. Slip is the ratio between the slip speed and synchronous speed. S = Where, Ns = synchronous speed in rpm. N = actual motor speed in rpm. Equivalent circuit of three phase induction motor. Generally three phase induction motor is treated as rotating transformer. Has two winding named by primary and secondary. Similarly in induction motor, stator → primary, rotor → secondary. Equivalent circuit of induction motor. N.Balamurugan AP. EEE Page 9 Unit-3. Three Phase Induction Motor. Parameters V1 = supply voltage in volts. E1 = Induced emf in stator/phase (due to self inductance) E2 = Induced emf in rotor/phase (due to mutual inductance) R1 = stator resistance in ohms. R2 = rotor resistance in ohms X1 =stator reactance X2r =rotor reactance. E2r =rotor induced emf at running condition.(E2r) From the diagram, under no load condition the no load current I0, I0 = Iw+Iµ Where, Iw =Working component which supplied no load losses. Iµ =Magnetic component which sets up flux at core and airgap. Ro = No load resistance/phase (represents no load losses) Ro = X0 = No load reactance/phase (represents flux setup in the core) Xo = X2r = sX2 X2 is constant. S varies with respect to rotor speed. N.Balamurugan AP. EEE Page 10 Unit-3. Three Phase Induction Motor. So the above equivalent circuit will becomes, Under running condition, Rotor current I2r = = From this , Motor load changes → speed changes. Speed changes → slip changes. Slip changes → reactance changes. Reactance X gets differ with speed which is affect the flux, so far rotor reactance indicated as variable element. Equivalent circuit for rotor. I2r = N.Balamurugan AP. EEE Page 11 Unit-3. Three Phase Induction Motor. From the above expression, X2 and E2 are constant values. R2 varies with slip. Now the variable Resistance, = = ) From this R2/s has two parts, o R2 = Rotor copper loss. o R2( = RL which is vary with slip. So it is indicated as variable element. N.Balamurugan AP. EEE Page 12 Unit-3. Three Phase Induction Motor. Equivalent circuit referred to stator K = Transformer ratio. K = Rotor parameter referred to as stator, o E2' = E2/K o I2r' = I2r/K (Already we have I2r = ) 2 o X2' =X 2/K 2 o R2' = RL/K = = R2' ( Approximate equivalent circuit N.Balamurugan AP. EEE Page 13 Unit-3. Three Phase Induction Motor. For simplify the calculation the exciting circuit is transferred to the left of R1, X1. The inaccuracy because of this shifting is negligible. For the further simplification, R01 = R1+R'2 X01 = X1+X'2 R01 = R1+ X01=X1+ I1 = I0+ I2r' I1 = (IW+I ) Torque equation of three phase induction motor. Torque is proportional to flux per pole. N.Balamurugan AP. EEE Page 14 Unit-3. Three Phase Induction Motor. 2 2 2 ) 2 ) K = 3 / 2П ns If , S= 1 )2 (Or refer class notes) Condition for maximum running torque: Condition: (dT/dR2) = 0; 2 2 R2 = X2 Condition for maximum running torque: Torque equation, )2 Condition: = 0; Sm = is the slip at which torque is maximum. 2 Tmax= K i. The maximum torque is independent of rotor resistance N.Balamurugan AP. EEE Page 15 Unit-3. Three Phase Induction Motor. ii. Maximum torque is directly proportional to the square of the induced emf at standstill. iii. Maximum torque is inversely proportional to the rotor reactance. Torque – Slip characteristics It consists of three regions (1) Stable operating region. (2) Unstable operating region. (3) Normal operating region. TORQUE(+) e e e MOTORING m m m Tem PLUGGING (max torque or GENERATING pull-out torque) zero slip T (starting torque) es (sync.speed) e 0 unity slip SPEED (standstill) rated slip SLIP,s TORQUE(-) Stable region In stable region, the slip value„s‟ is very small. i.e. the term ( 2 is very small as compared to .hence neglecting ( 2 Torque increases slip also increases, motor speed decreases. N.Balamurugan AP. EEE Page 16 Unit-3. Three Phase Induction Motor. Unstable region When the slip is further increases, the region is unstable. Here, Slip increases torque decreases, motor speed increases, this region is called high slip region Normal operating region Motor is continuously operated in this region. Starting torque, maximum torque or pull load torque, full load torque. Losses in an Induction Motor. 1. Magnetic losses. 2. Mechanical losses. }constant losses 3. Electrical losses. → variable losses. Magnetic losses Also called as core loss (or) iron loss These losses occur in stator and rotor core There are Two types Hysteresis Eddy current Hysteresis losses Due to alternates change in magnetic field in the stator core To reduce, select silicon for stator for stator &rotor core N.Balamurugan AP.
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