Electrical World

For the motoring action, there must exist two fluxes which interact with each other to produce the torque. In d.c. motors, field winding produces the main flux while d.c. supply given to armature is responsible to produce armature flux. The main flux and armature flux interact to produce the torque.        In the single phase induction motor, single phase a.c. supply is given to the stator winding. The stator winding carries an alternating current which produces the flux which is also alternating in nature. This flux is called main flux. This flux links with the rotor conductors and due to transformer action e.m.f. gets induced in the rotor. The induced e.m.f. drives current through the rotor as rotor circuit is closed circuit. This rotor current produces another flux called rotor flux required for the motoring action. Thus second flux is produced according to induction principle due to induced e.m.f. hence the motor is called induction motor. As against this in d.c. mot

According to this theory, any alternating quantity can be resolved into two rotating components which rotate in opposite directions and each having magnitude as half of the maximum magnitude of the alternating quantity.        In case of single phase induction motors, the stator winding produces an alternating magnetic field having maximum magnitude of Φ 1m .        According to double revolving field theory, consider the two components of the stator flux, each having magnitude half of maximum magnitude of stator flux i.e. (Φ 1m /2). Both these components are rotating in opposite directions at the synchronous speed N s which is dependent on frequency and stator poles.        Let Φ f  is forward component rotating in anticlockwise direction while Φ b  is the backward component rotating in clockwise direction. The resultant of these two components at any instant gives the instantaneous value of the stator flux at the instant. So resultant of these two is the original stator

Consider a single phase induction motor with standstill rotor as shown in the Fig. 1. The stator winding is excited by the single phase a.c. supply. This supply produces an alternating flux Φ s which acts along the axis of the stator winding. Due to this flux, e.m.f., gets induced in the rotor conductors due to transformer action. As rotor is closed one, this e.m.f. circulates current through the rotor conductors. The direction of the rotor current is as shown in the Fig. 1. The direction of rotor current is so as to oppose the cause producing it, which is stator flux Φ s . Fig.  1        Now Fleming's left hand rule can be used to find the direction of the force experienced by the rotor conductors. It can be seen that when Φ s acts in upward direction and increasing positively, the conductors on left experience force from left to right while conductors on right experience force from right to left. Thus overall, the force experienced by the rotor is zero. Hence no tor

In practice some arrangement is provided in the single phase induction motors so as that the stator flux produced becomes rotating type rather than the alternating type, which rotates in particular direction only. So torque produced due to such rotating magnetic field is unidirectional as there is no oppositely directed torque present. Hence under the influence of rotating magnetic field in one direction, the induction motor becomes self starting. It rotates in same direction as that of rotating magnetic field. Thus depending upon the methods of producing rotating stator magnetic flux, the single phase induction motors are classified as, Split phase induction motor Capacitor start induction motor Capacitor start capacitor run induction motor Shaded pole induction motor         To produce rotating magnetic field, it is necessary to have minimum two alternating fluxes having a phase difference between the two. The interaction of such two fluxes produces a resultant flux whi

This type of motor has single phase stator winding called main winding. In addition to this, stator carries one more winding called auxiliary winding or starting winding. The auxiliary winding carries a series resistance such that its impedance is highly resistive in nature. The main winding is inductive in nature.                    Let                  I m = Current through main winding                     and                  Ist = Current through auxiliary winding        As main winding is inductive, current I m lags voltage by V by a large angle Φ m while Ist is almost in phase in V as auxiliary winding is highly resistive. Thus three exists a phase difference of α between the two currents and hence between the two fluxes produced by the two currents. This is shown in the Fig.1(c). The resultant of these two fluxes is a rotating magnetic field. Due to this, the starting torque, which acts only in one direction is produced. Fig. 1   Split phase induction motor        Th

The construction of this type of motors is similar to the resistance split phase type. The difference is that in series with the auxiliary winding the capacitor is connected. The capacitive circuit draws a leading current, this feature used in this type to increase the split phase angle α between the two currents I m and Ist.        Depending upon whether capacitor remains in the circuit permanently or is disconnected from the circuit using centrifugal switch, these motors are classified as,  1. Capacitor start motor and       2. Capacitor start capacitor run motors         The connection of capacitor start motor is shown in the Fig. 1(a). The current I m lags the voltage by angle Φ m while due to capacitor the current Ist leads the voltage by angleΦ st . Hence there exists a large phase difference between the two currents which is almost 90 o , which is an ideal case. The phasor diagram is shown in the Fig.1(b). Fig 1.  Capacitor start motor        The starting torque is pr

This type of motor consists of a squirrel cage rotor and stator consisting of salient poles i.e. projected poles. The poles are shaded i.e. each pole carries a copper band on one of its unequally divided part called shading ban Fig.1(a) shows 4 pole shaded pole construction while Fig. 1(b) shows a single pole consisting of copper shading band. Fig 1 Key point : When single phase a.c. supply is given to the stator winding, due to shading provided to the poles, a rotating magnetic field is generated.        The production of rotating magnetic field can be explained as below :         The current carried by the stator winding is alternating and produces alternating flux. The waveform of the flux is shown in the Fig. 2(a). The distribution of this flux in the pole area is greatly influenced by the role of copper shading band. Consider the three instants say t 1 , t 2 and  t 3 during first half cycle of the flux as shown, in the Fig 2(a). Fig. 2 (a)  Waveform of stator flux    

The double revolving field theory can be effectively used to obtain the equivalent circuit of a single phase induction motor. The method consists of determining the values of both the fields clockwise and anticlockwise at any given slip. When the two fields are known, the torque produced by each can be obtained. The difference between these two torques is the net torque acting on the rotor.        Imagine the single phase induction motor is made up of one stator winding and two imaginary rotor windings. One rotor is rotating in forward direction i.e. in the direction of rotating magnetic field with slip s while other is rotating in backward direction i.e. in direction of oppositely directed rotating magnetic field with slip 2 - s.        To develop the equivalent circuit, let us assume initially that the core loss is absent. 1. Without core loss Let the stator impedance be Z Ω                             Z = R 1 + j X 1 Where                  R 1 = Stator resistance      

Similar to a three phase induction motor, the various tests can be performed on single phase induction motor. The results of these tests can be used to obtain the equivalent circuit parameters of a single phase induction motor. The tests usually conducted are : 1. No load test or open circuit test 2. Blocked rotor test or short circuit test 1. No load test        The test is conducted by rotating the motor without load. The input current, voltage and power are measured by connecting the ammeter, voltmeter and wattmeter in the circuit. These readings are denoted as V o , I o and W o . Now                 W o = V o I o cosΦ        The motor speed on no load is almost equal to its synchronous speed hence for practical purposes, the slip can be assumed zero. Hence r 2 /s becomes ∞ and acts as open circuit in the equivalent circuit. Hence for forward rotor circuit, the branch r 2 /s + j x 2 gets eliminated.        While for a backward rotor circuit, the term r 2 /(2 - s) tends

The characteristics depends on whether generator is cumulatively compound or differentially compound generator. In cumulatively compound, Φ 2r = Φ 2r + Φ 2r . As load current increases,  I a increases hence  I se  also increases producing more flux. But as  I a increases, the various voltage drops and armature reaction drop also increases. Hence there is drop in the terminal voltage.        If drop in V t due to increasing  I L is more dominating than increase in V t due to increase in flux then generator is called under compounded and its characteristics is dropping in nature, as shown in the Fig. 1. Fig. 1 Characteristics of compound generator        If drop in V t due to armature reaction and other drops is much less than increase in V t due to increase in flux then generator is called over compound and its characteristics is rising in nature, as shown in the Fig. 1. If the effects of the two are such that on full load current V t is same as no load induced e.m.

Consider a series generator shown in the Fig. 1        In case of series generator,         I a = I se =  I L        As load current increases,  I se increases. The flux Φ is directly proportional to. So flux also increases. The induced e.m.f. E is proportional to  I se flux hence induced e.m.f. also increases. Thus the characteristics of E against i.e. internal characteristics is of increasing nature. As increases I a , armature reaction increases but its effect is negligible compared to increase in E. But for high load current, saturation occurs and flux remains constant. In such case, due to the armature reaction E starts decreasing as shown by dotted line in the Fig. 2. Fig. 1 Characteristics of d.c. series motor        Now as I L = I a  increases, thus the drop I a  (R a  +R se ) increases.       V t = E - I a  (R a  +R se )       Thus the external characteristics is also of rising nature as E increases but it will be below internal characteristic

Consider the d.c. shunt generator shown in the Fig. 1. The internal characteristics is E V s I L while the external characteristics is V t  against I L . Fig. 1 Internal characteristics        Let us see the nature of these two characteristics.        Ideally the induced e.m.f. is not dependent on the load current I L or armature current I a . But as load current increases, the armature current I a increases to supply load demand. As I a increases, armature flux increases. Note : The effect of flux produced by armature on the main flux produced by the field winding is called an armature reaction.        Due to the armature reaction, main flux pattern gets distorted. Hence lesser flux gets linked with the armature conductors. This reduces the induced e.m.f. Note : Thus the armature conductors. This reduces the induced e.m.f.        This is shown in the Fig. 2. Fig. 2  Internal characteristics 1.2 External Characteristics         For d.c. shunt generator we know that, E = V t + I a

The characteristics is separately excited d.c. generator are divided into two types, 1) Magnetization   and         2) Load characteristics. 1.1 Magnetization or Open Circuit Characteristics         The arrangement to obtain this characteristics is shown in the Fig. 1. Fig. 1  Obtaining O.C.C. of separately excited generator        The rheostat as a potential driver is used to control the field current and the flux. It is varied from zero and is measured on ammeter connected.        E o  = (ΦPNZ) / (60A)        As I f is varied, then Φ change and hence induced e.m.f. E o  also varies. It is measured on voltmeter connected across armature. No Load is connected to machine, hence characteristics are also called no load characteristics which is graph of E o  against field current I f as shown in the Fig. 2. As I f increases, flux Φ increases and E o increases. After point A, saturation occurs when Φ becomes constant and hence E o  saturates. Fig. 2  Open circuit characteristics

The d.c. generators have following characteristics in general, 1) Magnetization characteristics 2) Load characteristics 1.1 Magnetization Characteristics        This characteristics is the graph of generated no load voltage E against the field current I f , when speed of, generator is maintained constant. As it is plotted without load with open output terminals it is also called No load characteristics or Open circuit characteristics.        E o V s I f  is magnetization characteristics        Where  E o = No load induced e.m.f.        But for generator,        E = (ΦPNZ) / (60A) . . .     E α   Φ                 with (PNZ) / (60A)  constant . . .     E  α   I f                    as   Φ  α   I f        Thus induced e.m.f. increases directly as I f  increases. But after certain I f  core gets saturated and flux also remain constant though I f  increases. Hence after saturation, voltage also remains constatnt. Note : Thus characteristics is linear till saturation and after tha

In this type, the part of the field winding is connected in parallel with armature and part in series with the armature. Both series and shunt field windings are mounted on the same poles. Depending upon the connection of shunt and series field winding, compound generator is further classified as : i) Long shunt compound generator, ii) Short shunt compound generator. 1.1 Long Shunt Compound Generator        In this type, shunt field winding is connected across the series combination of armature and series field winding as shown in the Fig. 1. Fig. 1 Long shunt compound generator        Voltage and current relations are as follows.        From the Fig. 1.        I a = I se         and  I a = I sh +  I L        Voltage across shunt field winding is V t .        I sh = V t /R sh       where R sh = Resistance of shunt field winding        And voltage equation is,        E = V t + I a R a + I a R se + V brush        Where R se = Resistance of series field winding 1.2 Short Shunt Co