Presentation on theme: "Electromechanical Systems Asinchronous (induction) machines Types of machines with alternating current Types of induction machines with alternating current."— Presentation transcript:
Electromechanical Systems Asinchronous (induction) machines Types of machines with alternating current Types of induction machines with alternating current Components of asinchronous (induction) machines, (squirel cage and slip-ring induction machines) How does it works! Mathematical model Equivalent circuit Vector (phasor diagram)
Literature 1.R. Wolf: Osnove električnih strojeva, Školska knjiga, Zagreb, (72-95, , dijelovi ), in Croatian 2.B. Jurković: Elektromotorni pogoni, Školska knjiga, Zagreb, (Statička stanja elektromotornih pogona s asinkronim motorima, str.49-62), in Croatian 3. D. Ban: Mirna, pulzirajuća i okretna magnetska polja, predavanja (pogledati dodatnu literaturu na web stranicama), in Croatian
Electrical machines- types Stator with 3 phase winding Rotor, squirel cage or slip-ring type Stator with electromagnet or permanent magnet Rotor with winding, (armature winding) Stator with winding Rotor with permanent magnets Stator with winding on the pole Iron rotor; different reluctance in different axces ! Asynchro nous machine Synchro nous machine DC current machine Reluctant machine
ASINCHRONOUS (INDUCTION) MACHINES Induction machine (IM) Stator with three symetrical (balanced) distributed phases, a, b. c ( windings) air gap Rotor winding Stator windings Fig.1.Cross section of IM a), Spatial stator winding distribution, b)
ASINCHRONOUS MACHINES – industrial construction Fig.2. Two types of induction motors – industrial products
Squirel cage Induction machine (motor), IM - squirrel cage construction, the rotor winding consists of a number of rotor bars, short-cut by rings from both rotor side, see figures below ASINCHRONOUS induction machines bars ring Fig. 3. Squirrel cage rotor of induction motor, rings and bars a), squirrel cage rotor industrial product b). ring a) b)
Slip-ring asynchronous (induction, IM) machine stator is identical as squirrel cage induction motor rotor has clasical winding, not a bars usualy 3 windings (phases) on the rotor rotor winding ends connected to the stationary rings, see figure below ASINCHRONOUS induction machines rings resistors Fig. 4. Stator and rotor connections of a slip-ring a), squirrel cage rotor industrial product b).
Stator Sliced iron, slices electrically isolated from conductors (windings) placed in slots. There are 3 isolated balanced phase (windings), spaced with 120° (for 2-pole machine). 3-phase symmetrical stators winding is supplied by 3- phase symmetrical voltage supply 120° Rotor Sliced iron, slices electrically isolated from rotor conductors (windings), placed in rotor. Rotor winding is usually 3-phase, in star connection. The ends of 3- phase winding are short connected altogether from one side in one point. Three others ends of windings are usually connected, to three slip rings, see Fig. 4. Those rings are connected then on stator connection box. For squirrel cage type rotor, conductors are made from cooper (Cu) or aluminium (Al). Air gap It must be as small as possible, taking into account bearings specifications, as well as a mechanical stress. Smaller air gap resulting in small magnetizing current needed for magnetic field. That field is important for effective electromechanical conversion.
Physical concept of IM Three phase (3f) IM motor supplied from stator side by symmetrical 3f voltage supply, results with SYMMETRICAL ROTATING FIELD. This field rotate with synchronous speed s (1) Rotational field is cutting rotor conductors by relative speed s - (slip, (2), inducing in conductors (windings) voltage E 2 =s·E 20, (3) In short connected rotor winding (squirrel cage rotor) induced voltage (3) will generate current, which will together with rotational field produce tangentional force on the rotor, ie. torque. Developed torque will accelerate rotor, and after reaching desired speed, (steady state), rotor speed will be close to the synchronous speed, (1) pnumber of pole pairs (see explanation at the end) slip slip (%) (2) (1) Synchronous speed
Rotor voltage – dependence of slip When rotor is blocked (s=1, speed=0), rotational field induce in rotor winding voltage E 20, see Fig.5. When rotor start to move, relative speed is changing, as well as relative speed between rotational (stator) field against rotor, and voltage E 2 is changing according (3) When the relative speed is zero, ie. s=0, there is no voltage in rotor winding, no current, nor force, no torque!! It means that motor cannot work when s=0. Conclusion is that motor can work only when different speed between rotor and rotational speed exist!!! This phenomena define term ASINCHRONOUS MACHINE. (3) Fig.5. Rotor voltage vs rotor speed
Rotor voltage and current frequencies are depending of relative speed between rotor and rotational (stator) field. i.e. slip. Those variables have frequency determined by relative speed between rotor and rotational (stator) field. Rotor current frequency vs slip Reminder !!!!
Rotor speed– vs. slip rotor rotates with synchronous speed s = 0 rotor blocked, zero speed s=1 rotor rotates faster than rotational speed s < 0 rotor rotates opposite than rotational field speed s > 1 The sam units are used for the synchronous speed n s
Number of pole pairs- Explanation The term 1 pair poles defines the region in the stator of machine where three windings (phases) are simetrically spaced inside stator slots. It is said that the angle between axces of the phases are 120 geometricly, Fig.1. a) In the a) this space is 360, in b) it is 180 geometricly. For one supply stator voltage period, rotating field always passing 1 pair poles space!!!. That means, for one cycle T, rotating field will pass in case a) 360, but in case b) only half space, i.e. 180 geometricly Conclusion 1: rotating field speed in case a) is 2 times larger than in case b) Conclusion 2. In the machine with p-pole pairs, rotating field will pass in one T cycle 360 /p parts of machine stator space. a) 1par polovac) 2 para polovab) 1par polova
Physical process with one pole pairs machine doesnt changed increasing the number of poles. In that case, all analysis can be performed on one pair poles machines. In this case the term electrical angle ( el ), is defined and it is identical to the geometric angle ( g ) for 2-pole machine, p=1. Generally, for the case of p- pair poles machine, relation between electrical and geometric angle is (4) Number of pole pairs- Explanation
INDUCTION MACHINE – HOW DOES IT WORK Fig.6. Animation of PULSATING field Initial position of pulsating field is maximal field (maximal current) (maximal sinusoid) the circles are maximal red, vector is maximal right oriented. When the field is zero, vector is in the middle of circle (point!), "circles are red, current in conductors is zero. Next position is maximum fields in another (left) side, vector is maximal and on the left, circles are red (maximal negative current)
Fig.7. Animation of SYMMETRICAL ROTATIONAL field (black) and PULSATING fields of each phase (red, green, blue) Thru each of 3 winding SYMMETRICAL Y spaced in stators slot (namot A, B i C) flow one of the 3f currents, (delayed each other in120°). The picture shows that each of the fields are PULSATING, only the amount is changing in one position. Resulting field is ROTATIONAL field, (BLACK), the sum of pulsating fields of all 3 phases, with maximal amount 50%,greater than maximum of one phase pulsating field. INDUCTION MACHINE – HOW DOES IT WORK
Fig.8. Animation of ROTATIONAL field (black) and PULSATING fields of each of the 3 phase of IM
The principle of work is based on the force (i.e. torque) generation Torque is result of rotational field and rotor current. Rotor voltage is induced by rotational stator field Questions: Why rotor cannot reach the speed of rotational field? How rotor could reach the speed of rotational field? Explain! Fig. 9. Rotational field speed (n s ), rotor speed (n), and rotor speed relative to rotational field speed (n s -n) INDUCTION MACHINE – HOW DOES IT WORK
One phase equivalent induction machine circuit Induction machine – equivalent circuits E 1,I 1 - induced stator voltage and current U, U 1 - stator voltage (supply voltage) R 1 -stator winding (coil) resistance R 2 -rotor winding resistance X 1 -stator leakage reactance X 2 - rotor leakage reactance E 2 - inducied rotor voltage, E 20 - induced rotor voltage, (rotor locked, stator connected to suply voltage, U) f 1 - stator voltage frequency, f 2 - rotor voltage frequency, N 1, N 2 - stator and rotor number of coils
Fig.10. Equivalent circuit per phase of induction motor with rotor parameters relative to the stator side Recalculation of rotors parameters to the stator side with parameter (k) Induction machine – equivalent circuits (5)
Magnetic field generated from primary side and coupled with secondary side and magnetic field generated from secondary side and coupled with primary side are the main (coupled) magnetic field ( 12 or 21 ). Magnetic field which couple only primary winding is leakage field 1. Magnetic field which couple only secondary winding is leakage field 2. Primary winding secondary winding Main path Leackage path Explanation of the main and leakadge path - transformer Fig.11.
Induction machine_ vector-dijagram with k=1 Fig.12. Vector diagram of induction machine
Rotor current and leakage reactance Rotor current is defined by induced voltage E 2 and rotor impedance Z 2 : In standstil E 2 = E 20, see (3) This formalism can be applied on leakage reactance, X 2σ,, so, X 2σ0 is leakage reactance in standstil, n=0. Leakage reactance is defined for 50Hz (standstill), and influence of the frequency f 2 can be involved multiplying by slip s. For s=0, rotor current is I 2 (s)=0 (SYNCRONISM !!!) (6)
Assumption: Magnetic (rotating) field in the air gap induce in stator winding voltage e 1, defined by For small slip and small current (load) it can be wrote: neglect Electromagnetic torque-dependence of a voltage and frequency How torque is changing by stator voltage and frequency?
Electromagnetic torque M em can be expressed as Electromagnetic torque - derivation (10) (11)
Torque speed characteristics-derivation Primary impedance Z 1 =R 1 +jX σ1 is neglected in equivalent circuits. It shoud be emphasized that motor torque in each working point is proportional to the square of the motor voltage Machine torque dependence of voltage supply can be described using energy balance, Detailed derivation can be found in course textual material on the web pages (12) (13)
Machine torque characteristics-Kloss equation Kloss equation describes general torque-speed characteristics of induction machine. Functionally, Kloss equation involving two working points: arbitrary working point and working point with maximal slip. In the example below, developed torque at maximal and nominal (rated) torque are used for calculation Which simplification is used in Kloss-equation? (14) (15)
Torque vs speed characterestics of IM (It doesnt worth for motors less than 1kW)!! important 3 points: s= 1, n=0- standstil torque, M k s= s n, n= n n - rated (nominal) torque, M n s= s max, n= n max - maximal torque, M max (M pr ) Fig.13. Motor torque vs speed induction motor (IM) characteristics
Derivation for torque (1) – (4) has been done with assumption that recalculation factor, see (5), is k=1 From (10)–(13) it can be seen quadratic relation between torque and magnetic field (voltage). Expression (14), represent simplified Kloss-equation and can be used for slip-ring motors and squirrel-cage motors without skin effect in rotor slots. If the skin effect is present, Kloss equation (14) can be used only in the region of the small slip. Electromagnetic torque-dependence of a voltage and frequency
Fig.14. Simulation results given from mathematical model x 10 Electrical and mechanical power[kW] Ulazna (električna) snaga Izlazna (mehanička) snaga speed[rpm] speed [rpm] Electromagnetic torque [Nm] Stator current[A] speed [rpm]
P 1 is electrical power (power supply) P 2 is power on the motor shaft (mechanical Power)!!! Induction machine– energy balance Fig.15. Energy balance in induction motor
For magnetic field getting, IM taking reactive power Total power of IM is Active power (on the motor shaft!) P=P 2. m 1 is the number of phases Example of motor Data: 3f induction motor, P= 1000 kW Voltage 6000 V, frequency 50 Hz nominal speed1485,(rpm), cosφ=0,88, =0.8 nominal current 115 A Nominal data- Total, Active and Reactive power of IM
The amount of slip is directly indicator of the amount of losses in induction motors (see energy balance). It is needed to set working point in the way that slip must be very low. Nominal slip is usually between 0.1 i 5 %. Low power machine (up to cca 1kW), has larger slip. Take into account the problem of overheating.High losses means high heating, conductors isolation getting badly, it is possible dielectric breakdown! Induction motors - Slip and Losses
Working range of induction squirel cage motor s = 0 n = n s unloaded machine s = 0.01 n = 0.99 n s working region of large machines (over 100kW) s = 0.04 n = 0.96 n s working region of medium and small machines s = 1 n = 0 blocked rotor s > 1 revers current braking, pluging s < 0 generatory braking Magnetic field rotation Rotor rotation Generator braking PlugingMotoring Fig quadrant operation