Unit III A.C. Machines - Principle of operation of 3-phase Induction Motor Torque, slips characteristics- Speed control methods Single-phase Induction motor starting methods Principle of operation of Alternators. 1. Explain the construction of induction motor. General principle The conversion of electric power into mechanical power takes place in the rotating part of an electric motor. In dc motors the electrical power is conducted directly to the armature (i.e., rotating part) through brushes and commutator. Hence in this sense. a dc motor can be called a conduction motor. But in ac motors the rotor receives electric power, not by conduction but by induction.this is in the same way as the secondary of a 2 - winding transformer receives its power from the primary that is why such motors are known as induction motors. In fact an induction motor can be treated as a rotating transformer i.e., one in which primary winding is stationary but the secondary is free to rotate. Construction An induction motor consists essentially of 2 main parts (1) Stator and (2) Rotor Stator: The stator of the induction motor consists of a stationary frame and laminated steel core. The outermost part of the motor is called frame. The laminated core is bolted together with the frame and its inner periphery has slots cut in to accommodate 3 phase stator winding wound for specific number of poles. The exact number of poles for which the stator winding is wound is being determined by the requirements of speed. Greater the number of poles, lesser the speed and vice versa.
Fig.1.Stator of Induction motor The stator windings, when supplied with 3-phase currents produce a magnetic flux which is of constant magnitude, but which revolves at synchronous speed. The revolving magnetic flux induces an emf in the rotor by mutual induction. Rotor: 3ɸ induction motors are classified according to its rotor construction as 2 types, Squirrel-cage induction motor Wound-rotor or slip ring or phase wound induction motor Squirrel Cage Rotor: Almost 90% of Induction motors are squirrel - cage type because this type of rotor has the simplest and most rugged in construction imaginable and is almost indestructible. The rotor consists of a cylindrical laminated core with parallel slots for carrying rotor conductors which it should be noted clearly, are not wires but consist of heavy bars of copper, aluminium or alloys. One bar is placed in each slot, rather the bars are inserted from the end when semi-closed slots are used. The rotor bars are brazed or electrically welded or bolted to heavy and stout short circuiting end rings, thus giving us, what is so picturesquely called, a squirrel construction.
It should be noted that 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. As shown in Fig.2. the rotor bars are slightly inclined to the shaft axis due to the skew provided while stacking the rotor stampings. This is useful in 2 ways. It helps the motor to run quietly by reducing magnetic hum. 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. Fig.2.Squirrel Cage rotor Slip-Ring Rotor: Fig.3. External resistance connection
Phase wound rotor is also made of steel laminations. This type of rotor is provided with 3- phase, double layer, distributed winding consisting of coils as used in alternators. The rotor is wound for as many poles as the number of stator poles and is always wound 3- phase even when the stator is wound two phase. The 3 phases are started internally. The other 3 winding terminals are brought out and connected to 3 insulated slip rings mounted on-the shaft with brushes resting on them.these 3 brushes are further externally connected to a 3 - phase star connected rheostat. This makes possible the introduction of additional resistance in the rotor circuit during the starting period for increasing the starting torque of the motor and for changing its speed torque/current characteristics. When running under normal conditions, the slip - rings are automatically short - circuited by means of a metal collar, which is pushed along the shaft and connects 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. The other parts include the following: l. Cast iron frame 2. High quality low loss silicon steel laminated stator & rotor core 3. Stator and rotor windings 4. Shafts and bearings 5. Fans 6. Slip ring and slip ring enclosures and 7. Air gap. Advantages Cheap Simple and rugged in construction. High efficiency, as frictional losses are reduced due to the absence of brushes. Works at a reasonable and good power factor. Requires less maintenance. Does not require extra starting devices. Disadvantages For an induction motor, speed decreases with increase in load. Its speed can' t be varied without sacrificing some of its efficiency
2. Explain the production of rotating magnetic field in three phase induction motor. Principle of operation The Principle when current carrying conductors are present within a magnetic field, the conductors experience a force which causes them to effect torque on the shaft. Production of rotating magnetic field: When stationary coils, wound for 3phases, are supplied by a 3ɸ supply, a uniformly rotating magnetic flux of constant value is produced. The direction of rotation of the field depends upon the phase sequence of the supply and the speed of rotation is given by the equation. Where, f= Supply frequency in Hz. P= No.of poles N s =120f/P N s = Synchronous speed of the rotating magnetic field in rpm. When 3ɸ windings displaced in space by 120 0 are fed by 3ɸ currents, displaced in time by 120 0 they produce resultant magnetic flux, which rotates in space as if actual magnetic poles were being rotated mechanically. The flux is assumed sinusoidal due to 3ɸ windings. Let Maximum value of flux due to any one of the three phases be ɸ m. the resultant flux ɸ r at any instant is given by the vector sum of the individual fluxes, ɸ 1, ɸ 2 and ɸ 3 due to 3 phases. We will consider values of ɸ r, at 4 instants l/6 the time period apart corresponding to points marked 0,1, 2, and 3 in Fig.5.
Fig.5.(a) and (b) Case (i) θ = 0 0, point 0 The vector for ϕ 2 in Fig.6 (a) is drawn in a direction opposite to the direction assumed positive in Fig.5(b). Fig.6.(a)
Case (ii) θ = 60 0, point 1 Fig.6.(b) Case (iii) θ = 120 0, point 2 The resultant vector has further rotated clockwise through an angle of 60 0
Fig.6.(c) Case (iv) θ = 180 0, point 3 Fig.6.(d) Therefore, ɸ r = 1.5ɸ m again and has rotated clockwise through an additional angle 60. Hence, we conclude that, The resultant flux is of constant value = 3/2 ɸ m i.e., 1.5 times the maximum value the flux due to any phase The resultant flux rotates around the stator at synchronous speed N s = 120 f/p
3. Draw and explain the torque slip characteristics of three phase induction motor. Slip The difference between the synchronous speed N s and the actual speed N of the rotor is known as slip. Though it may be expressed in so many revolutions/second, yet it is usual to express it as a percentage of the synchronous speed. Actually, the term slip is descriptive of the way in which the rotor slips back from synchronism. % slip S N S N 100 N S Sometimes, N s N is called the slip speed. Obviously, rotor (or motor) speed is N = N s (1 s). It may be kept in mind that revolving flux is rotating synchronously, relative to the stator (i.e. stationary space) but at slip speed relative to the rotor. Torque-Slip Characteristics As the induction motor is located from no load to full load, its speed decreases hence slip increases. Due to the increased load, motor has to produce more torque to satisfy load demand. The torque ultimately depends on slip as explained earlier. The behaviour of motor can be easily judged by sketching a curve obtained by plotting torque produced against slip of induction motor. The curve obtained by plotting torque against slip from s = 1 (at start) to s = 0 (at synchronous speed) is called torque-slip characteristics of the induction motor. It is very interesting to study the nature of torque-slip characteristics. We have seen that for a constant supply voltage, E 2 is also constant. So we can write torque equations as, Now to judge the nature of torque-slip characteristics let us divide the slip range (s = 0 to s = 1) into two parts and analyse them independently. i) Low slip region : In low slip region, 's' is very very small. Due to this, the term (s X 2 ) 2 is so small as compared to R 2 2 that it can be neglected.
Hence in low slip region torque is directly proportional to slip. So as load increases, speed decreases, increasing the slip. This increases the torque which satisfies the load demand. Hence the graph is straight line in nature. At N = N S, s = 0 hence T = 0. As no torque is generated at N = N S, motor stops if it tries to achieve the synchronous speed. Torque increases linearly in this region, of low slip values. ii) High slip region : In this region, slip is high i.e. slip value is approaching to 1. Here it can be assumed that the term R 2 2 is very very small as compared to (s X 2 ) 2. Hence neglecting from the denominator, we get So in high slip region torque is inversely proportional to the slip. Hence its nature is like rectangular hyperbola. Now when load increases, load demand increases but speed decreases. As speed decreases, slip increases. In high slip region as T 1/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 1/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. So torque - slip characteristics has two parts, 1. Straight line called stable region of operation 2. Rectangular hyperbola called unstable region of operation. In low slip region, as load increases, slip increases and torque also increases linearly. Every motor has its own limit to produce a torque. The maximum torque, the motor can produces as load increases is Tm which occurs at s = s m. So linear behaviour continues till s = s m. If load is increased beyond this limit, motor slip acts dominantly pushing motor into high slip region. Due to unstable conditions, motor comes to standstill condition at such a load. Hence i.e. maximum torque which motor can produce is also called breakdown torque or pull out torque. So range s = 0 to s = s m is called low slip region, known as stable region of operation. Motor always operates at a point in this region. And range s = s m to s = 1 is called high slip
region which is rectangular hyperbola, called unstable region of operation. Motor can not continue to rotate at any point in this region. At s = 1, N = 0 i.e. start, motor produces a torque called starting torque denoted as T st. The entire torque - slip characteristics is shown in the below Fig. Full load torque Fig. Torque slip characteristics When the load on the motor increases, the torque produced increases as speed decreases and slip increases. The increases torque demand is satisfied by drawing motor current from the supply. The load which motor can drive safely while operating continuously and due to such load, the current drawn is also within safe limits is called full load condition of motor. When current increases, due to heat produced the temperature rise. The safe limit of current is that which when drawn for continuous operation of motor, produces a temperature rise well within the limits. Such a full load point is shown on the torque-slip characteristics torque as T F.L. The interesting thing is that the load on the motor can be increased beyond point C till maximum torque condition. But due to high current and hence high temperature rise there is possibility of damage of winding insulation, if motor is operated for longer time duration in this region i.e. from point C to B. But motor can be used to drive loads more than full load, producing torque upto maximum torque for short duration of time. Generally full load torque is less than the maximum torque. So region OC upto full load condition allows motor operation continuously and safely from the temperature point pf view. While region CB is possible to achieve in practice but only for short duration of time and not for continuous operation of motor. This is the difference between
full load torque and the maximum or breakdown torque. The breakdown torque is also called stalling torque. Therefore, T Full load < T m Generating and Braking Region When the slip lies in the region 0 and 1 i.e. when 0 s 1, the machine runs as a motor which is the normal operation. The rotation of rotor is in the direction of rotating field which is developed by stator currents. In this region it takes electrical power from supply lines and supplies mechanical power output. The rotor speed and corresponding torque are in same direction. When the slip is greater than 1, the machine works in the braking mode. The motor is rotated in opposite direction to that of rotating field. In practice two of the stator terminals are interchanged which changes the phase sequence which in turn reverses the direction of rotation of magnetic field. The motor comes to quick stop under the influence of counter torque which produces braking action. This method by which the motor comes to rest is known as plugging. Only care is taken that the stator must be disconnected from the supply to avoid the rotor to rotate in other direction To run the induction machine as a generator, its slip must be less than zero i.e. negative. The negative slip indicates that the rotor is running at a speed above the synchronous speed. When running as a generator it takes mechanical energy and supplies electrical energy from the stator. Thus the negative slip, generation action takes place and nature of torque - slip characteristics reverses in this generating region. The fig. shows the complete torque - slip characteristics showing motoring, generating and the braking region. Fig. Regions of torque - slip characteristics
4. Discuss in detail the various methods by which speed control of induction motor is achieved in stator side. Speed control A three phase induction motor is practically a constant speed machine, more or less like a dc shunt motor. However, there is one difference of practical importance between the two whereas dc shunt motors can be made to run at any speed within wide limits, with good efficiency and speed regulation, merely by manipulating a simple field rheostat, the same is not possible with induction motors. In their case, speed reduction is accompanied by a corresponding loss of efficiency and good speed regulation. The equation for rotor (motor) speed N of a 3-phase induction motor is, Different methods of speed control may be grouped under 2 main headings. Control from Stator Side: 1. By changing the applied voltage 2. By changing the supply frequency 3. By changing the number of stator poles Control from Rotor Side: l. Rotor rheostat control 2. By operating 2 motors in concatenation or cascade 3. By injection of emf in the rotor circuit Control from Stator Side: (i) By changing the applied voltage: Fig.Speed control by supply voltage variation
It is seen that the electro-magnetic torque of a polyphase induction motor is proportional to square of the supply voltage [T V 2 ]. The torque-speed characteristics of a 3 phase induction motor for varying supply voltage Advantages: are shown in Fig. from which it can be observed that for a given load, the speed of the motor can be varied within a smalf range by this method. 1. Cheap 2. Easy method Disadvantages: Applications: fan type loads. 1. A large change in voltage is required for a relatively small change in speed. 2. A large change in voltage will result in a large change in the flux density thereby seriously disturbing the magnetic conditions of the motor. 3. The developed torque reduces greatly with the reduction in supply voltage. 4. The range of speed control is very limited in the downward direction i.e.,from rated speed to lower speeds. Application of this method is restricted to very small motors, particularly to those driving (ii) By changing the number of stator poles: This method ef speed control provides change in speed at 2 or 4 discrete levels by changing the number of poles of the rotating magnetic field. The squirrel cage rotor can adjust itself to the rotating. magnetic field for different poles whereas the slip ring rotor is to be wound for the same number of poles as that of the stator winding. Hence this method of speed control is applicable only for SCIM. We know that N s = 120f/P, from this equation it becomes evident that the synchronous speed (and hence the running speed) of an induction motor could also be changed by changing lhe number of stator poles. The number of stator poles can be changed by, (a) Multiple stator winding (b) Method of consequent poles
(a)multiple stator winding The stator has 2 or more entirely independent windings with different number of poles the same slot and only one winding is energised at a time. For example, a 36 slot stator may have two, 3ϕ windings with 4 and 6 poles respectively with a supply frequency of 50Hz, 4 pole winding will give N s = (120 50)/4 = 1500 rpm and the 6 pole winding will give N s = (120 50)/6 =1000 rpm. Hence the motor can be operated at two different speed Demerits Application 1. Such a machine tends to be more costly 2. Less efficient and hence used only when absolutely necessary Elevator motor, traction motor, motors for machine tools (b) Method of consequent poles Fig. Multiple Stator winding In this method, the number of poles can he changed in the ratio 2: 1 by change in the connection of coils. The below figure shows 4 coils of one phase of a stator winding with alternate coils in series and terminals X 1, Y 1, X 2, Y 2 brought out as shown. If these terminals are connected in the order X 1 Y 1 X 2 Y 2, there will be a total of 4 poles giving a synchronous speed of 1500 rpm. If these terminals are connected in the order X 1, Y 1, X 2, Y 2 all the 4 coils will magnetise in the same direction and form N poles (directions of instantaneous current flow in coils are the same). For completion of magnetic circuit, consequent poles of opposite polarity (i.e., S) will be formed in between the N poles giving a total of 8 poles and a synchronous speed of 750 rpm. It is also possible to connect these coils in parallel for the low and high speeds.
Fig.Coils of one phase of a stator winding with alternate coils in series (iii) By changing the supply frequency The synchronous speed (and hence the running speed) of the induction motor can be changed by changing the supply frequency, f as per the equation,n s = l20f/p However, this method could only be used in cases where the induction motor happens to be the only load on the generatorsin this case, the supply frequency could be controlled by controlling the speed of the prime movers of the generators. Limitations l. The range over which the motor speed may be varied is limited by the economical speeds of the prime mover. 2. This method is rarely used. Application Electrically driven ships.
5. Discuss in detail the various methods by which speed control of induction motor is achieved in rotor side. Control from Rotor Side (i) Rotor Rheostat Control Fig. Rotor rheostat control In this method, which is applicable to slip ring motors alone the motor speed is reduced by introducing an external resistance in the rotor circuit. For this purpose, the rotor starter may be used provided it is continuously rated, this method is in fact similiar to the armature rheostat control method of dc shunt motors. Near synchronous speed (i.e., for very small slip value)t s/r 2. It is obvious that for a given torque, slip can be increased i.e., speed can be decreased by increasing the rotor resistance R 2 Disadvantages With increase in rotor resistance, I 2 R losses also increase, which decrease the operating efficiency of the motor. In fact, the loss is directly proportional to the reduction in the speed. Double dependence of speed, not only on R 2, but on load as well. Because of the waste fullness of this method, it is used where speed changes are needed for short periods only. (ii) Cascade / Concatenation / Tandem operation This method requires 2 motors, the first of which must have a wound rotor. It should also have a one-to-one voltage ratio. So that in addition to cascading, each motor may be run from the supply mains separately. That is, at standstill with rotor circuit open, the voltage across the sliprings should be equal to that across the stator terminals. The stators of both motors should be wound for the same voltage. The second motor may be of the squirrel cage type or have a wound rotor with external resistance. The rotor shafts are directly coupled, so that both run at the same speed.
Fig.Cascade /Concatenation /Tandem operation The stator of the first motor A is connected to the 3ϕ supply. Then the rotor of the first motor is connected to the stator of the second motor B. The starting resistance is connected to the rotor circuit of the second motor. In cascade method, there are 4 ways to obtain different speeds by the combination of the motors. Motor 1 may be run separately from the supply, Synchronous speed, N SA = 120f/P A [for motor A] Where, f= supply frequency P A = no. of stator poles of the motor A Motor 2 may be allowed to run separately from the supply, Synchronous speed of motor B, N SB = 120f/P B [for motor B] Where, P B = no. of stator poles of the motor B Cumulative Cascade Motor A and motor B are allowed to operate in cumulative cascade. In this, the stator fields of the motor A and B are having the phase rotation in the same direction. The synchronous speed of the cascade is
Differential Cascade In this case, the rotating magnetic fields of motors A and B are in opposite direction i.e, the phase rotation of stator field of A and B are opposing. This reversal of phase rotation is obtained by inter-changing any of its 2 leads. In this case, the synchronous speed obtained is, (iii) Injecting an emf in the rotor circuit In this method, the speed is controlled by Injecting a voltage in the rotor circuit. It being of course, necessary for the injected voltage to have the same as frequency as the slip frequency. There is however no restriction as to the phase of the injected emf. When we insert a voltage which is in phase oeposition to the induced rotor emf it amounts to increasing the rotor resistance, whereas inserting a voltage which is in phase with the induced rotor emf is equivalent to decreasing its resistance. Hence, by changing the phase of the injected emf and hence the rotor resistance, the speed can be controlled. (a) Kramer System Fig. Kramer system This scheme is used in the case of large motors of 4000 kw or above. It consists of rotary converter C which converts the low slip frequency ac power into dc power, which is used to drive dc shunt motor D, mechanically coupled to the main motor M.
The main motor is coupled to the shaft of the dc shunt motor D. The slip rings of M are connected to those of the rotary converter C. The dc output of C is used to drive D. Both C and D are excited from the dc bus bars or from an exciter. There's a field regulator which governs the back emf E b of D and hence the dc potential at the commutator of C which further controls the slip ring voltage and therefore the speed of M. Advantages Any speed, within the working range, can be obtained instead of only 2 or 3. If the rotary converter is over excited, it will take a leading current which compensates for the lagging current drawn by main motor M and hence improves the pf of the system. (b) Scherbius scheme In this case, the slip energy is not converted into de and then fed to a de motor, rather it is fed directly to a special 3-phase ac commutator motor called a scherbius machine. The polyphase winding of machine C is supplied with the low-frequency output of machine M through a regulating transformer (RT). The commutator motor C is a variable speed motor and its speed (and hence that of M) is controlled by either varying the tapping on RT or by adjusting the position of brushes on C. Fig.Scherbius scheme
6. Explain the working principle of single phase induction motor with circuit diagram and phasor diagram. Single phase induction motors Single phase motors are fractional kw motors, which are used for a number of applications such as ceiling fans, mixers, portable drilling machines refrigerators, hair dryers, sewing machines, vacuum cleaners etc. It has very much lower efficiency and lower pf compared to 3ϕ induction motors. Also, it weighs more, occupies more space and costs more per rated output compared to 3ϕ motors. Single phase induction motors are classified as, (a) Split-phase motors (b) Capacitor motors (c) Shaded pole motors Construction: Fig.Plain single phase induction motor Constructionally, this motor is more or less similiar to a polyphase induction motor except that 1. Its stator is provided with a single phase winding. 2. A centrifugal switch is used in some types of motors in order to cut out a winding used only for starting purposes It has distributed stator winding and a squirrel cage rotor. When fed from a single-phase supply its stator winding produces a flux / field which is only alternating.i.e. one which alternates along
one space axis only. It is not a synchronously revolving/rotating flux, as in the case of a 2 or a 3 phase stator winding fed from a 2 or 3phase supply. Now alternating or pulsating flux acting on a stationary squirrel cage rotor can't produce rotation (only a revolving flux can). That's why a single-phase motor is not self starting. However, if the rotor of such a machine is given an initial start by hand (or by small motor) or otherwise in either direction, then immediately a torque arises and the motor accelerates to its final speed. This peculiar behaviour of the motor has been explained below with the help of double -field revolving theory. Double-field Revolving Theory: This theory makes use of the idea that an alternating uni-axial quantity can be represented by 2 oppositely - rotating vectors of half magnitude. Accordingly, an alternating sinusoidal flux can be represented by 2 revolving fluxes, each equal to half the value of the alternating flux and each rotating synchronously in opposite direction. As shown in Fig.(a) let the alternating flux have a maximum value of ϕ m. Its component fluxes A and B will each be equal to ϕ m /2, revolving in anticlockwise and clockwise directions respectively. After some time, when A and B would have rotated through angel +θ and θ in fig.(b).the resultant flux would be = 2 ϕ m /2 cos 2θ/2 = ϕ m cosθ After a quarter cycle of rotation, fluxes A and B will be oppositely directed as shown in Fig.(c) so that the resultant flux will be zero [Fig.(d)]. After half a cycle, fluxes A and B will have a resultant of -2 ϕ m /2 = - ϕ m. After 3 quarters of a cycle, again the resultant is zero, as shown in Fig.(e). and so on.
If we plot the values of resultant flux against θ between limits θ = 0 0 to θ = 360, then a curve similiar to the one shown in Fig.(f) is obtained. That is why an alternating flux can be looked upon as composed of 2 revolving fluxes, each of half the value and revolving synchronously in opposite directions. Making Single-phase Induction Motor Self-starting As discussed above, a single-phase induction motor is not self-starting. To overcome this drawback and make the motor self-starting, it is temporarily converted into a two-phase motor during starting period. For this purpose, the stator of a single-phase motor is provided with an extra winding, known as starting (or auxiliary) winding, in addition to the main or running winding. The two windings are spaced 90 0 electrically apart and are connected in parallel across the single-phase supply as shown in Fig. lt is so arranged that the phase-difference between the currents in the two stator windings is very large (ideal value being 90 0 ). Hence. the motor behaves liken two phase motor. These two currents produce a revolving flux and hence make the motor self-starting.
7. Explain detail about the starting of 1 phase induction motor. Starting Methods for Single-Phase Induction Motors Single-phase induction motor inherently has no starting torque, and it must be started by an auxiliary winding, by being displaced in phase position from the main winding, or by some similar device. Once started by auxiliary means, the motor will continue to run. Thus, nearly all single-phase induction motors are actually two-phase motors, with the main winding in the direct axis adapted to carry most or all of the current in operation, and an auxiliary winding in the quadrature axis with a different number of turns adapted to provide the necessary starting torque. The various forms of a single-phase induction motor are grouped into three principal types, depending on how they are started. 1. Split-phase or resistance-split-phase motors Split-phase motors have two stator windings (i) a main winding and (ii) an auxiliary winding with their axes displaced 90 (electrical) in space. R The auxiliary winding has a higher X ratio than the main winding. The motor is equivalent to an unbalanced two-phase motor. The rotating stator field produced by the unbalanced two-phase winding currents causes the motor to start. The auxiliary winding is disconnected by a centrifugal switch or relay when the motor comes up to about 75% of the synchronous speed. (a) (b) Figure: Split-phase motor. (a) Schematic diagram. (b) Phasor diagram at starting.
2.Capacitor motors (C) Figure: Split-phase motor. (c) Typical torque speed (or slip) characteristic. Capacitor motors have a capacitor in series with the auxiliary winding and come in three varieties:(i) capacitor start, (ii) two-value capacitor, and (iii) permanent-split capacitor. The first two use a centrifugal switch or relay to open the circuit or reduce the size of the starting capacitor when the motor comes up to speed. A two-value-capacitor motor, with one value for starting and one for running, can be designed for optimum starting and running performance; the starting capacitor is disconnected after the motor starts. The relevant schematic diagrams and torque speed characteristics are shown in Figures i,ii, and iii. Motors in which the auxiliary winding and the capacitor are not cut out during the normal running conditions operate, in effect, as unbalanced two-phase induction motors. (a) (b)
(c) Figure(i): Capacitor-start motor. (a) Schematic diagram. (b) Phasor diagram at starting. (c) Typical torque speed characteristic Fig ii: Two-value-capacitor motor (a) Schematic diagram. (b) Typical torque speed characteristic
(a) (b) Fig.iii: Permanent-split-capacitor motor(a) Schematic diagram. (b) Typical torque speed characteristic. 3. Shaded-pole motors The auxiliary winding consists of one short-circuited copper straps (shading bands) wound on a portion of the pole and displaced from the center of each pole, as shown in Fig.iv(a). Induced currents in the shading coil cause the flux in the shaded portion of the pole to lag the flux in the other portion in time. The result is then like a rotating field moving in the direction from the unshaded to the shaded portion of the pole. A low starting torque is produced. A typical torque-speed characteristic is shown in Fig.iv(b). Shaded-pole motors have a rather low efficiency. (a) (b) Fig.iv: Shaded-pole motor. (a) Schematic diagram. (b) Typical torque speed characteristic.
8. Explain the construction and working principle of alternator. Alternator The machine which produces 3 phase power from mechanical power is called an Alternator or Synchronous generator. An alternator works on the same fundamental principle of electromagnetic induction as a dc generator i.e., when the flux linking a conductor changes, an EMF is induced in the conductor. Like a DC generator, an alternator also has an armature winding and a field winding. But there is one important difference between the two. In a DC generator, the armature winding is placed on the rotor in order to provide a way of converting alternating voltage generated in the winding to a direct voltage at the terminals through the use of a rotating commutator. The field poles are placed on the stationary part of the machine. Construction In any alternator standard construction consists of armature winding mounted on a stationary element called stator and field winding on a rotating element called rotor. The frequency of output ac voltage of a synchronous generator is directly proportional to the rotor speed. To maintain the frequency constant, the rotor must always move at synchronous speed. An alternator has 3 phase winding on the stator and a dc field winding on the rotor Stator It is the stationary part of the machine and is built up of sheet-steel laminations having slots on its inner periphery. A 3 phase winding is placed in these slots and serves as the armature winding of the alternator.
Rotor The rotor carries a field winding which is supplied with direct current through slip rings by a separate dc source. Rotor construction is of two types, namely, 1. Salient (or) projecting pole type 2. Non-salient pole (or) cylindrical type. Salient pole type The rotor of this type is used almost entirely for slow and moderate speed alternators, since it is least expensive. Salient poles cannot be employed in high speed generators on account of very high peripheral speed and the difficulty of obtaining sufficient mechanical strength. The salient poles are made of thick steel laminations riveted together and are fixed to rotor by a dove-tail joint. The pole faces are usually provided with slots for damper windings. These dampers are useful in preventing hunting. The pole faces are so shaped that the radial air gap length increases from the pole centre to the pole tips so that the flux distribution over the armature is sinusoidal and waveform of generated emf is sinusoidal. The field coils are placed on the pole-pieces and connected in series. The ends of the field windings are connected to a dc source through slip-rings carrying brushes and mounted on the shaft of the field structure. The salient-pole field structure has the following special features, i) They have large diameter and short axial length. ii) The pole shoes cover about 2/3 of pole pitch. iii) Poles are laminated in order to reduce eddy current losses. iv) They are employed with hydraulic turbines or diesel engines. The speed is from120 to 400 rpm.
Fig. Salient pole rotor Fig. Smooth cylindrical rotor Smooth cylindrical or non-salient pole type The rotors of this type are used in very high speed alternators driven by steam Turbines. To reduce the peripheral velocity, diameter of the rotor is reduced and axial length is increased. Such rotors have two or four poles. It consists of cylindrical steel forging which is suitably fabricated mechanically and treated thermally. The forging has radial slots in which the field copper, usually in strip form is placed. The coils are held in place by steel or bronze wedges and the coil ends are fastened by metal rings. The slots over certain portions of the core are omitted to form pole faces. The regions forming the poles are usually left unslotted as shown in figure. The non salient pole field structure has the following special features, i) They are of small diameter and of very long axial length. ii) Less windage loss. iii) The speed employed is from 1000 to 3000 rpm iv) Better in dynamic balancing and quieter in operation.
Working principle of an Alternator