Induction Machines PDF

Induction Machines Contents  Introduction  Construction  Rotating magnetic field  Principles of operation  Synchronous speed and slip  Speed control of IM  Starting of IM 01/23/25 2  Introduction  The induction machine is the most rugged and t...

Induction Machines Contents  Introduction  Construction  Rotating magnetic field  Principles of operation  Synchronous speed and slip  Speed control of IM  Starting of IM 01/23/25 2  Introduction  The induction machine is the most rugged and the most widely used machine in industry.  The induction machine has a stator and a rotor mounted on bearings and separated from the stator by an air gap.  However, in the induction machine both stator winding and rotor winding carry alternating current.  The alternating current (ac) is supplied to the stator winding machine.  The induction machine can operate both as a motor and as a generator.  However, it is seldom used as a generator supplying electrical power to a load. 01/23/25 4  cont’d…  The performance characteristics as a generator are not satisfactory for most applications.  The induction machine is extensively used as a motor in many applications.  Of all the a.c motors the poly-phase induction motor is the one which is extensively used for various kinds of industrial drives.  It has the following main advantages and also some disadvantages. 01/23/25 5  cont’d… Advantages: 1.It has very simple and extremely rugged, almost unbreakable construction (especially squirrel cage type) 2.Its cost is low and it is very reliable 3.It has sufficiently high efficiency. In normal running condition, no brushes are needed, hence frictional losses are reduced. 4.It has a reasonably good power factor 5.it requires minimum of maintenance 6.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. 01/23/25 6  cont’d… Disadvantage 1.Its speed cannot be varied without sacrificing some of its efficiency. 2.Just like a d.c. shunt motor, its speed decreases with increase in load 3.The speed is not easily controlled 4.Large starting current 5.They run at low and lagging power factor when lightly loaded. 6.Its starting torque is somewhat inferior to that of a d.c shunt motor 01/23/25 7  cont’d…  The induction motor is used in various sizes:  Large three-phase induction motors (in tens or hundreds of horsepower) are used in pumps, compressors, paper mills, textile mills and so forth.  Small single-phase induction motors (in fractional horsepower rating) are used in many household appliances, such as blenders, lawn mowers, juice mixers, washing machines, refrigerators, and stereo turntables, fans.  The linear version of the induction machine has been developed primarily for use in transportation systems. 01/23/25 8  Construction  An induction motor has two main parts Stator Rotor  A stationary stator - consisting of a steel frame that supports a hollow, cylindrical core - core, constructed from stacked lamination sheets 0.4 - 0.5 mm thickness (why?), having a number of evenly spaced slots, providing the space for the stator winding Stator of IM 01/23/25 9  cont’d…  Laminations are insulated from each other by means of varnish coating or oxide.  It is made up of a number of stampings which are slotted to receive the windings.  A three-phase winding is put in slots punched out on the inner surface of the stator frame.  The stator carries a 3-phase winding and is fed from a 3-phase supply.  It is wound for a definite number of poles, the number of poles being determined by the requirements of speed.  Greater the number of poles, lesser the speed and vice versa.. 01/23/25 10  Cont’d…  The stator windings, when supplied with 3-phase currents , produce a magnetic flux which is of constant magnitude but which revolves ( or rotates) at synchronous speed.  This revolving magnetic flux induces an emf in the rotor by mutual induction  A revolving rotor - composed of punched laminations, stacked to create a series of rotor slots punched out on the outer surface, providing space for the rotor winding - The windings used in the rotor can be 01/23/25 11 Cont’d… - 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) - The frequency of the rotor flux is very low; as a result thicker laminations can be used without excessive iron losses. 01/23/25 12 Cont’d…  Stator and rotor laminations 01/23/25 13  Cont’d…  Two types of rotor construction is normally used for three phase induction motor, depending on the rotor design. - Squirrel-cage - Wound-rotor  squirrel-cage rotor: - Almost 90 percent of induction motors are squirrel-cage type, because this type ‘rotor' has the simplest and most rugged construction and is almost indestructible. - conducting bars of copper, aluminum or alloys are laid into slots and shorted at both ends by shorting rings. - the rotor bars are permanently short-circuited on themselves, hence it is not possible to add any external resistance in series with the rotor circuit for starting purposes. 01/23/25 14 Cont’d… - The rotor slots are usually not quit parallel to the shaft but are purposely give a slight skew. - This is useful in two ways : it helps to make the motor run quietly by reducing the magnetic hum and it helps in reducing the locking tendency of the rotor i.e. the tendency of the rotor teeth to remain under the stator teeth due to direct magnetic attraction between the two.  This type of induction motor is  Simple in construction,  Low cost,  Robust,  Requires low maintenance, etc 01/23/25 15 Cont’d… Cage type of rotor 01/23/25 16 Cont’d… Cut away view of squirrel cage induction motor 01/23/25 17  Cont’d…  wound-rotor:  It is provided with a complete set of three-phase windings exactly as the stator.  The three phases are Usually Y-connected, the ends of the three rotor wires are connected to 3 insulated slip rings mounted on the rotor shaft with brushes resting on them.  In this way, the rotor circuit is accessible in order to connect externally to a three phase star connected rheostat  This makes possible the introduction of additional resistance in the rotor circuit during the starting period, which helps for. increasing the starting torque decreasing starting current and changing its speed-torque characteristic. 01/23/25 18 Cont’d… 01/23/25 19 Cont’d…  When running under normal condition , the slip-rings are automatically short-circuited by means of a metal collar which is pushed along the shaft and connect all the rings together.  Next, the brushes are automatically lifted from the slip-rings to reduce the frictional losses and the wear and tear.  Hence , it is seen that under normal running conditions, the wound rotor is short –circuited on itself just like the squirrel-cage rotor.  This type of motor is  Easy to control its speed, Wound rotor  More expensive 01/23/25 Notice the 20 slip rings Comparison of squirrel cage and wound rotors  The squirrel cage motor has the following advantages as compared with the wound rotor machine - No slip rings, brush gear, short circuiting devices, rotor terminals for starting rheostats are required. The star delta starter is sufficient for staring. - It has slightly higher efficiency. - It is cheaper and rugged in construction. - It has better space factor for rotor slots, a shorter overhang and consequently a smaller copper loss. - It has bare end rings, a larger space for fans and thus the cooling conditions are better. - It has smaller rotor overhang leakage which gives a better power factor and greater pull out torque and overload capacity. 01/23/25 21 Cont’d… - The greatest disadvantage of squirrel cage rotor is that it is not possible to insert resistance in the rotor circuit for the purpose of increasing the starting torque. - The cage rotor motor has a smaller starting torque and larger starting current as compared with wound rotor motor. 01/23/25 22  Principle of operation  When the 3-phase stator winding are fed by a 3-phase supply, then a magnetic flux of constant magnitude but rotating at synchronous speed , is set up.  This rotating magnetic field, pass through the air gap and cuts the rotor windings.  Due to the relative speed between the rotating flux and the stationary rotor conductors, an emf. is induced in the latter according to Faraday’s laws of electro-magnetic induction  Its magnitude is proportional to the relative speed between the flux and the conductors and its direction is given by Fleming’s Right-hand rule 01/23/25 23  Induction motor speed and slip  As discussed in the previous topic, the stator magnetic field with constant magnitude rotates at synchronous speed which is given as: P 50 Hz 60 Hz 120 f e nsync  rpm 2 3000 3600 p 4 1500 1800 6 1000 1200  It is dependent on the 8 750 900 no. of poles and frequency 10 600 720 12 500 600 Table: Synchronous speed for different no. of poles 01/23/25 24  Induction motor speed  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 as a result 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 01/23/25 25  Cont’d…  Therefore, the IM will always run at a speed lower than the synchronous speed  The difference between the motor speed and the synchronous speed is Known as the Slip speed nSlip nsync  nm Where:  nslip= slip speed  nsync= speed of the magnetic field (synchronous speed)  nm = mechanical shaft speed of the motor 01/23/25 26  Slip  The other term used to describe the relative motion is slip, which is the relative speed expressed on a per-unit or a percentage basis. That is, slip is defined as: nslip S x100% nsync Where S is the slip nsync  nm S x100% nsync  This equation can also be expressed in terms of angular velocity w (radians per second) as:   S sync m x100% sync 01/23/25 27  Cont’d…  Notice that : if the rotor runs at synchronous speed S 0  If the rotor is stationary S 1  It is possible to express the mechanical speed of the rotor shaft in terms of synchronous speed and slip as: nm 1  S  nsync Or m 1  S sync  These equations are useful in the derivation of induction motor torque and power relationships 28 01/23/25 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 01/23/25 29  Starting of IM  Most induction motors large and small are rugged enough that they could be started across the line without incurring any damage to the motor windings, although about five to eight times the rated current flows through the stator at rated voltage at standstill.  However, in large induction motors, large starting current are objectionable in two respects:  First, the mains supplying the induction motor may not be of a sufficiently large capacity.  Second, because of large starting current, the voltage drops in the lines may be excessive, resulting in reduced voltage across the motor.  Because the torque varies approximately as the square of the voltage, the starting torque may become small at the reduced line voltage that the motor might not even start on load. 01/23/25 30 Cont’d…  Thus we formulate the basic requirement for starting:  The line current should be limited by the capacity of the mains, but only to the extent that the motor can develop sufficient torque to start (on load , if necessary).  A number of methods is available of for starting both cage-rotor and wound-rotor motors: Starting of squirrel-cage motors  For cage motors, the choice of any particular method of starting depend on: Ø on size and design of the motor Ø capacity of the power lines and Ø type of the driven load. 01/23/25 31  Cont’d…  There are primarily two methods of starting of squirrel-cage induction motors: a) full-voltage starting and b) reduced-voltage starting  The full-voltage starting consists of DOL (direct-on-line) starting only.  The reduced-voltage starting has the advantage of reducing the starting current, but it produces an objectionable reduction in the starting torque, on account of the fact that torque is proportional to square of voltage.  Despite this, reduced-voltage starting is the most popular method of starting three-phase squirrel-cage induction motors and consists of  stator resistor (or reactor) starting,  auto-transformer starting and  star-delta starting. 01/23/25 32  Cont’d… Starting of wound rotor motors The methods used for starting squirrel-cage motors can also be employed for starting wound-rotor motors, but it is usually not done so because then the advantages of wound-rotor induction motors can't be fully realized. The simplest and cheapest method of starting wound-rotor induction motors is by means of added rotor resistance, with full-line voltage across the stator terminals. It has already been discussed that at the time of start, the addition of external resistance in the rotor circuit of a wound-rotor induction motor  decreases its starting current  increases its starting torque (for a suitable external resistance) and  improves its starting power factor. 01/23/25 33  Cont’d…  At the time of start, the entire external resistance is added in the rotor circuit.  As the rotor speeds up, the external resistance is decreased in steps so that motor torque tends to remain maximum during the accelerating period.  Finally, under normal operation, the external resistance is fully cut off and the slip rings are short-circuited so that motor now develops full- load torque at low value of slip for which it is designed. 01/23/25 34  Transformer 01/23/25 35