3 phase induction motor for Electrical.pptx
- 2. Contents ⮚ Constructional feature ⮚ Working principle of three phase induction motors, types ⮚ Torque equation, torque slip characteristics ⮚ power stages; efficiency ⮚ Sarters (auto transformer starter , star delta starter) ⮚ methods of speed control and industrial applications
- 4. Introduction • Three-phase induction motors are the most common and frequently encountered machines in industry • simple design, rugged, low-price, easy maintenance • wide range of power ratings: fractional horsepower to 10 MW • run essentially as constant speed from no-load to full load • Its cost is very low and it is very reliable • It has high efficiency .No brushes are needed and hence frictional losses are reduced It requires minimum of maintenance • Its speed depends on the frequency of the power source • not easy to have variable speed control • requires a variable-frequency power-electronic drive for optimal speed control
- 5. Construction • An induction motor has two main parts • a stationary stator • consisting of a steel frame that supports a hollow, cylindrical core • core, constructed from stacked laminations (why?), having a number of evenly spaced slots, providing the space for the stator winding • Stator frame, stator core, stator winding Stator of IM
- 6. Stator Frame Stator of IM
- 9. Construction • a revolving rotor • composed of punched laminations, stacked to create a series of rotor slots, providing space for the rotor winding • one of two types of rotor windings • conventional 3-phase windings made of insulated wire (wound-rotor) » similar to the winding on the stator • aluminum bus bars shorted together at the ends by two aluminum rings, forming a squirrel-cage shaped circuit (squirrel-cage) • Two basic design types depending on the rotor design • squirrel-cage: conducting bars laid into slots and shorted at both ends by shorting rings. • wound-rotor: complete set of three-phase windings exactly as the stator. Usually Y-connected, the ends of the three rotor wires are connected to 3 slip rings on the rotor shaft. In this way, the rotor circuit is accessible.
- 10. Construction Squirrel cage rotor Wound rotor Notice the slip rings
- 12. • It consists of a laminated cylindrical core having semi closed circular slots at the outer periphery. • Copper or aluminum bar conductors are placed in these slots and short circuited at each end by copper or aluminum rings called short circuiting rings. • The rotor winding is permanently short circuited and it is not possible to add any external resistance.
- 13. • The rotor slots are not parallel to the shaft but skewed to – Reduce humming . Provide smoother torque for different positions of rotor. Reduce magnetic locking of stator and rotor.
- 19. • It is also called SLIP RING ROTOR • Consists of a laminated core having semi closed slots at the outer periphery and carries a 3-phase insulated winding. • The rotor is wound for the same number of poles as that of stator. • The three finish terminals are connected together forming a star point and the three star terminals are connected to three slip rings fixed on the shaft.
- 21. Rotating Magnetic Field • Balanced three phase windings, i.e. mechanically displaced 120 degrees form each other, fed by balanced three phase source • A rotating magnetic field with constant magnitude is produced, rotating with a speed 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)
- 22. Synchronous speed P 50 Hz 60 Hz 2 3000 3600 4 1500 1800 6 1000 1200 8 750 900 10 600 720 12 500 600
- 23. Production Of RMF: • The three phase windings are displaced from each other by 120° e. The windings are supplied by a balanced three phase ac supply. • The three phase currents flow simultaneously through the windings and are displaced from each other by 120° electrical. • Each alternating phase current produces its own flux which is sinusoidal. So all three fluxes are sinusoidal and are separated from each other by 120°. • If the phase sequence of the windings is R-Y-B, then mathematical equations for the instantaneous values of the three fluxes ΦR , ΦY ,ΦB can be written as, • ΦR = Φmsin(ωt) • ΦY = Φmsin(ωt - 120) • ΦB = Φmsin(ωt - 240)
- 24. As windings are identical and supply is balanced, the magnitude of each flux is Φm .
- 25. • Case 1 : ωt = 0 ΦR = Φmsin(0) = 0 ΦY = Φmsin(-120) = -0.866 Φm ΦB = Φmsin(120) = +0.866 Φm • Case 2 : ωt = 60 ΦR = Φmsin(60) = +0.866 Φm ΦY = Φmsin(- 60) = -0.866 Φm ΦB = Φmsin(- 180) = 0 • Case 3 : ωt = 120 ΦR = Φmsin(120) = +0.866 Φm ΦY = Φmsin(180) = 0 ΦB = Φmsin(-120) = -0.866 Φm • Case 4 : ωt = 180 ΦR = Φmsin(180) = 0 ΦY = Φmsin(60) = +.866 Φm ΦB = Φmsin(-60) = -0.866 Φm By comparing the electrical and phasor diagrams we can find the flux rotates one complete 360 degree on the 180 degree displacement of flux.
- 26. Conditions for Production of RMF • The stator 3- phase winding should be placed at 120 degrees is space • The current supply to these winding should be balanced. • The direction of rotation of the magnetic field can be varied according to the phase sequence. • A three-phase winding displaced in space by 1200 is fed by a three-phase current displaced in time by 1200 : • It produces a resultant magnetic flux which rotates in space as if actual magnetic poles were being rotated mechanically.
- 27. Conclusions: • The magnitude of the Rotating magnetic field is always constant i.e. its value remains the same at any instant of time. • The direction of RMF is decided according to the phase sequence of the winding • The speed of rotation of the RMF is equal to the angular frequency of the supply voltage which in a way depends on the synchronous speed of the machine.
- 32. Working principle • Alternating flux is produced around the stator winding due to AC supply. This alternating flux revolves with synchronous speed. The revolving flux is called as "Rotating Magnetic Field" (RMF). • The relative speed between stator RMF and rotor conductors causes an induced emf in the rotor conductors, according to the Faraday's law of electromagnetic induction. The rotor conductors are short circuited, and hence rotor current is produced due to induced emf. That is why such motors are called as induction motors. (This action is same as that occurs in transformers, hence induction motors can be called as rotating transformers.) • Now, induced current in rotor will also produce alternating flux around it. This rotor flux lags behind the stator flux. The direction of induced rotor current, according to Lenz's law, is such that it will tend to oppose the cause of its production. • As the cause of production of rotor current is the relative velocity between rotating stator flux and the rotor, the rotor will try to catch up with the stator RMF. Thus the rotor rotates in the same direction as that of stator flux to minimize the relative velocity. However, the rotor never succeeds in catching up the synchronous speed.
- 33. Induction motor speed • At what speed will the IM run? • Can the IM run at the synchronous speed, why? • If rotor runs at the synchronous speed, which is the same speed of the rotating magnetic field, then the rotor will appear stationary to the rotating magnetic field and the rotating magnetic field will not cut the rotor. So, no induced current will flow in the rotor and no rotor magnetic flux will be produced so no torque is generated and the rotor speed will fall below the synchronous speed • When the speed falls, the rotating magnetic field will cut the rotor windings and a torque is produced
- 34. Induction motor speed • So, the IM will always run at a speed lower than the synchronous speed • The difference between the motor speed and the synchronous speed is called the Slip Where Ns= speed of the magnetic field N = mechanical shaft speed of the motor
- 35. The Slip Where s is the slip Notice that : if the rotor runs at synchronous speed s = 0 if the rotor is stationary s = 1 Slip may be expressed as a percentage by multiplying the above eq. by 100, notice that the slip is a ratio and doesn’t have units
- 36. Induction Motors and Transformers • Both IM and transformer works on the principle of induced voltage • Transformer: voltage applied to the primary windings produce an induced voltage in the secondary windings • Induction motor: voltage applied to the stator windings produce an induced voltage in the rotor windings • The difference is that, in the case of the induction motor, the secondary windings can move • Due to the rotation of the rotor (the secondary winding of the IM), the induced voltage in it does not have the same frequency of the stator (the primary) voltage
- 37. Frequency • The frequency of the voltage induced in the rotor is given by Where fr = the rotor frequency (Hz) P = number of stator poles n = slip speed (rpm)
- 38. Emf induced in rotor in running condition •
- 40. Rotor current •
- 41. Power Factor •
- 56. Torque • While the input to the induction motor is electrical power, its output is mechanical power • Any mechanical load applied to the motor shaft will introduce a Torque on the motor shaft. This torque is related to the motor output power and the rotor speed
- 57. Torque equation • Torque of a three phase induction motor is proportional to flux per stator pole, rotor current and the power factor of the rotor. • T ɸ I ∝ 2 cosɸ2 • OR • T = k ɸ I2 cosɸ2 . • where, ɸ = flux per stator pole, I2 = rotor current at standstill, ɸ2 = angle between rotor emf and rotor current, k = a constant. • Now, let E2 = rotor emf at standstill we know, rotor emf is directly proportional to flux per stator pole, i.e. E2 ɸ. therefore, ∝ • T E ∝ 2 I2 cosɸ2 • OR T = k1 E2 I2 cosɸ2.
- 59. Torque Under Running Condition •
- 60. Maximum Torque Under Running Condition •
- 61. Maximum Torque Under Running Condition •
- 62. • Maximum Torque Under Running Condition
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- 68. • Now when load increases, load demand increases but speed decreases. As speed decreases, slip increases. In high slip region as T α1/s, torque decreases as slip increases. • But torque must increases to satisfy the load demand. As torque decreases, due to extra loading effect, speed further decreases and slip further increases. Again torque decreases as T α1/s hence same load acts as an extra load due to reduction in torque produced. Hence speed further drops. Eventually motor comes to standstill condition. The motor can not continue to rotate at any point in this high slip region. Hence this region is called unstable region of operation.
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- 71. A three phase 3 kV, 24 pole, 50Hz, star connected slip ring induction motor has a rotor resistance of 0.015 Ω and a stand still reactance of 0.265 Ω per phase. Full load torque is obtained at a speed of 245 rpm. Calculate (i) ratio of maximum to full load torque. (ii) Speed at maximum torque. Neglect stator impedance. • • Soln: Synchronous speed Ns = 120f/p = 120 x 50/24 = 250 rpm • Full load slip Sf = (Ns – N) / Ns = (250 - 245)/250 = 0.02 • (i) ratio of R2 / X2 = 0.015/0.265 = 0.0566 • Ratio of full load to maximum torque (Tmax / Tf ) = (a2 + Sf 2 )/2a Sf • = (0.05662 + 0.022 )/2x0.0566x0.02 = 1.59 • (ii) at maximum torque R2 = SX2 • Hence slip at maximum torque S = R2/ X2 = 0.0566 • Speed at maximum torque N = (1-S) Ns = 235.85 rpm
- 72. A three phase slip ring induction motor has a star connected rotor. It has an induced emf of 60 volts on open circuit between the slip rings at stand still when the rated voltage is supplied to the stator. The resistance and stand still reactance of rotor per phase are 0.5 Ω and 5 Ω respectively. Determine the rotor current per phase (i) when the rotor is at stand still and connected to a star connected rheostat of resistance 5 Ω and reactance of 0.5 Ω per phase. (ii) when running at 4 % slip with rheostat short circuited. • Soln: Current through rotor at stand still = current at starting • As external resistance is connected in series with rotor per phase R2 = 5.5 Ω; X2 = 5.5 Ω • (i) I2 = E2/ √(R2 2 + X2 2 ) • I2 = (60/√ 3) / √ (5.52 + 5.52 ) = 4.454 amps • (ii) When running at 4% slip I2 = sE2/ √(R2 2 + (sX2 )2 ) = (0.04 x 60/√ 3) / √ (0.52 + (0.04 x 5)2 ) = 2.573 amps
- 73. Power losses in Induction machines • Copper losses • Copper loss in the stator (PSCL) = I1 2 R1 • Copper loss in the rotor (PRCL) = I2 2 R2 • Core loss (Pcore) • Mechanical power loss due to friction and windage • How this power flow in the motor?
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- 75. The power supplied to a three-phase induction motor is 32 KW and the stator losses are 1200 W. If the slip is 5% .Determine (a) The rotor copper losses, (b) The total mechanical power developed by the rotor, (c) The output power of the motor if the friction and windage losses are 750 W, and (d) The efficiency of the motor, neglecting rotor iron loss. •
- 76. A 6 pole, 50 Hz, 3-phase induction motor runs at 960 rpm when torque on the shaft is 200 N-m. if the stator losses are 1500 W and Friction and windage losses are 500 W, find (i) rotor copper loss and (ii) efficiency Of the motor • Ns=1000, S= 0.04 • Output power = 2π N Tsh/60 = 20096W • Gross power developed Pm = 20096+500 =20596 • Rotor cu loss = Pc • Pc/Pm =s/1-s, Pc= 858W • Rotor input = 20596+858=21454W • Stator input = Rotor input +stator loss=22954W • Efficiency =20096/22954 =87.5%
- 77. If the power input to 500 V,50Hz,6 pole three phase induction motor running at 975 rpm is 40 kW Stator losses are 1 kW & Friction and windage losses are 2 kW , find (i) rotor copper loss and (ii) efficiency Of the motor • i) Ns 120f/p= 1000 rpm • Slip s = - / 𝟏𝟎𝟎𝟎 𝟗𝟕𝟓 𝟏𝟎𝟎𝟎 = 0.025 = 2.5 % • ii) Motor input = 40 kW, Stator loss = 1 kW • Rotor input = 40-1 = 39 kW Rotor Cu loss = s x Rotor input • = 0.025 x 39= 0.975 kW • iii) Power output = Rotor input – rotor Cu loss-Friction loss • = 39 – 0.975 – 2 = 36.025 kW • % Efficiency = (Power output/motor input) x 100 =(36.025 kW / 40 kW) x 100 • = 90.06 %
- 78. Starting of an Induction Motor •
- 79. Autotransformer Starter • An autotransformer is used to apply a low voltage to the stator winding at the time of starting. When the motor speed reaches the desired level, autotransformer is disconnected and motor is connected directly across the supply. • The stator of the motor is connected through a 6-way double throw switch. • While starting, the switch is thrown to ‘Start’ side so that a reduced voltage is applied to stator. This keeps the starting current safe limits. • Once motor take up the speed, the switch is throw to ‘Run’ side so that full supply voltage is applied to stator. A specific advantage of this starter is that reduction in voltage during starting, can be done to any desired level by selecting proper tapping of the autotransformer
- 80. Star-delta starter • This is the most common form of starter used for three phase induction motors. • It achieves an effective reduction of starting current by initially connecting the stator windings in star configuration which effectively places any two phases in series across the supply. Starting in star not only has the effect of reducing the motor’s start current 3 Phase Induction motor but also the starting torque. • Once up to a particular running speed a double throw switch changes the winding arrangements from star to delta whereupon full running torque is achieved. Such an arrangement means that the ends of all stator windings must be brought to terminations outside the casing of the motor
- 81. Speed control • The Speed can be changed from Both Stator and Rotor Side. • From stator side ⮚V / f control or frequency control. ⮚Changing the number of stator poles. ⮚Controlling supply voltage. ⮚Adding rheostat in the stator circuit. • From rotor side ⮚Adding external resistance on rotor side. ⮚Cascade control method. ⮚Injecting slip frequency emf into rotor side
- 82. Speed Control from Stator Side • Speed Control by frequency variation: • By varying supply frequency (on small amount), we can vary the speed. • But a decrease in supply frequency decreases the speed and increases the flux, core losses which leads heating and low efficiency. • Increase in frequency increases the speed and reduces the torque. • A separate costlier auxiliary equipment is required to provide a variable frequency. • So this method is not used in practical. • V/F control • If we change frequency synchronous speed changes but with decrease in frequency flux will increase and this change in value of flux causes saturation of rotor and stator cores which will further cause increase in no load current of the motor . • So, its important to maintain flux , φ constant and it is only possible if we change voltage. i.e if we decrease frequency flux increases but at the same time if we decrease voltage flux will also decease causing no change in flux and hence it remains constant. • So, here we are keeping the ratio of V/f as constant. Hence its name is V/ f method. For controlling the speed of three phase induction motor by V/f method we have to supply variable voltage and frequency which is easily obtained by using converter and inverter set.
- 83. Speed Control By Pole Changing: • The change of number of poles is done by having two or more entirely independent stator windings in the same slots. • Each winding gives a different number of poles, so we will get different speeds. • Due to cost and complex switching arrangements, it is not practical to provide more than two arrangements of poles (ie, two normal speeds). • This method is applicable to squirrel cage induction motor only. • It is not practically applicable with wound rotor.
- 84. Speed control by varying Supply voltage • The speed of induction motor can be varied by changing supply voltage. • The torque developed in this method is proportional to the square of the supply voltage. T V ∝ 2 • This is the cheapest and easiest method, but it is rarely used because of the below reasons. ⮚A small change in speed requires a large change in voltage. ⮚This large change in voltage will result in a large change in the flux density.
- 85. Adding Rheostat in Stator Circuit • In this method of speed control of three phase induction motor rheostat is added in the stator circuit due to this voltage gets dropped • In case of three phase induction motor torque produced is given by T sV ∝ 2 2 . If we decrease supply voltage torque will also decrease. • But for supplying the same load, the torque must remains the same and it is only possible if we increase the slip and if the slip increase motor will run reduced speed.
- 86. Speed control by varying Rotor Resistance • This method is applicable to three-phase slip-ring induction motor only. • By introducing external resistance in the rotor circuit, the speed of the motor can be reduced. • The change in speed depends upon both rotor circuit resistance and load. • Due to power loss in the resistance ,this method is used where speed changes are required for short period only. • This method is similar to armature rheostat control method of DC shunt motors.
- 87. Speed control by Cascade Connection: •This method needs two motor, one of them is wound motor. •The two motors are mechanically coupled together to drive a common load. •The starter of slip ring induction motor is connected to three-phase supply and its rotor is connected to stator of the other machine. •There are four possible speeds can be obtained by this arrangement. Where f = supply frequency P1 = No of poles in slip ring motor P2 = No of poles in other motor
- 88. Speed control by injected EMF • nstead of applying the resistance into the rotor circuit of the motor, the speed can be varied by applying EMFs into the circuit. • These EMFs are applied at the rotor by a suitable source whose frequency should be same as slip frequency. • Inserting the EMF in phase with the rotor induced EMF is equivalent to decreasing the rotor resistance . • Inserting the EMF in phase opposition to the induced rotor EMF is equivalent to increasing its resistance. • Thus by injecting EMF into the rotor the speed can be controlled.