ACTION OF THE COMMUTATOR

                                                    ACTION OF THE COMMUTATOR      

      A commutator performs two important functions in a DC. machine-the first one is to convert alternating quantities to direct quantities or vice-versa and the second function is to keep the rotor or armature m.m.f. stationary in space. The first commutator function of rectifying the alternating quantities in the armature winding to direct quantities at the output brushes Ofa D.C. generator, has already been explained in Art. 3.4.4. In case of a DC. motor, the direct quantities at the brushes are converted to alternating quantities (voltage and current) in the armature winding. Thus a commutator can act both as a convertor or rectifier (AC. to DC.) and as an inverter (DC. to A.C.).. 

        The stator of the DC. machine carries salient poles, which produce stationary air gap flux. For the development of electromagnetic torque, the armature m.m.f. wave must also be stationary in space. The commutator in a DC. machine carries out this second important function of rendering the armature m.m.f. wave stationary in space, inspite of the armature rotation. In the 'present article, this action of the commutator is examined in detail. 

         Fig. 4.3 shows a 2-pole D.C. machine, with four full-pitched coils housed in 4 slots. The four commutator segments, numbered 1 to 4, are insulated from each other and from the shaft. The centre line of the poles around which the field coil (or coils) is wound, is called the direct axis of the machine. The shape of the coil end-connections, makes it necessary to place the two carbon brushes along the direct axis or field axis, as depicted in Fig. 4.3. 

         In an actual D.C. machine, there are a large number of coils and slots and the number of poles is usually greater than 2, except for small D.C. machines. Therefore the DC. machine illustrated in Fig. 4.3 differs much from a real machine, but it serves well in explaining the commutator action. It may also be noted that the brushes make contact with the outside periphery of the commutator, though in Fig. 4.3, these are shown inside the commutator segments, for the sake of clarity. 

        Coil sides in the slots are represented by small circles with crosses and dots in them, indicating currents away and towards the reader, respectively. In Fig. 4.3 (a), there are two parallel paths between the two brushes. One parallel path may be traced as follows-terminal A to right hand carbon brush and commutator segment 1, upper coil-side in slot 1, back end connection 1, lower coil-side in slot 3, commutator segment 2, upper coil-side in slot 2, coil 2, lower coil-side in slot 4, commutator segment 3 and back to the terminal B. The second parallel path is from commutator segment 1, lower coil-side in slot 2, back coil end-connection 4, top coil-side in slot 4, commutator segment 4,10wer coil-side of slot 1 and so on till terminal B is reached. The current entering at the right hand carbon brush divides equally between the two parallel paths described above. The effect of current in the armature coils is to set up an armature m.m.f. that is vertically upward along an axis 90° away from d-axis. This axis, which is space displaced by an angle of 90° from the d-axis, is called the quadrature axis. At the rotor surface, armature m.m.f. creates N-pole at the top and S-pole at the bottom of the rotor. The S-pole on the stator, attracts rotor N -pole and repels rotor S-pole. Similarly N -pole on the stator attracts rotor S-pole and repels rotor N -pole. The torque produced by the interaction of field and armature poles is called electromagnetic torque. If this machine is working as a motor, the rotor would rotate in the direction of electromagnetic torque, i.e. in the clockwise direction in Fig. 4.3. In case the machine is working as a generator, then the rotor must be driven by the prime-mover against the direction of electromagnetic torque for proper energy conversion. In Fig. 4.3, this direction of rotation for generator operation must be anticlockwise. 

            Fig. 4.3 (b) illustrates the conditions when the rotor has turned through 45° clockwise There are again two parallel paths, one through coil 1 and the second through coil 3. It maybé seen that coils 1, 3 (housed in slots 1, 3) have maximum e.m.fs. generated in them ; because these coils are cutting the maximum flux. Coils 2, 4 are short-circuited by the brushes and carry no currents, because e.m.fs. induced in these coils are zero. Currents in coil-sideg pertaining to slots 1, 3 in Fig. 4.3 (b), again establish armature m.m.f. that is direc Vertically upward. Note that coil current in each parallel path is again Ia/2 as it is in Fig. 4.3(a). 

            Fig. 4.3 (c) depicts the condition when the rotor has turned through by further 45° from the position of Fig. 4.3 (b). One parallel path is now through coils 4, 1 and the second parallel path is through coils 3, 2. Coils in each parallel path carry again Ia/2 as it is in Fig. 4.3 (a) and (b). The armature m.m.f. is again directed vertically upward. Hence it can be concluded that armature m.m.f. is always stationary in space and is directed along the interpolar 0r quadrature axis, even though the armature rotates. For a motor, the electromagnetic torque is always acting in the clockwise direction, (Fig. 4.3) and the rotation continues. For a generator, the armature must be driven in anticlockwise direction (Fig. 4.3), for the necessary energy conversion from' mechanical to electrical. 

         An examination of Fig. 4.3 reveals that just before the coils 2, 4 are short-circuited by the brushes, these coils carry current Ia/2 (or in general Ia/a) in one direction. Soon after the short-circuit is over, these two coils carry current Ia/2 (or in general Ia/a) in the reversed direction as shown in Fig. 4.3 (c). Here a is the number of parallel paths and in Fig. 4.3, a is equal to 2. Thus during the time of short-circuit, the currents in the short-circuited coils, in general, must be reduced from Ia/a to zero and then built up to Ia/a. This process of current reversal in the coils short-circuited by the brushes is called commutation and the time during Which it takes place is called commutation period. If the current variation with time is uniform from Ia/a to Ia/a as depicted in Fig. 4.4, a linear commutation is obtained. 

        Another important conclusion from the foregoing analysis is that the magnitude of current in the coils under each pole, remains constant and equal to Ia/a, (Fig. 4.4). But the magnitude of e.m.fs. induced in the coils under each pole is not constant coil under the pole centre has maximum e.m.f. induced in it, whereas the other coils have their e.m.f. magnitudes proportional to the flux density wave present there. 

       The coils short-circuited by the brushes during commutation process must have their coilsides in the zero flux density region so that e.m.f.s induced in them are zero. In other words, the coils undergoing commutation must have their coil sides in the magnetic neutral axis of the field poles, i.e. in the interpolar or quadrature axis. For example, coils 2 and 4 undergoing commutation in Fig. 4.3 (bmm in the magneW neutral axis or interpolar axis. The shape of the coil end connections is such that the brush' axis is aligned along the field pole or direct axis. For convenience in the schematic diagram of a DC. machine, the brushes are shown along the quadrature axis, i.e. in the position of the cons which are short-circuited by the brushes and are undergoing commutation-this schematic diagram for a 2-p01e D.C. machine is shown in Fig. 4.5 (a). Hence it may be stated that the armature m.m.f. is always directed along the brush axis, which now coincides with the interpolar or quadrature axis of the DC. machine. The circuit representation, more commonly employed for a DC. machine with any number of poles is as illustrated in Fig. 4.5 (b). In this figure, circular symbol represents the armature and two small rectangles or squares at the opposite sides of circle represent the two brushes. 

General Concepts of DC Machines for B.tech

            According to Faraday's laws of electromagnetic induction, whenever a conductor is placed in a varying magnetic field (OR a conductor is moved in a magnetic field), an emf (electromotive force) gets induced in the conductor.

            D.C. machine is a highly versatile energy conversion device. It can meet the demand of loads requiring high starting torques, high accelerating and declerating torques. At the same time, D.C. machine is easily adaptable for drives requiring wide-range speed control and quick reversals. These inherent characteristics can further be modified, if desired, by feedback ‘ circuits. In view of these outstanding features, D.C. machine possesses a high degree of flexibility. These are therefore widely used in industry, particularly for tough jobs as are encountered in steel-mill drives-inspite of their higher initial cost. 

           D.C. machines discussed in this chapter have hetropolar field system (alternate N and S poles) and armature-commutator system. In normal D.C. machines, stator core is not laminated ; armature core is, however, always laminated to reduce eddy-current losses. Direct-current machines used in control systems have their magnetic circuit completely laminated. This is done to minimise the effect of eddy-current damping on the fast response required in DC. machines employed in controlled systems. 

           At present, the annual production of DC. machines is about 40% of the rupee volume in electrical-machine production and sales. This is on account of the fact that most highway vehicles use batteries for the storage of electric energy. In these vehicles and automobiles, DC. motors are used as starter motors, windshield-wiper motors, fan motors and for driving other accessories in the vehicles. For these purposes, almost millions of DC. motors are built each year. In industrial applications requiring accurate control of speed and/or torque, DC. motor is unrivalled. Therefore, DC. motors are almost universally employed in steel and aluminium rolling mills, power shovels, electric elevators, railroad locomotives and large earth-moving equipment. 

DC machines - Label all its parts and mention the matertial

           The constructional features of DC. machines have already been described in Chapter 3, where it has been stated that field winding is a concentrated winding on salient poles bolted to the stator frame and armature Winding is a distributed winding housed in the Slots around the periphery of the cylindrical rotor. Basic principles underlying the torque production and e.m.f. generation in DC. machines are also outlined in Chapter 3. The object if this chapter is to present the physical concepts regarding the steady state behaviour of DC. machines. ‘ 

    The object of this example is to supplement the constructional details already described in Art 3.2.3. The reader must go through this article before studying the following presentation. 

(a) Sketch of a 6-pole D.C. machine is shown in Fig.1. In this figure, iron from the bottom of armature teeth to the shaft diameter is the armature core. The flux paths for the six poles are also shown. It is observed from this figure that 

                                            1. Flux paths in a 6-pole D.C. machines

(i) each pole carries a flux ɸ (say), 

(ii) yoke handles half of the pole flux, i.e. ɸ/2, 

(iii) armature core also handles a flux ɸ/2. 

        Reveals that main flux 4) starts from a north pole, crosses the air gap and then travels down to the armature core. There, it divides into two equal (¢/2) halves, each half enters the nearby south pole, each half then passes through the yoke and reaches the starting point of north pole so as to complete the flux path. Each flux line crosses the air gap twice. Some flux lines may not enter the armature ; this flux, called the leakage flux, is not shown in Fig.1. 

 (b) Various parts of a 6-pole D.C. machine are shown in Fig.1. commutator forms the most important component of a DC. machine. In Fig.2, various parts of a 4ole D C machine along with its commutator are labelled.

                                                                    2. 4-pole D.C. machines

       Stator of a DC. machine consists of yoke (or frame), field windings, interpoles, compensating Winding, brushes and end covers. Rotor consists of armature core, armature winding, commutator and shaft. Stator components are described first. 

Yoke -

             It has two functions : -

 (i) it provides path for the pole flux ɸ and carries half of it, i.e ɸ/2

 (ii) it prevides mechanical support to the whole machine. Since the flux carried by yoke is stationary (i.e. constant), it is not laminated. As stated before, case iron is used for small, D.C.machines and fabricated steel for large D.C. machines. In case DC. motor is to be operated through a power-electronics converter, the yoke is laminated to reduce the eddy/ current losses. 

Field poles -

                       Field pole consists of pole core and pole shoe. The pole core is made from cast steel but the pole shoe is laminated and fixed to the pole core appropriately. 

 The present-day practice, however, favours laminated pole. Thus, both pole core and pole shoe are made from thin laminations of sheet steel to reduce the eddy-current losses. The laminated pole is welded 0r bolted to the yoke. 

Field winding -

                          The pole is excited by a winding wound around the pole core. This winding, called field or exciting Winding, is prepared from copper. The number of turns and cross-section of field Winding depend upon the type of DC. machine as under : 

' (i) For D.C. shunt machine, large number of turns of small cross-section are used. 

(ii) For D.C. series machine, small number of turns of large cross-section are used. 

(iii) For D.C. compound machine, both shunt (thin Wire) and series (thick wire) field windings are used. 

Interpoles -

                    These are fixed to the yoke in between the main poles of a DC. machine. These are usually tapered With sufficient sectional area at the root to avoid magnetic saturation. The interpole Winding, consisting of a few turns of thick wire, is connected in series With the armature so that its magnetomotive force is proportional to armature current. 

Compensating windings -

                                            These windings are placed in the slots cut in the pole faces of a DC. machine. Compensating winding is also connected in series With the armature circuit. This winding is, however, used in large D.C. machines only. 

Brushes -

                   Brushes are housed in box-type brush holders attached to the stator end cover, Fig.2 , or the stator yoke. A small spring keeps the brushes pressed on to the commutator surface. The brush pressure on the commutator surface must be carefully adjusted. Too small a brush pressure may lead to excessive arcing between the brushcommutator contact. If the brush pressure is too high, it may cause excessive wear of the commutator surface and the brushes. Brushes are made of carbon for small D.C. machines, electrographite for all D.C. machines and copper-graphite for low-voltage high-current D.C. machines. Rotor components are now described below.

Armature core -

                            It serves the twin purpose of (i) housing the armature coils in the slots and (ii) providing the low-reluctance path to the magnetic flux ¢l2. It is made from 0.35 to 0.50 111m thick laminations of silicon steel to keep down the iron losses. For larger sizes of DC. machines, the armature core is placed on the spider as shown in Fig.2. 

 Armature winding -

                                  The armature winding is made from copper. It consists of large number of insulated coils, each coil having one or more turns. The coils are usually former Wound. These are placed in slots and appropriately connected in series and parallel depending “Poll the type of winding required. There are basically two winding types : (i) lap Winding and (”Wave winding. Two coil ends of each coil are then connected to the riser of segments of a cmmutator, Fig.2. 

Commutator -

                           It 1s of cylindrical structure. It is built up of wedge-shaped segments of

high conductivity hard-drawn ccpper to reduce its wear and tear. Segments. are insulated from each other by 0.8 mm thick mica sheets. The segments are tapered as shown in Fig.3. so that their assemblyresults in circular shape. Hub H and ring R are insulated from commutator segments by mica sheet M and V-shaped so as to prevent the segments from flying out due to centrifugal forces, Fig.2. Each commutator segment, Fig. 3, has a riser where conductors from the armature winding are connected.

                                                      3. One commutator segment

Shaft -

            On armature shaft are mounted 

(i) hub H of commutator, 

(ii) spider in big machines or armature core in smallmachines and (iii) bearings. End covers are connected to the yoke on one side and to the bearings and shaft on the other side,Fig.2.

  

MRSPTU B. TECH. ELECTRCAL ENGINEERING SYLLABUS 2019

After 4th semester, student will go for 6 Weeks Institutional / Industrial Training in which he/she should cover complete knowledge of at least one of the following software:
MATLAB/LabVIEW/C/C++/Automation/AutoCAD (Electrical)/Data Analysis using Excel.

n-type and p-type materials in a semiconductor conduct electricity

n-TYPE AND p-TYPE MATERIALS

                 Because Si is the material used most frequently as the base (substrate) material in the con- struction of solid-state electronic devices, the discussion to follow in this and the next few sections deals solely with Si semiconductors. Because Ge, Si, and GaAs share a similar covalent bonding, the discussion can easily be extended to include the use of the other materials in the manufacturing process. As indicated earlier, the characteristics of a semiconductor material can be altered sig- nificantly by the addition of specific impurity atoms to the relatively pure semiconductor material. These impurities, although only added at 1 part in 10 million, can alter the band structure sufficiently to totally change the electrical properties of the material. A semiconductor material that has been subjected to the doping process is called an extrinsic material. sureable importance to semiconductor device fabrication: n-type and p-type materials. Each is described in some detail in the following There are two extrinsic materials of immea subsections. 

                                           

n-Type Material

                  Both n-type and p-type materials are formed by adding a predetermined number of impuri atoms to a s ilicon base. An n-type material is created by introducing impurity elements th have five valence electrons (pentavalent), such as antimony, arsenic, and phosphorus. Each a member of a subset group of elements in the Periodic Table of Elements referred to as Gro V because each has five valence electrons. The effect of such impurity elements is indicated Fig. 1.7 (using antimony as the impurity in a silicon base). Note that the four covalent bor are still present. There is, however, an additional fifth electron due to the impurity atom, wh is unassociated with any particular covalent bond. This remaining electron, loosely boun its parent (antimony) atom, is relatively free to move within the newly formed n-type mater Since the inserted impurity atom has donated a relatively "free" electron to the structure: Diffused impurities with five valence electrons are called donor atoms. It is important to realize that even though a large number of free carriers have been es lished in the n-type material, it is still electrically neutral since ideally the number of tively charged protons in the nuclei is still equal to the number of free and orbiting negat charged electrons in the structure.

                                                  n-type material

      

           The effect of this doping process on the relative conductivity can best be described through the use of the energy-band diagram. Note that a discrete enenerg level (called the donor level) appears in the forbidden band with an Eg significantly less than that of the intrinsic material. Those free electrons due to the added impurity sit at this energy level and have less difficulty absorbing a sufficient measure of thermal energy to move into the conduction band at room temperature. The result is that at room temperature, there are a large number of carriers (electrons) in the conduction level, and the conductivity of the material increases significantly. At room temperature in an intrinsic Si material there is about one free electron for every 10¹² atoms. If the dosage level is 1 in 10 million (10⁷),the radio 10¹²/¹⁰⁷= 10⁵ indicates that the carrier concentration has increased by a ratio of 100,000:1.

                                        Effect of donor impurities

 p-Type Material

   

                  The p-type material is formed by doping a pure germanium or silicon crystal with impurity atoms having three valence electrons. The elements most frequently used for this purpose are boron, gallium, and indium. Each is a member of a subset group of elements in the odic Table of Elements referred to as Group III because each has three valence electrons. The effect of one of these elements, boron, on a base of silicon is indicated. 

        Note that there is now an insufficient number of electrons to complete the covalent band of the newly formed lattice. The resulting vacancy is called a hole and is represented by a  small circle or a plus sign, indicating the absence of a negative charge. Since the resulting vacancy will readily accept a free electron: 

           The diffused impurities with three valence electrons are called acceptor atoms.

                                                   p-type material

     The resulting p-type material is electrically neutral, for the same reasons described for the n-type material

Basic of Transformers Construction

    Transformer Construction:-

                                                  There are two general types of transformers, the core type and the shall type. These two types differ from each other by the manner in which the windings are wound around the magnectic core.

  The magnetic core is a stack of thin silicon-steel laminations about 0.35mm thick for 50Hz transformers. In order to reduce the eddy current losses, these laminations are insulated from one another by thin layers of varnish. For reducing the core losses, nearly all transformers have their magnetic core made from cold-rolled grain-oriented sheel-steel (C.R.G.O). This material, when magnetized in the rolling direction, has core loss and high permeability.

        In the core-type, the winding surround a considerable part of steel core as shown in fig (a). In the shell type the steel core surround a major part of the windings as shown in fig (b).

(a)

(b)

      For a given output and voltage rating, core-type transformer requires less iron but more conductor material as compared to a shell-type transformer. The vertical portions of the core are usually called limbs or legs and the top and bottom portions are called the yoke. This means that for single phase transformers, core-type has two legged core whereas shell type has three legged core. In iron-core transformers, most of the flux is confined to high permeability core. Leaks flux through  the core legs and non-magnetic material surrounding the core. This flux called leakage flux links one winding and not the other. A reduction in this leakage flux is desirable as it improves the transformer performance considerably. Consequently, an effort is always made to reduce it. In the core-type transformer, this is achieved by placing half of the low voltage winding over leg and other half over the second leg or limb. For the high voltage winding also, half of the winding is over one leg and the other half over the second leg as shown in fig (c)

                                       (c)

  low voltage winding is placed adjacent to the steel core and H.V. winding outside, in order to minimise the amount of insulation required.

            In the shell type transformer, the L.V. and H.V. windings are wound over the central limb and are interleadved or sandwiched as shown in fig (d).

(d)

 Note that the botton and top L.V. coils are of half the size of other L.V. coils. Shell-type transformers are preferred for low voltage low-power levels, whereas core type construction is used for high voltage . In core type transformer, the flux, has a single path around the legs or yokes. In the shall type transformer, the flux in the central limb divides equally and returns though the outer two legs.There are two types of windings employed for transformers. The connecntic coils are used for core-type transformer. One type of laminations for the core and shell-type of transformers is illustranted  as shown in fig (d)

(d)

The steel core is assembled in such a manner that the butt joints in adjacent layers are staggened as illustrated. The staggering of the joints avoids continuous air gap and therefore, the reluctance of the magnetic circuit is not increased. At the same time, a continuous air gap would reduce the mechanical strength of the core and therefore, the staggering of the butt joint is essential.

    During the transformer construction first the primary and secondary winding are wound, then the laminations are pushed through the coil openings, layer by layer and the steel core is prepared. The laminations are then tightened by means of clamps and bolts. Low power transformers are air-cooled whereas large power transformers are immersed in oil  for better cooling. In oil-cooling transformers, the oil serves as a coolant and also as an  insulating medium.

    For power frequency range of 25 Hz to 400 Hz, transformers are construnted with 0.35mm thick silicon-steel laminations. For audio-frequency range of 20Hz to 20,000Hz, iron core with suitable refinements is used. For high frequencies employed in communication circuits, core is made up  of powderd ferromagnetic alloy. In special cases, the magnetic circuit of a transformer may be made of non-magnetic material and in such a case, the transformer is referred to as an air-core transformer. The air-core transformer is primarily used in radio devices and in certain types of measuring and testing instruments. Cores made of soft ferrites are also used for pulse transformers as well as for high frequency electronic transformers.

Permanent magnet moving coil

                               Permanent magnet moving coil

                                                  (PMMC)

         The instruments of this type, the deflection of the instrument provides a basis for determining the quantity under measurement. The measured quantity produces some physical effect with deflects produces a mechanical displacement of the moving system of the instrument.
         An  opposing effect is built in the instrument which tries to oppose the deflection or the mechanical displacement of the moving system. The opposing effect is closely related to the defection or mechanical displacement which can be directly observed. The opposing effect is so designed that it magnitude increases with the increase of eflection or mechanical displacement of the moving system caused by the quantity under measurement. The balance is achieved when opposing effect equal to cause producing the deflection or mechanical movement. The defection or mechanical movement. The value of the measured quantity can then be  inferred from the deflection or mechanical displacement at the point of balance.

                                                          Permanent magnet moving coil
                                               
Example PMMC :- 
                               PMMC in a use ammeter, the deflection of the moving coil is proportionl of the cunnent I the quantity under measurement. The torque acting on the moving coil is proportionl to current or which is  dependent upon flux density, number of turns and area of moving coil. The opposing effect is produced by a spring whose torque is proportionl to deflection or where is the spring contant whose value depends upon materil and the dimension of  the spring. Under the condition of balance or deflection and current.         
    The value of measured quantity, cunnent, in this case dependent upon the value of deflection and the constants of the meter. thus, in order to make the instrument direct reading, it reads the value of current direction in terms of terms of the deflection, it will have to be calibrated to make into consideration by value G and  K.
       we conclude in a deflection  type instrument the value of measured quantity depends upon the calibration of the  instrument. 

Basic A. C machines

 

 These are futher classified as transfomers , synchronous machines, induction machines  and A.C commutator machines.

1. Transformers :-
                               A transformer is not an electromechanical converter. It is used in almost all the devies energised from  ac source. It usually steps up or steps down electrical energy from one voltage level to another voltage level. However, it is also employed with unity turns ratio for isolation purposes. In small sizes single phase transformers are used. For large ratings, 3-phase transformers are more common.

2. Synchronous machines:-
                                             In these machines, the field poles may be on stator or rotor. In bigger sizes, field poles are always on the rotor. Field windig on  rotor is excited with D.C . whereas stator winding handles three-phase A.C. power. Rotor runs at synchronous speed Ns . Frequency f  of the e.m.f. generated in armature is given by
                                                                 f = PNs / 120  Hz
    A synchronous machine is a doubly excited machine. It is employed as an alternator for the generation of three phase power at all the generating stations. It is also used as a synchronous motor for special applications.

3. Induction machines:-
                                        These are of two types, three-phase induction machiase  and single phase induction machines. Both types are singly excited in the sense that stator of 3-phase induction motor is fed from 3-phase A.C. source whereas stator of single phase induction motor is energised from 1-phase A.C. source.
A). Three phase induction machines :- 
                                                                  The stator winding of a three phase induction motor, connected to 3-phase mains, carries both load current and field-producing exciting current. The field produced rotates at synchronous speed in the air gap but rotor speed  is always less than synchronous
speed. Two types of 3-phase induction motor is energised from 1-phase A.C. source.
a) . SCIM:-
                  Rotor carries copper, or aluminium, bars embedded in rotor slots. these bars are short-circuited by two end rings, one at each end. SCIM has the most rugged construction and is used where control of starting torque or speed is not needed.
b) . SRIM:-
                   Its stator is similar to the stator of SCIM, but rotor carries 3-phase winding with three leads connected to three slip rings mounted on the shaft. Brushes pressing on the slip rings allow the insertion of external resistance in the rotor circuit. SRIMs are used where speed, or torque, control is required. A 3-phase induction motor, connected to 3-phase source, can operate as a 3-phase induction generator when driven above synchronous speed. This, however has not become commercially popular due to certain limitation.
B). Single phase induction machines:-
                                                                 In addition to 3-phase induction machines discussed above, single phase induction motors are used where single phase low voltage is available, as in homes, offices, class rooms,  shops, etc.

4. A.C. Commutator machines:-
                                                      As the name suggests, these machines are fed from A.C. source and are fitted with commutators. A large variety of A.C commutator machines are reported in the literature. Only three phase schrage motors and single phase series motors have withstood the onslaught of time . For wide speed control, 3-phase schrage motor is reliable and introduces no harmonics into the supply system as are produced by inverter-fed three phase induction motors.
     Single phase series motors are indispensable where high starting torque and light weight is desirable as in sewing machines, food processor, vacuum cleaners and tools. In small sizes, this motor is gives the highest power output per cost, though it makes more noise.  In addition, this motor is easily adaptable to speed or torque control though the use of power electronics based 1 phase voltage controllers.