Presentation on theme: "1. Induction Machines 1.1 Introduction"— Presentation transcript:
11. Induction Machines 1.1 Introduction • Induction machines are rotating electromechanicalenergy converting devices.• Their operating principle is similar to all rotatingelectrical machines.
2Operating principle of all rotating electrical machines is based on the two electromagnetic laws. Generator actione = BLV, voltsWhere,e - induced emf , vB - magnetic field density, wb L - length of the conductor ,mV - velocity of the conductor, m/sB, L,V, are orthogonal to eachother.2. Motor actionF = BLI, Nwhere,I - current , AFor rotating object, torque is expressed as:-T = Fxr = BLIr, N-mr - Radius of the rotatingobject.
3• An induction machine is an ac machine which can work as an induction generator and as induction motor.• Application of induction machine as a generator is limited to some special purposes, whereas induction motors are widely used for various industrial and domestic applications. They are the Workhorses of industries.• Generally induction motors are of two types1. Poly phase IM. (usually 3-phase)2. Single phase IM.
4It has low cost compared with other motors of the same power out put. Some Advantages and Disadvantages In using IMAdvantages:Its losses are reduced.It has a very simple and extremely rugged, almost unbreakable construction (especially squirrel cage type), thus requires minimum of maintenance.It has low cost compared with other motors of the same power out put.It has sufficiently high efficiency. Squirrel cage types use no brushes, hence frictional losses are minimum and reasonably good power factor.It starts up from rest and needs no extra starting motor and has not to be synchronized. Its starting arrangement is simple especially – for squirrel- cage type motor.DisadvantageIts speed cannot be varied without sacrificing some of its efficiency.Just like a d.c. shunt motor, its speed decreases with increase in loadIts starting torque is somewhat inferior to that of a d.c shunt motor
52. Construction of poly phase Induction Motor A typical motor consists of two parts:1- An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field,2- An inside rotor attached to the output shaft that is given a torque by the rotating field.- The rotors of induction motors are of twotypes:-- squirrel cage rotor.- Wound rotor
9•Stator construction– Stator of an IM consists of:-- stator frame, stator core, 3-phase/single phasedistributed winding, two end covers, bearings,etc.It is a stack of steel laminations (0.35-o.5 mm thick) with slots similar to a stator of a synchronous machine. –Coils are placed in the slots to form a three or single phase winding.
12INDUCTION MOTORS MAGNETIC CIRCUIT Stator iron coreStatorSlotsRotor iron coreRotorSlot
13Squirrel-Cage Rotor–Rotor is made from punched laminations ( mm thick) of steel core with slots to provide rotor windings.–Metal (Aluminum) bars are molded in the slots instead of a winding.–Two rings short circuits the bars.–Most of single phase induction motors have Squirrel-Cage rotor.–One or 2 fans are attached to the shaft in the sides of rotor to cool the circuit.
14Squirrel cage Rotor of a large induction motor. (Courtesy Siemens).
15Star-Delta starter is sufficient for its starting Advantages of squirrel cage rotorNo slip rings, brushes, brush holders, & rotor terminals; thus less operating troubles.Star-Delta starter is sufficient for its startingIts construction is robust and cheepIt has higher efficiencyRotor to slots space factor is better, shorter overhang, thus smaller copper lossWith bare end rings, it has better ventilation opportunity.With smaller overhang leakage, it has better power factor, greater pull-out torque and overload capacity.
16DisadvantagesIt is not possible to insert external resistance- to increase starting torque and- to decrease starting currentThey have small starting torque but large starting current as compared to the wound rotor.From the rotor side, speed control is impossible.
17Wound Rotor It is usually for large 3 phase induction motors. • Rotor has a winding the same as stator and the end of each phase is connected to a slip ring.• Three brushes contact the three slip-rings to three connected resistances (3-phase Y) for reduction of starting current and speed control.• Wound rotor induction motor was the standard form for variable speed control before the advent of semiconductor devices.
193-phase wound –rotor induction motor with external starting rheostat.
20Advantages of wound rotor Disadvantages The locked-rotor current can be reduced by inserting external resistances in series with the rotor windingsThe speed can be varied by varying the external resistances.The wound rotor motor is ideally suited to accelerate high inertia loadsDisadvantagesHas slip rings, brushes, brush holders, & rotor terminals; thus greater operating troubles and require frequent maintenance.It has large overhang leakage, more copper loss, thus, less power factor and inferior efficiency compared to squirrel cage motor.Compared to squirrel cage rotors, wound rotor motors are expensive, so it is not so common in industry applications
21Basic operating principles • An AC current is applied in the stator armaturewhich generates a flux in the stator magneticcircuit.• This flux induces an emf in the conducting bars of rotor as they are “cut” by the flux while the magnet is being moved (E = BVL (Faraday’s Law))• A current flows in the rotor circuit due to theinduced emf, which in tern produces a force,(F = B I L ) can be changed to the torque as theoutput.
22Rotating Magnetic Field Operation of an IM is based on the development and existence of rotating magnetic field.The 3 windings in the stator of a 3-phase IM are positioned from each other by 120o electrical.• When a balanced three-phase voltage is applied to the stator windings, currents ia, ib and ic, each of equal magnitude, but differing in phase by 120° flow in the stator winding.• Each phase current produces a magnetic flux and there is physical 120 °shift between each flux.
23The total flux in the machine is the sum of the three fluxes. The summation of the three ac fluxes results in a rotating resultant flux, which turns with constant speed and has constant amplitude.• Such a magnetic flux produced by balanced three phase currents flowing in thee-phase windings is called a rotating magnetic flux (RMF).RMF rotates with a constant speed (Synchronous Speed).• Existence of a RFM is an essential condition for the operation of an induction motor.
24Graphical analysis of resultant rotating magnetic field Positive direction of fluxFlux wave formLet the maximum value of flux of each phase be mThe resultant flux r, at any instant, is given by thevector sum of the individual fluxes A, B and c
25Let us consider values of r at four instants 1/6 time- period apart, corresponding to points marked 0,1,2 and 3.i) when = 0o, i.e. corresponding to point 0,
26ii) When = 600 , i.e. corresponding to point 1,
27iii) When = 1200, i.e. corresponding to point 2
28iv) When = 1800, i.e. corresponding to point 3,
29SUMMARYIn all four cases, it is found that the resultant flux is1.5 m ; but has rotated clockwise through an angle of 600 in each of the case.
30The resultant flux is of constant value ; i. e. 1 The resultant flux is of constant value ; i.e. 1.5 times the maximum value of the flux of each phase.The resultant flux rotates around the stator at synchronous speed given by:-Where, f - is supply frequencyP – is number of poles
31Operating Principles of Induction Motors Rotating magnetic field andOperating Principles of Induction Motors1. If stator is energized by an ac current, RMF is generated due to the applied current to the stator winding.2. This flux produces magnetic field and the field revolves in the air gap between stator and rotor.3. So, the magnetic field induces a voltage in the short-circuited bars of the rotor. This voltage drives current through the bars.4. The interaction of the rotating flux of the stator and the flux in the rotor developed by the rotor current generates a force that drives the motor and a torque is developed consequently.5. The torque is proportional with the flux density and the rotor bar current (F = BLI, N ).6. The direction of the rotation of the rotor is the same as the direction of the rotation of the revolving magnetic field in the air gap7. The rotor speed is less than the synchronous speed of the rotating magnetic field. WHY?
32An induction motor running at no load will have a speed very close to synchronous speed and therefore e.m.f. in the rotor winding will be very small .This small e.m.f. gives a small current producing a torque just sufficient to overcome the losses such as due to friction and windage and maintain the rotor in rotation.As the mechanical load is applied on the motor shaft, it must slow down because the torque developed at no load will not be sufficient to keep the rotor revolving at the no load speed against the additional opposing torque of load.As the motor slows down, the relative motion between the magnetic field and the rotor is increasedThis results in greater rotor e.m.f., rotor current and greater developed torque. Thus, as the load is increased, the motor slows down until the relative motion between the rotor and the rotating magnetic field is just sufficient to result in the development of the torque necessary for that particular load.
33SlipIn practice the rotor never succeeds in catching up with the stator field. If it really did so, then there would be no relative speed between the two hence no rotor e.m.f. no rotor current and so no torque to maintain rotation. It is due to this relative motion that torque is developed.The difference between the synchronous speed Ns and the actual speed N of the rotor is known as slip speed.ns = Ns - Nand, the relative speed expressed in % is called slip.Where, Ns – synchronous speed of RMFN – speed of the rotorS% - slip
34N0tice that, if the rotor turns at synchronous speed, S = 0; while if the rotor is stationary, S = 1.All normal motor speeds fall some what between these two limits.Thus, S = (1-nrot/nsync)100%nrot = (1-S) nsyncand, ωrot = (1-S) ωsync