Induction Motors: Induction Motors Introduction: 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 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 Construction: 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 of IM Construction: 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. Construction: Construction Squirrel cage rotor Wound rotor Notice the slip rings Construction: Construction Cutaway in a typical wound-rotor IM. Notice the brushes and the slip rings Brushes Slip rings Rotating Magnetic Field: 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 e is the supply frequency and P is the no. of poles and n sync is called the synchronous speed in rpm (revolutions per minute) Synchronous speed: 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 Rotating Magnetic Field: Rotating Magnetic Field Rotating Magnetic Field: Rotating Magnetic Field Rotating Magnetic Field: Rotating Magnetic Field Rotating Magnetic Field: Rotating Magnetic Field Rotating Magnetic Field: Rotating Magnetic Field Principle of operation: Principle of operation This rotating magnetic field cuts the rotor windings and produces an induced voltage in the rotor windings Due to the fact that the rotor windings are short circuited, for both squirrel cage and wound-rotor, and induced current flows in the rotor windings The rotor current produces another magnetic field A torque is produced as a result of the interaction of those two magnetic fields Where ind is the induced torque and B R and B S are the magnetic flux densities of the rotor and the stator respectively Induction motor speed: 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 Induction motor speed: 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 n slip = slip speed n sync = speed of the magnetic field n m = mechanical shaft speed of the motor The Slip: 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 Induction Motors and Transformers: 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 Frequency: Frequency The frequency of the voltage induced in the rotor is given by Where f r = the rotor frequency (Hz) P = number of stator poles n = slip speed (rpm) Frequency: Frequency What would be the frequency of the rotor’s induced voltage at any speed n m ? When the rotor is blocked ( s= 1) , the frequency of the induced voltage is equal to the supply frequency On the other hand, if the rotor runs at synchronous speed ( s = 0), the frequency will be zero Torque: Torque While the input to the induction motor is electrical power, its output is mechanical power and for that we should know some terms and quantities related to 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 and Horse power: Horse power Another unit used to measure mechanical power is the horse power It is used to refer to the mechanical output power of the motor Since we, as an electrical engineers, deal with watts as a unit to measure electrical power, there is a relation between horse power and watts Example: Example A 208-V, 10hp, four pole, 60 Hz, Y-connected induction motor has a full-load slip of 5 percent What is the synchronous speed of this motor? What is the rotor speed of this motor at rated load? What is the rotor frequency of this motor at rated load? What is the shaft torque of this motor at rated load? Solution: Solution Equivalent Circuit: Equivalent Circuit The induction motor is similar to the transformer with the exception that its secondary windings are free to rotate As we noticed in the transformer, it is easier if we can combine these two circuits in one circuit but there are some difficulties Equivalent Circuit: Equivalent Circuit When the rotor is locked (or blocked), i.e. s =1, the largest voltage and rotor frequency are induced in the rotor, Why ? On the other side, if the rotor rotates at synchronous speed, i.e. s = 0, the induced voltage and frequency in the rotor will be equal to zero, Why ? Where E R0 is the largest value of the rotor’s induced voltage obtained at s = 1(loacked rotor) Equivalent Circuit: Equivalent Circuit The same is true for the frequency, i.e. It is known that So, as the frequency of the induced voltage in the rotor changes, the reactance of the rotor circuit also changes Where X r0 is the rotor reactance at the supply frequency (at blocked rotor) Equivalent Circuit: Equivalent Circuit Then, we can draw the rotor equivalent circuit as follows Where E R is the induced voltage in the rotor and R R is the rotor resistance Equivalent Circuit: Equivalent Circuit Now we can calculate the rotor current as Dividing both the numerator and denominator by s so nothing changes we get Where E R0 is the induced voltage and X R0 is the rotor reactance at blocked rotor condition ( s = 1) Equivalent Circuit: Equivalent Circuit Now we can have the rotor equivalent circuit Equivalent Circuit: Equivalent Circuit Now as we managed to solve the induced voltage and different frequency problems, we can combine the stator and rotor circuits in one equivalent circuit Where Power losses in Induction machines: Power losses in Induction machines Copper losses Copper loss in the stator ( P SCL ) = I 1 2 R 1 Copper loss in the rotor ( P RCL ) = I 2 2 R 2 Core loss ( P core ) Mechanical power loss due to friction and windage How this power flow in the motor? Power flow in induction motor: Power flow in induction motor Power relations: Power relations Equivalent Circuit: Equivalent Circuit We can rearrange the equivalent circuit as follows Actual rotor resistance Resistance equivalent to mechanical load Power relations: Power relations Power relations: Power relations 1 s 1- s Example: Example A 480-V, 60 Hz, 50-hp, three phase induction motor is drawing 60A at 0.85 PF lagging. The stator copper losses are 2 kW, and the rotor copper losses are 700 W. The friction and windage losses are 600 W, the core losses are 1800 W, and the stray losses are negligible. Find the following quantities: The air-gap power P AG. The power converted P conv . The output power P out . The efficiency of the motor. Solution: Solution Solution: Solution Example: Example A 460-V , 25-hp , 60 Hz, four-pole , Y-connected induction motor has the following impedances in ohms per phase referred to the stator circuit: R 1 = 0.641 R 2 = 0.332 X 1 = 1.106 X 2 = 0.464 X M = 26.3 The total rotational losses are 1100 W and are assumed to be constant. The core loss is lumped in with the rotational losses. For a rotor slip of 2.2 percent at the rated voltage and rated frequency, find the motor’s Speed Stator current Power factor P conv and P out ind and load Efficiency Solution: Solution Solution: Solution Solution: Solution Torque, power and Thevenin’s Theorem: Torque, power and Thevenin’s Theorem Thevenin’s theorem can be used to transform the network to the left of points ‘a’ and ‘b’ into an equivalent voltage source V TH in series with equivalent impedance R TH + jX TH Torque, power and Thevenin’s Theorem: Torque, power and Thevenin’s Theorem Torque, power and Thevenin’s Theorem: Torque, power and Thevenin’s Theorem Since X M >>X 1 and X M >>R 1 Because X M >>X 1 and X M +X 1 >>R 1 Torque, power and Thevenin’s Theorem: Torque, power and Thevenin’s Theorem Then the power converted to mechanical ( P conv ) And the internal mechanical torque ( T conv ) Torque, power and Thevenin’s Theorem: Torque, power and Thevenin’s Theorem Torque-speed characteristics: Torque-speed characteristics Typical torque-speed characteristics of induction motor Comments: Comments The induced torque is zero at synchronous speed . Discussed earlier. The curve is nearly linear between no-load and full load . In this range, the rotor resistance is much greater than the reactance, so the rotor current, torque increase linearly with the slip. There is a maximum possible torque that can’t be exceeded. This torque is called pullout torque and is 2 to 3 times the rated full-load torque. Comments: Comments The starting torque of the motor is slightly higher than its full-load torque , so the motor will start carrying any load it can supply at full load. The torque of the motor for a given slip varies as the square of the applied voltage . If the rotor is driven faster than synchronous speed it will run as a generator , converting mechanical power to electric power. Complete Speed-torque c/c: Complete Speed-torque c/c Maximum torque: Maximum torque Maximum torque occurs when the power transferred to R 2 / s is maximum. This condition occurs when R 2 / s equals the magnitude of the impedance R TH + j ( X TH + X 2 ) Maximum torque: Maximum torque The corresponding maximum torque of an induction motor equals The slip at maximum torque is directly proportional to the rotor resistance R 2 The maximum torque is independent of R 2 Maximum torque: Maximum torque Rotor resistance can be increased by inserting external resistance in the rotor of a wound-rotor induction motor. The value of the maximum torque remains unaffected but the speed at which it occurs can be controlled . Maximum torque: Maximum torque Effect of rotor resistance on torque-speed characteristic Example: Example A two-pole, 50-Hz induction motor supplies 15kW to a load at a speed of 2950 rpm. What is the motor’s slip? What is the induced torque in the motor in N.m under these conditions? What will be the operating speed of the motor if its torque is doubled? How much power will be supplied by the motor when the torque is doubled? Solution: Solution Solution: Solution In the low-slip region, the torque-speed curve is linear and the induced torque is direct proportional to slip. So, if the torque is doubled the new slip will be 3.33% and the motor speed will be Example: Example A 460-V, 25-hp, 60-Hz, four-pole, Y-connected wound-rotor induction motor has the following impedances in ohms per phase referred to the stator circuit R 1 = 0.641 R 2 = 0.332 X 1 = 1.106 X 2 = 0.464 X M = 26.3 What is the maximum torque of this motor? At what speed and slip does it occur? What is the starting torque of this motor? If the rotor resistance is doubled, what is the speed at which the maximum torque now occur? What is the new starting torque of the motor? Calculate and plot the T - s c/c for both cases. Solution: Solution Solution: Solution The corresponding speed is Solution: Solution The torque at this speed is Solution: Solution The starting torque can be found from the torque eqn. by substituting s = 1 Solution: Solution If the rotor resistance is doubled, then the slip at maximum torque doubles too The corresponding speed is The maximum torque is still max = 229 N.m Solution: Solution The starting torque is now Determination of motor parameters: Determination of motor parameters Due to the similarity between the induction motor equivalent circuit and the transformer equivalent circuit, same tests are used to determine the values of the motor parameters. DC test: determine the stator resistance R 1 No-load test: determine the rotational losses and magnetization current (similar to no-load test in Transformers). Locked-rotor test: determine the rotor and stator impedances (similar to short-circuit test in Transformers). DC test: DC test The purpose of the DC test is to determine R 1 . A variable DC voltage source is connected between two stator terminals. The DC source is adjusted to provide approximately rated stator current, and the resistance between the two stator leads is determined from the voltmeter and ammeter readings. DC test: DC test then If the stator is Y-connected, the per phase stator resistance is If the stator is delta-connected, the per phase stator resistance is No-load test: No-load test The motor is allowed to spin freely The only load on the motor is the friction and windage losses, so all P conv is consumed by mechanical losses The slip is very small No-load test: No-load test At this small slip The equivalent circuit reduces to… No-load test: No-load test Combining R c & R F+W we get…… No-load test: No-load test At the no-load conditions, the input power measured by meters must equal the losses in the motor. The P RCL is negligible because I 2 is extremely small because R 2 (1- s )/ s is very large. The input power equals Where No-load test: No-load test The equivalent input impedance is thus approximately If X 1 can be found, in some other fashion, the magnetizing impedance X M will be known Blocked-rotor test: Blocked-rotor test In this test, the rotor is locked or blocked so that it cannot move , a voltage is applied to the motor, and the resulting voltage, current and power are measured. Blocked-rotor test: Blocked-rotor test The AC voltage applied to the stator is adjusted so that the current flow is approximately full-load value. The locked-rotor power factor can be found as The magnitude of the total impedance Blocked-rotor test: Blocked-rotor test Where X’ 1 and X’ 2 are the stator and rotor reactances at the test frequency respectively Blocked-rotor test: Blocked-rotor test X 1 and X 2 as function of X LR Rotor Design X 1 X 2 Wound rotor 0.5 X LR 0.5 X LR Design A 0.5 X LR 0.5 X LR Design B 0.4 X LR 0.6 X LR Design C 0.3 X LR 0.7 X LR Design D 0.5 X LR 0.5 X LR Midterm Exam No.2: Midterm Exam No.2 Example: Example The following test data were taken on a 7.5-hp, four-pole, 208-V, 60-Hz, design A, Y-connected IM having a rated current of 28 A. DC Test : V DC = 13.6 V I DC = 28.0 A No-load Test : V l = 208 V f = 60 Hz I = 8.17 A P in = 420 W Locked-rotor Test : V l = 25 V f = 15 Hz I = 27.9 A P in = 920 W Sketch the per-phase equivalent circuit of this motor. Find the slip at pull-out torque, and find the value of the pull-out torque.